# PMC IMP Build log and WIP



## GailInNM (Mar 15, 2009)

About 10 days ago I made the first run of a sort of replica version of a PMC IMP. A couple of photos of it on the test stand and a short video of it's second run were documented at:
http://www.homemodelenginemachinist.com/index.php?topic=4338.0
I promised a construction series on it and this is the start of that. There will be details left out, mostly because I forgot to put them in. If there is something specific that you would like more information on, just ask. I may not have the answer, but you can ask.

This whole thing started in early November 2008. As I often do, I was peeking around on the ModelEngineNews.org web site and happened across the PMC IMP. Here are a couple of links to information on the IMP from there. If you are interested, do a search on the site and there is more information.

http://modelenginenews.org/cardfile/pmcimp.html
http://modelenginenews.org/ed.2005.09.html#3

In addition, Ron had prepared CAD drawing by reverse engineering an IMP, and those drawings appear in the "members only" area of the site and are included in the DVD that he has prepared for members of the site. The drawings have a few minor omissions in them, but nothing that would prevent construction. After looking at them for a few days, I sketched up 3 versions of the IMP. On all the versions, Imperial screws were substituted tor the BA screws on the original.

Version 1 is fairly much as the IMP was originally constructed. It is a fair representation of the IMP, but there were quite a few variations of the IMP. As only a few were built, and they were hand built, it is probably fair to say that almost every one was a little bit different.

Version 2 of the IMP is mostly the same as Version 1, except the front and back plates for the crankcase are screwed on, beam mounts added, and the crankcase has a rounded bottom. In addition the cylinder and head are held on by 4 screws instead of 2.

Version 3 is the same as Version 2 except the exhaust and intake positions are reversed, so the engine more resembles the EmBee which the IMP design has as an ancestor.

The common parts of all three versions have been built at the same time. At this point, only Version 2 is complete and running. Versions 1 and 3 have a few parts to go, some being made over using some of the information learned from Version 2. 

I am showing this as a "Work in Progress" as Versions 1 and 3 will be completed (I hope) during the life of this thread.

Here is a photo of Version 2. Followed by a photo or the original engine from ModelEngineNews.org so you can see some of the diffenences between Version 1 and version 2.











Gail in NM,USA


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## GailInNM (Mar 15, 2009)

Lets start off by putting things in perspective. Because I work in the imperial system, all dimensions will be inches unless other wise specified.
The cylinder fins measure 0.875 inch (22mm) diameter and the overall height is 2.7 inch (68mm). Width over the mounting lugs is 1.375 inch (35mm). Prop shaft is 0.190 diameter threaded 10-32.
The fasteners are all either 0-80 or 2-56. 
Weight is 3.1 ounces or 89 grams.

Bore is 0.313 inch (0.8mm) and stroke is 0.488 inch (12.4mm). 
Displacement is 0.038 cubic inches or 0.62 cc.  Thats about 1/16 the displacement of the Maryak 10 that Bob is building or 80 percent of the 0.049 engines that many of us played with as kids (and some of us still play with).

So it's oversize and overweight for it displacement. And while I don't think ANY engine is ugly, the original that this is based on comes close.
Version 2 performance was 9060 RPM with a 7-4 propeller and 10900 RPM with a 6-3 propeller. With such a long stroke to bore ratio, it will never turn a very high RPM, but it will swing a big club.

Gail in NM,USA

Edited to add displacement.


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## Maryak (Mar 15, 2009)

Gail,

Great start. :bow: and thanks for all your help with M10. :bow:

Best Regards
Bob


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## GailInNM (Mar 28, 2009)

Sorry for the delay in getting started on this. Seems the big bad wolf tried to eat the bluebird of happiness, but I think all is resolved for now.

A few notes to get started with. As mentioned, there are three versions being built more or less in parallel, so some of the photos will be of different versions, so don' let it fool you. Where differences are important, I will denote the versions as V1, V2 and V3. If not mentioned, things will probably apply to all versions. 

I normally work in imperial measurements, so unless other wise noted, dimensions will be in inches. The original had a mixture of BA and imperial threads. I have used all imperial threads.

For openers, here is the general arrangement drawing as extracted from the plans. It is reproduced here with permission of Ron Chernich of modelenginenews.org. The plans for this engine are published by Ron on his website. They are available free to members of his site or are available from him for US$ 15 mailed anywhere in the world. The plans are 5 sheets of letter size (ANSI A).
Gail in NM,USA


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## GailInNM (Mar 28, 2009)

On most of my model aircraft style engines I start with the crankcase. Then I start adding parts to it as they are made so I can see it to start looking like an engine. This build is no exception. 

I started by saw cutting 6061 alumninum for the 3 crankcases and then milling to the maximum dimensions of the 3 crankcases. The height and depth are the same< 1.667 X 0.773) for all three engines, but the width is 1.0 for V1 and 1.375 for V2 and 3. The extra width is to accomodate the mounting lugs.






All the crankcases were then drilled and bored for the front bearing and rear covers. I first drilled all with a 1/2 inch drill, and then rough bored all to .740 diameter. Each was then finish bored to 0.750 inch diameter. The original was 0.756 inch diameter, but as it was not critical excepting for the mating parts I reduced it to a standard fractional size. 

Not visible in the photo is that I marked the upper right corner for registration so I could return the parts to my fixed vice stops with out having to indicate the parts in for the following operations.






On V2 and V3 six mounting screw holes for both the front bearing and rear cover were center drilled, drilled and tapped 0-80. Four on each side would have be adequate but I liked the look of six. On V1, only two 4-40 holes were drilled on the rear of the crankcase and the front side was left plain. On the plans, the holes were 6BA. No holes were used on the front as the front bearing will be glued in place as was the original. While this seems a little crude, several different brands of engines used this techinque during this era and I don't know of any problems resulting from it. Key was a close fit between the crankcase and front bearing and a fairly large surface area for the glue. 






Gail in NM,USA


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## GailInNM (Mar 29, 2009)

Before continuing, a brief bit about the machine tools I use. My milling machine is a ancient Bridgeport CNC. For this project, I used it in a manual mode for all but two operations. The CNC mode was not necessary for either, but was more convenient at the time. My lathe is an import tool room lathe with a DRO. 11 inch swing and 19 inches between centers. I normally use collets on it. I do have 3 and 4 jaw chucks for it, but rarely use them. None of this is important except it determines my style of machining.

The next operations on the crankcase are to reduce the upper portion to a "double D" shape and putting a 7/16 hole from the top to intersect with the 3/4 hole. I did this operation using the CNC mill, but it could be just as easily done in the lathe using a 4 jaw chuck.

First the top was milled to an 0.85 diameter with two flats remaining from the 0.773 depth of the blank. 






While still set up, the 7/16 hole was drilled and reamed. This hole needs to have a good finish as the cylinder liner will fit in it. The liner needs to be a close fit to minimize leakage around the liner from the intake and exhaust ports to each other and to the crankcase lower cavity.











At the same setup, but not photographed, I center drilled, drilled and tapped the holes for the cylinder head mounting bolts. V1 has 2 4-40 holes to substitute for the 2 6BA holes on the original. V2 and V3 have 4 2-56 holes, only because 2 holes just looks wrong. These holes can be seen in the second and third photographs as they were put in before the 7/16 hole was put in.
Gail in NM,USA


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## GailInNM (Mar 31, 2009)

Only one operation left on the crankcase of V1. Drilling and tapping the port opening for the fuel intake assembly and the opening for the exhaust. On the original, the exhaust opening was just a 1/4 inch drilled hole matching the port openings on the cylinder. I elected to tap this hole also to permit a short exhaust pipe for cosmetic purposes. The same operations are also done on V2 and V3. At this point V2 and V3 crankcases become different as the exhaust and intake are reversed. The exhaust hole ia about 0.10 inches higher than the intake hole. The positioning of these holes is not critical as the actual timing is controlled by the port holes in the cylinder, so these holes just need to cover these small holes.

I did deviate from the original a little bit. On the original, the intake hole was threaded 1/4-32. I have a 1/4-32 tap, but I like fine threads and since I had a 1/4-40 tap and die also I changed to 1/4-32. If someone were to be building this engine and did not have either, I would recommend that they stay with the original 1/4-32 as this is a common size thread for a lot on model engineering parts, in particular glow plugs and small spark plugs. It is also sometimes used for small steam fittings.

Lookng at the photo, you can see that I spotfaced the surface on the curved side with a 3/8 diameter end mill so the nut would have a smooth surface to seat on.
Gail in NM,USA


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## Maryak (Mar 31, 2009)

Gail,

It's coming along very nicely. :bow: :bow:

Best Regards
Bob


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## kvom (Apr 1, 2009)

Gail,

Is that a metal plate fixed to the table? For keeping chips out of the t-slots?


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## GailInNM (Apr 1, 2009)

KVOM,
The plate you see in the photos above is a 15 X 21 X 1 piece of MIC6 ground tooling plate that is bolted to the table. The vices are bolted to it. Beyond the tooling plate on each side are pieces of 1/16 sheet aluminum to keep chips out of the T slots. The Chick vices have 4 holes through them for bolts and 2 precision holes in the bottom for location. The tooling plate has precision location holes bored to match the vices. There are 3 sets on holes spaced on 8 inch centers to take 3 vices. The plate also has 4 locating pins to fit the table so it could be removed and replaced if necessary. The photos only show 2 vices as I loaned one of them to my son. It is home again, but I have not remounted it. Since all the vice jaws are removable and can be reconfigured I can clamp anything from small parts up to 12 inch wide, and by using all three vices I can cover the 12 X 25 inch travel of the machine with only a small overhang on the outsides of the outer vices. I put the fixture plate on over 20 years ago and have never had to remove it.


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## GailInNM (Apr 1, 2009)

The V2 and V3 crankcases need the mouting holes drilled in the mounting lugs and the bottom of the crankcase rounded.

The mounting holes are drilled first while it is still easy to clamp the crankcase square in the vice before the bottom is rounded. They only have to be drilled about 1/8 inch deep. Prior to drilling the holes, I milled away the material down to the top surface of the mounting lug, then drilled the holes in the same setup.






I used the CNC to round the bottom of the crankcase. CNC is by no means necessary to do this. Bob shows how he rounded his Maryak 10 using a rotary table mounted vertical on his milling machine in:
http://www.homemodelenginemachinist.com/index.php?topic=3712.msg39398#msg39398
I have also rounded crankcases by planeing in the lathe using a wide parting tool mounted to cut horizontally and moving it with the carriage hand wheel.







Here is short video of the CNC machining the bottom of the crankcase. When doing it this way, it should be noted that the path of the cutter is not circular. This is because the cutter path is described at being at the center of the cutter, but it is the edge of the end of the cutter that we want to move in a circular path. I did this by writing a parametric program for the CNC so the cutter path is controlled by an equation. The program is only about a dozen lines long and took about 5 minutes to write. If someone is really interested in how that is done, I will make a thread about it in the CNC section sometime.
Gail in NM,USA

[youtube=425,350]KWvmAtzbQ6I[/youtube]


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## GailInNM (Apr 1, 2009)

A couple of posts back I said the V1 crankcase was done.  I lied. ;D

While it is not shown on the drawings, the junction of the of the 7/16 inch and the 3/4 inch bores in the crankcase need to be relieved for clearance of the connecting rod. Rather than calculate and try to machine this clearance, it is easier to do this later after other parts have been made and assembly is started. I cut the clearance with a Dremel rotary tool using a long 3/16 diameter pear shaped cutter with an 1/8 inch shank.

Even though it will be done later, I included this step here to keep all the crankcase operations in one spot.


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## GailInNM (Apr 2, 2009)

The recess between the upper and lower part of the crankcase can be cut at any time it is convenient. As I had "turned" the upper part of the crankcase with the CNC mill, I put it in near the last of the crankcase machining and did it on the lathe. If I had turned the upper part using a 4 jaw and the lathe, I would have put it in then. The recess is purely cosmetic and the only benefit it gives is to reduce the weight. These engines are so over weight anyway that is a dubious benefit. I put it in because the original had it and anything to improve the looks of this engine is helpful.
Gail in NM,USA


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## Maryak (Apr 2, 2009)

Gail,

Looking good. :bow: :bow:
Not knowing the 1st thing about CNC, I was fascinated to watch the mill in action rounding off the bottom of the crankcase.

Best Regards
Bob


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## GailInNM (Apr 2, 2009)

Thanks Bob -- Glad you enjoyed the video.
Now on with the show.

With the crankcases finished it is time to either go vertical to the cylinder group or horizontal to finish up the bottom end. I decided to go horizontal. 

The parts to be made are the front bearing, the crankshaft and the rear crankcase cover. The order that these parts are made in has some importance. As I intend to ream the front bearing, I have little control over the hole size. So it will be made first. Then I can turn and lap or polish the crankshaft to a nice running fit in the front bearing. Planing ahead a little more, I will want to make the connecting rod before I turn the crankpin, so only the front part of the crankshaft will be finished at this time and the crankpin wiil be turned later after the conrod is made and the crankpin can be fitted to the reamed conrod. 

Two different front bearings need to be made. For V1, the flange diameter is 0.875 and has no mounting holes. For V2 and V3 the flange diameter is 0.990 and each has six 0.062 clearance holes for 0-80 screws to mount the front bearing to the crankcase. For V1 no holes are needed as the bearing is glued into the crankcase.

Starting with 1.0 diameter 6061 aluminum I turned the nose part 0.437 diameter for a length of 0.562. The original had a much shorter note section, but I wanted more bearing surface, and this gave a little more conventional look to the engine. The end was chamfered about 0.015 at 45 degrees. 

For the bearings for V2 and V3, a skim cut was made t0 reduce the 1 inch diameter to 0.990 and the edge was chamfered slightly with a file. Then the blank was parted off to an overall length of 0.900. This left 0.025 inch to cleanup on the cutoff end for a finished length of 0.875.

For the V1 bearing, everything was identical except the 0.99 dimension became 0.875.

Gail in NM,USA


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## rake60 (Apr 2, 2009)

Looking great Gail!

Having run CNC machines I know the pucker power of
the rapid motions in a new program.
Nice work!

Rick


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## GailInNM (Apr 2, 2009)

The bearing blanks are reversed in the lathe so they are held by the 0.437 diameter. It is not extremely important that they are absolutely running true, but they should be running within a few thousands indicated runout on the turned surface. I was using a known good 7/16 collet so I did not bother to check.

A skim cut was made to face the end and then the part was removed and the length measured. The part was then reinstalled and an amount equal to the measured length - 0.875 was faced off to bring the bearing to length. The face was then reduced to 0.500 diameter for a length of 0.015. This is the thrust bearing face. Then the larger diameter was reduced to 0.749 for an additional 0.204 to make a smooth fit in the crankcase. I put a small relief at the junction to the flange so the front bearing would seat on the crankcase. I put a slight chamfer on the edge of the 0.75 diameter to make sure it would start in the crankcase easily. The bearing was center drilled, drilled 0.242 (Letter C drill) and reamed with a 0.250 reamer using lots of cutting fluid. It was drilled and reamed at this stage instead of the first operation to make sure the bearing hole was centered on the 0.75 diameter and perpendicular to the crankcase seating flange. 







Now back to the mill. Holding the part in a V block, 6 mounting holes were put in the V2 and V3 bearings and an oil groove 1/16 wide x 0.005 deep was cut in the thrust bearing face. I put all the holes in with a 0.059 drill and cut the groove with a 1/16 end mill. I aligned the groove to fall between two of the mounting holes.

On the V1 bearing, only the oil groove was necessary as there are no mounting holes. 

Not visible in the photos, I filed a small notch in the edge of the 0.75 diameter in line with the oil groove. This is so I can face the notch up during assembly, and the crankshaft will obscure the passage at that time. It probably does not make any difference, but I thought the bearing might get a little more oil that way. After all oil, like other well known substances, flows down hill.










Gail in NM,USA


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## GailInNM (Apr 2, 2009)

Thanks for the comments Rick. 
My Bridgeport rapids are slow at 200 ipm. My Mori Seki, which my son now has, would put the fear of all holy things in to you with it's 1800 ipm rapids. Especially when rigid tapping a series of 0-80 holes at 6000 RPM with rapids between them. 
Gail in NM,USA


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## Maryak (Apr 2, 2009)

GailInNM  said:
			
		

> My Bridgeport rapids are slow at 200 ipm. My Mori Seki, which my son now has, would put the fear of all holy things in to you with it's 1800 ipm rapids. Especially when rigid tapping a series of 0-80 holes at 6000 RPM with rapids between them.



Don't tell me any more please, my heart can't take it.


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## GailInNM (Apr 3, 2009)

While I still had the 1 inch bar stock in the lathe I turned and parted off the backplates for V2 and V3. They are a simple top hat shape with 0.990 diameter for the brim of the hat and 0.750 top of the hat to fit in the crankcase. Later they were chucked on the 0.750 diameter and the back faced off and a recess bored to reduce weight. Then the mounting holes were drilled in the mill.








Although not done until later, the backplate for V1 was made the same way. On the original, the backplate was same as the the V2 and V3 backplates, but only two mounting holes were drilled for 6BA clearance. Since the engine also mounted with these same holes, the backplate was just clamped between the crankcase and the bulkhead that the engine was mounted to. Did I mention that the original was a little bit crude??

In defense of the manufacturer, they did offer an optional backplate that bolted to the crankcase and was large enough to radial mount the engine to a bulkhead from the front. I chose that option for V1. It is 1.5 inch diameter and the flange is thicker so the bolts mounting to the crankcase can be recessed. It is interesting to note that there are 4 mounting holes, but only 3 of them can be used. The 4th hole is covered up by the crankcase so there is no way to insert a bolt in it. That is the way the original was made so I put all 4 holes in mine.
Gail in NM,USA


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## GailInNM (Apr 6, 2009)

On to the crankshaft. Before starting, I drilled and reamed a 1/8 inch hole in a scrap of the same material I will be using for the connecting rod. This is just to give a check gage for the crankpin when it is turned. I could have made the connecting rods first as they are an indepentent part, but I was anxious to make the crankshaft.

I started by chucking a length of 5/8 inch diameter 12L14 in the lathe with enough protruding to cut the from the crankdisk to the front end of the shaft. After facing the end, the 0.188 (3/16) diameter for the propeller mounting shaft was roughed to about 0.02 oversize and the bearing area was roughed to about 3/8 diameter. The reason for roughing these first is to get rid of the stressed skin that most cold rolled material has. If the bearing area was not roughed before finish turning the prop shaft, there is a good chance that the shaft will warp a little bit when the bearing area is turned. I used a VBMT 35 degree carbide insert with a 0.015 tip radius for all turning operations. After roughing, I put a 60 degree center hole in the end of the shaft and finish turned the prop bearing area. I did not find it necessary to use a center during any turning operations, but the center is still necessary to be able to use a gear puller to remove the prop driver if/when the finished engineis disassembled. 








After the propshaft was turned, an additional section 0.156 long was turned to 0.203 diameter for the for the propdriver. The end of the shaft was chamfered at 45 degrees so the die would start true and the propshaft was threaded 10-32 using a die. The die was started true by applying pressure with the tailstock drill chuck. The chuck was opened so the jaws were inside the chuck body and pressure applied with the tailstock hand wheel while the first 4 or 5 turns of the thread were formed.






Not photographed, the area of the prop driver was lightly knurled with a single wheel straight knurl to raise the diameter to about 0.206. This probably should have been done before the finish turning of the propshaft because of the pressure of the knurling tool, but as it was a light knurl there was no bending of the shaft. This is a deviation from the drawings as the original used a taper on the shaft and a mating taper on the prop driver. Both work, so it is just a matter of preference and my being lazy.

The 0.250 diameter for the bearing was finish turned and at the junction of the of the bearing and the crankdisk the tool was fed in about 0.012 to form a radiused recess. This is to make sure the crankdisk can seat on the thrust bearing. The thrust bearing could have been recessed to accomplish the same results. In either method it is important that a radius be formed at the junction of the crankdisk and the shaft to help prevent stress cracks from forming at the junction. On a larger engine I would relieve the bearing rather than the shaft.

After forming the radius, I faced off the front of the crankdisk and polished it so it would run smoothly on the thrust bearing.

While turning the bearing surface of the shaft, I checked for a slightly tight fit of the crankshaft in the front bearing and then polished the crankshaft with abrasive paper starting with 600 grit and graduating up to 1200 grit using light oil.

The crankshaft was fed out of the chuck to allow the crankdisk section to be polished a little bit and then cutoff with sufficient allowance for the crankpin to be turned.






Gail in NM,USA


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## Maryak (Apr 7, 2009)

Nice work Gail, :bow: :bow:

You sure don't mess around.

Best Regards
Bob


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## GailInNM (Apr 7, 2009)

Before turning the crankpin, I made a fixture to hold the crankshaft. Crankpins are often turned by offseting the crankshaft in a 4 jaw chuck and that will work as well. I chose to make a fixture as I am making 3 crankshafts so it is quicker to make the fixture than to set up the 4 jaw three times. There is also less chance of damage to the bearing surface ot the chrakshaft. For me, an additional advantage is that I can use the fixture in a collet and not have to mount the 4 jaw. 

I started off with 7/8 inch diameter 12L14 steel rod. Faced the end of the rod and then turned down 3/4 inch of it to a diameter of 3/4 inch. I put a relief at the end ot the 3/4 in long so the fixture would sit in the collet with out any danger of the collet gripping on the radius formed by the cutting tool. I trued up an additional 3/16 inch of the 7/8 diameter to make sure it was concentric with the 3/4 diameter and put a finger nail groove in it about 1/16 inch from the end of the 3/4 section. Just makes it easier to remove from the collet. a 3/16 hole was drilled for 7/8 depth plus a little bit and the fixture parted off a little over 7/8 inch long. 

Reversing the part in the lathe, the cutoff end was then faced to make the fixture 7/8 inch long and sharp edges removed with a fine file.

I moved the part to a vee block in the milling machine, located the center using a dial test indicator in the 3/16 hole and zeroed the DRO. 

Then the fixture was center drilled, drilled 0.244 (Letter C) and reamed to 1/4 inch in two locations, each 0.244 from the center and opposite each other from the center.

After removing the fixture from the vice, two rods of the same diameter and a little under 1/4 inch were inserted in the 1/4 inch holes. These rods were placed on the vice jaws and the fixture was clamped on the ends at the end of the vice jaws. Since I was clamping off center in the vice, a 7/8 inch parallel was inserted in the vice jaws at the other end of the jaws. This makes sure that the part is clamped across the end faces and not just in one point. ALL vice jaws tilt a little bit when a part is clamped near the end of the jaws, so the load should be balanced.






Then the rods were removed from the fixture and a slitting saw set up to cut a slot from one edge through to the hole on the far side. I used a 0.040 wide blade, but it is not critical. A blade with fewer teeth than shown is easier, but I was too lazy to change blades. I often times just cut this slot with the bandsaw. The slitting saw looks a little nicer, but really does not do anything for the function of the fixture. It takes very little more time to use the slitting saw.






To finish up the fixture, I deburred all the edges of of the slot with a fine file and marked the fixture with the dimensions of the hole and the offset. I have quite a collection of these fixtures and they get reused on occasion. I used an acid etching pen to mark this fixture, but some times I engrave them with a vibrating engraver.
Gail in NM,USA


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## GailInNM (Apr 7, 2009)

With the both crankshaft blanks and the crankpin turning fixture done, it is time to turn the crankpin.

After making the crankshaft blanks, I faced off the cut off end so the thickness of the large section was equal to the thickness of the crankdisk plus the length of the crankpin.






Because of the interrupted cut, many people find this intimidating. I have had a problem with it. Just don't make any sudden moves and always turn the spindle of the lathe one turn by hand before starting the spindle under power.

The crankshaft is inserted into the hole of the fixture that has a slot to the outside. I put the fixture in a 3/4 inch collet and clamped firmly. It could be put in a 3 jaw chuck but the thin wall from the unused hole means that the fixture would be rotated 10 or 20 degrees away from the point where the unused hole is nearest a jaw. This puts the slot only approximately centered between the other two jaws. 

For turning, I use the same VBMT insert I used for turning the crankshaft blank. The holder I use mounts the insert with the insert rotated 3 degrees anti-clockwise (viewed from the top) from the point where the cutting edge is parallel to the work surface. This way, only the area near the point contacts the work. I start taking light cuts, about 0.020 or 0.030 deep and cut the length of the crankpin minus 0.003 to 0.005 to leave a small amount to clean up on the crankdisk after the the crankpin is turned.











After as the cutting progressed, I increased the depth of cut as the radius of the cut decreased and then finished the crankpin to be a snug fit in the test hole I had made early on with the reamer I intended to use for the connecting rods. I finished the crankpin to length and shaved the few thousands of extras length I had left on the crankdisk by cutting from the crankpin to the outisde of the crankdisk. I then finished the crankpin to a snmooth running fit on my test hole using 600 to 1200 grit abrasive paper backed with a piece of flat metal. 






Gail in NM,USA


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## GailInNM (Apr 8, 2009)

With the crankshaft done, only a few small parts and the bottom end of the engine can be assembled.

Needed are a prop driver, a washer and a nut. I am using commercial nuts, but will eventually make a longer acorn nut so I dont have to pack it out to use the acorn nut. The crankshaft is longer than it needs to be so a standard acorn nut will not screw on far enough. The washer is just a slice of 1/2 inch steel with with a 3/16 hole in it. I beveled the front edge for looks. 

The prop driver is a little more work, but not much. 

I started with 9/16 aluminum, cleaned up the end and drilled a 0.203 hole in it a 1/4 inch deep. 0.203 is the same size I turned the seat for it on the crankshaft before knurling the crankshaft. Then a pleasing profile was cut ending with a 7/16 diameter to match the front bearing.






The 0.203 hole was opened up to 0.015 deep with a small boring bar to provide some relief when knurling the front face.






The front face was knurled with a straight face knurling tool with a single knurl. I only went in about 0.005 inch after the knurl touched off. Different patterns can be cut by raising or lowering the knurling tool or by using a knurling wheel with that has the pattern cut at an angle. 






Then the driver is ready to be cut off. After cutoff, the 0.203 hole was chamfered about 0.005 deep with a countersink so it would start easy on the knurled portion of the crankshaft.






Gail in NM,USA


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## ksouers (Apr 8, 2009)

Gail,
This is a great thread! Thanks for all the detail. I'm learning a lot!


Kevin


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## GailInNM (Apr 8, 2009)

Thanks Kevin.  It's always a problem about how much detail to include. Don't want to bore people, but want to include enough detail to be helpful. If I stray too far either way, let me know. Now on with the show. 

Since all the bottom end parts are made, it's time to assemble them.

A simple tool needs to be made to press the prop driver in place. For it, I just drilled a a 0.189 hole in a short length of 1/2 diameter aluminum. Both ends were faced off so they were square. I made the length about the same a the protruding part of the crankshaft when the crankshaft was inserted into the front bearing.






The crankshaft was oiled with machine oil and inserted into the front bearing and worked back and forth a bit to distribute the oil. The prop driver was pressed on using the tool in the vice on the milling machine.






With the addition of the prop washer and a nut, the assembly is finished.






Gail in NM,USA


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## GailInNM (Apr 8, 2009)

Sliding things together it now starts to look a little bit like an engine.


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## Maryak (Apr 9, 2009)

ksouers  said:
			
		

> Gail,
> This is a great thread! Thanks for all the detail. I'm learning a lot!
> Kevin



Me too, thanks Gail. :bow:

Best Regards
Bob


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## rklopp (Apr 9, 2009)

Gail
What alloys are your front bearing and prop driver? I ran into galling issues on my Little Dragon when both were 6061. I learned the hard way that 2024 has better bearing properties.
RKlopp


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## GailInNM (Apr 9, 2009)

RKlopp,
I used 6061 for both the front bearing and the prop driver. 
I had the same problem as you with my Little Dragon. It was the hardest starting engine I have ever built and I made extensive use of an electric starter on it. Of course, once running there is no contact between the bearing and driver, but until then......

I seem to remember cleaning up the faces of the bearing and driver and inserting a thin washer of 12L14 between the two until I got the Dragon sorted out. I made a new bearing later. I was going to make a new crankshaft and bearing so I could make them longer, but by the time I got things running well I had enough of the Dragon slaying routine so I went on to another project. 

Did you get your Little Dragon running OK?

Gail in NM,USA


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## rklopp (Apr 9, 2009)

Gail,
My Little Dragon fired up on the second or third finger flick!! Subsequent starts required the electric motor, until the thrust from the motor chewed up the bearing behind he prop driver. Like you, I've moved on, and the Little Dragon is now a "shelf queen."
RKlopp


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## GailInNM (Apr 16, 2009)

Moving toward the top of the engine the connecting rod is next. On a compression ignition engine it is under a lot of stress because of the high compression ratio. On most CI model engines the compression ratio will be in the 18:1 or 20:1 range. 

I made the con rod out of 2024 alumninum. It has the necessary strength and wears well on the steel crankpin and the steel wrist pin in the piston. 

I started off with piece of 1/4 X 1/2 exttruded 2024 aluminum about 4 inches long. I split it in half lengthwise and milled each half down to 3/16 square. I figured that would let me make 4 rods with plenty left over to clamp on by making a rod on each end of the strip. And, if I really messed up I could get 2 more out of the center sections.

I rounded each end of each strip using the CNC mill and then drilled and reamed the holes for the crankpin and the wristpin.

The rod part was turned by chucking the 3/16 square in a collet I used a square collet, but a 17/64 round collet will also work very well if you take care to keep all the corners out of a slot in the collet. I used the right side of the insert to chamfer the rod to end transition.
Gail in NM,USA


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## Maryak (Apr 16, 2009)

Gail,

I like your conrod and the way you are making it. :bow: :bow:

Best Regards
Bob


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## GailInNM (Apr 16, 2009)

After turning the main part of the rod, I reduced the width of the small end. This could have been done when I was drilling the rods, but I did not think of it then.






I sawed the rods to the required length plus a little bit for cleanup. To work on the small end, I made a bushing to hold on to the turned portion of the rod and cleaned up the rod to length. I roughed the radius on the end using the CNC and then finished with a file. The wristpin end of the rod was shaped using the backside of the insert. The rest of the shaping of both ends was done with a file in the lathe using the bushing to hold the rod.

Gail in NM,USA


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## GailInNM (Apr 21, 2009)

Now for the fun parts. The cylinder,piston and contrapiston. The absolute diameter of these is not important, but the fit of these three parts to each other is very important. The smaller the engine displacement, it more important it becomes. Because of the high compression ration, approximately 18 to 1, the fit needs to be near perfect if easy starting and adequate performance is to be expected. 

The cylinder is the hardest, so it is done first and then the other two parts made to fit the cylinder. On small engines, such as this, it is typical to make the cylinder with a parallel bore in the upper part of the piston travel, and to taper the bore a small amount toward the bottom. This lets the piston move freely when it is below the exhaust ports and then almost jam in the cylinder in the compression area. Different builders have differing opinions about where this "pinch point" should be. 

On this engine, I use 12L14 steel for the cylinder and cast iron for the piston and contrapiston. 

I started by turning the lower portion of the cylinder up to the locating flanged and cleaning out the radius left by the toolbit with a square parting tool. 






The cylinder is then drilled about 0.015 inch under the finished size and parted off.






.

Gripping on the just turned section, the cylinder is faced to length and the upper part of the cylinder is turned to diameter and again the radius is cleaned out as on the lower part.

The cylinder is bored to about 0.0015 to 0.002 under the desired finished size. I bored to 0.311 for a finished size of 5/16 inch or 0.3125. The amount of stock left is dependent on how good a finish I am getting on the boring operation. I use a solid carbide boring bar and although the photo does not show it, I use lots of cutting oil. If the finish is good, it takes a lot less time to lap the cylinder to get the desired finish.






This ends the lathe operations for now. Next it is off to the milling machine.
Gail in NM,USA


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## GailInNM (Apr 21, 2009)

As usual, I lied. Before moving on to the mill, I put a countersink in the bottom end of the cylinder. The original had a deep narrow angle chamfer to clear the connecting rod, but as that serves no useful purpose, I shortened the cylinder and only put a small countersink in. It reduces the primary compression a little bit, but not enough to matter.
Gail in NM,USA


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## GailInNM (Apr 22, 2009)

Moving over to the milling machine, it is time to mill away part of the bottom end of the cylinder to provide a passage from the crankcase to the transfer port, and drill the ports. There are three ports. The transfer port to transfer the fuel-air charge into the cylinder from the crankcase, the intake port to draw air into the crankcase when the piston is at the top of its stroke, and the exhaust that is uncovered when the piston is at the bottom of the stroke. 

Each port consists of two holes drilled on a radial to the cylinder. Two holes are used instead of one big hole or a slot so the wrist pin in the piston is in contact with the the bridge between the two hole in the cylinder as it passes by a port. 

Because I am building three engines and the port orentation is different, it would seem logical that two different cylinders would need to be made. The difference is that the exhaust port is on the opposite side of the cylinder on one version. I took the simple way out and put exhaust ports on both sides of the cylinder. The unused port is blocked by the crankcase, so it has no effect on the operation of the engine. It also lets me have the option of having two exhaust ports on V3 of the engine. I have not decided if I am going to do this or not. I think it would look better, but would not do anything for the engine performance. 

Starting off, I mounted a 5C Spindex on the milling machine. I have modified my Spindex so it mounts in the vice with a keyed subplate on the Spindex. This way it is already parallel to the X axis of the milling machine. 

The cylinder is gripped on the top end in a collet and is indicated in with an edge finder in both the X and Y directions.






With the Spindex set at zero degrees, the transfer passage was milled into the end of the cylinder.






The X position set to the location of the transfer port and the Spindex is set at Plus and Minus 22 degrees for drilling the transfer port holes.

I used a 1/16 end mill to drill the transfer port holes. I use an endmill as the holes are starting on a slanted surface and produces less burr on the inside of the cylinder. 






The spindex is indexed 180 degrees and then offset plus and minus 22 degrees and the intake port holes are drilled with the 1/16 end mill. Of course the X position of the table is changed as the intake port is closer to the bottom of the cylinder.

The endmill was changed to a 3/32 end mill to drill the exhaust ports and after setting the X position to the exhaust port position. The Spindex was set at 90 degrees plus and minus 25 degrees and the holes drilled and then again at 270 degrees and repeated.






After removing the cylinder, the bore was deburred with a piece of 400 grit wet or dry abrasive paper wrapped around a 1/4 inch diameter rod. This is to prevent burrs from damaging the lap when lapping is started. Any burrs on the outside of the cylinder are also removed with 400 grit paper.

Gail in NM,USA


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## Maryak (Apr 22, 2009)

Gail,

Very nice work, you seem to be very well equipped with small tools and cutters. :bow: :bow:

Best Regards
Bob


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## GailInNM (Apr 25, 2009)

Thanks Bob,
I have built mostly small toys for 50 years, so I have had lots of time to collect small stuff. Most of my machine tools are standard size however. There have been some adaptations to assist in making small parts on some of them. I would love to build a Nano class engine (0.1 cc) but I doubt that the eyes and certain body extremities would be up to it any more. At 0.6 cc, the IMP will be the last of the small displacement engines that I build and future engines will be one cc or above per cylinder. 
Gail in NM,USA


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## GailInNM (May 2, 2009)

Please recognize that this post contains a lot of personal opinion. 

With all the machining done on the cylinder, the bore must be finished. For the hobby machinist there are two methods in common use, honing and lapping. So which did I use? I am not sure. The definitions of the two processes have been merging for some time, and differ some from one industry to another. 

Traditionally, honing was defined as a process using vitrified stones to finish a bore. Lapping was defined as using a loose abrasive in a slurry.

In more modern times, honing definitions have expanded to include abrasive, mostly diamond, that has been bonded to a metal core. Electroplating nickel, like a diamond file, is a common way to bond. Sometimes the tool is is adjustable, or is less critical applications is it fixed. A common name for this process is single pass honing. Oil is normally used to wash out removed metal particles.

Lapping has also expanded, and is sometimes referred to as wet or dry lapping. Wet lapping is where a slurry of abrasive is used between the lapping tool and the part. Dry lapping is where the abrasive is pressed into the tool to impregnate it. The tool is then cleaned of all loose abrasive. Despite the name, dry lapping is not done dry. Oil or other liquid is used to wash out the metal particles as they are removed from the part. 

So, at the overlapping point, the only difference between lapping and honing is is method used to secure the abrasive to the tool. I am going to call the process that I use lapping as it meets the definition of dry lapping.

I used a pair of commercial expanding lapping tools with a brass barrel. As the operations are identical only one will be shown in the photos. 

For abrasive, I use diamond lapping pastes. when dry lapping, I do not believe that diamond is worse about embedding in the work than the silicon carbide and other abrasives used in valve grinding compounds, or that it makes any difference. While some other abrasives will break down easier than diamond, if any abrasive of any kind is left in the bore, the piston/cylinder set will be worn out long before the abrasive breaks down. 

I use two different grades of diamond. Because the terminology used to grade abrasives can get confusing I am including more information than you probably want to know. The syringe in the photo is a 5 gram syringe. It is hard to see if any has been used. While I have not kept count, I have lapped over 20 cylinders from 1/4 to 3/4 inch bore with this syringe. A 5 gram syringe costs about US$20 at Enco. I do not expect to ever use up the contents of this syringe. The compounds I have are medium concentration. This is just how much diamond is mixed in. Light concentration would work just as well, but a little bit more of it would be used.

For roughing the cylinder, I use 600 mesh diamond. 600 mesh is roughly the equivalent of 380 grit if you compare it to wet-or-dry abrasive paper and the particle size is a nominal 30 micron, or about 0.0012 inch. This roughly is the same as Clover compound 1A.

To finish the cylinder, I use 1800 mesh diamond. 1800 mesh is roughly the same as 800 or 1000 grit wet-or-dry and the particle size is a nominal 10 micron or about 0.0004 inch. This roughly is the same as Clover compound 5A or 6A.

The Clover compounds referenced are silicon carbide, not diamond.

End of opinion and background information. Next post will be the how-to part.


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## GailInNM (May 2, 2009)

To charge the lap with diamond, a small amount of compound is placed on a hardened surface. I used an old odd size parallel that was in a box of junk that I bought at a machine shop auction. One of those deals where I wanted one thing in the box and the price was right.

The blob of paste is about 3/32 inch diameter. It does not take much.






The paste is spread out to form a patch about 1/2 inch long and the width of the parallel. If the paste is not spread out, then a lot gets pressed into the grooves of the lap. Then I used a 3/8 square ground tool bit to roll the lap back and forth in the paste using firm pressure. The position of the lap is changed every few strokes to make sure the entire surface of the lap is covered. Always roll the lap. If you slide it the lap will mostly just pick up the diamond in the slits in the lap. Probably 1/2 of the diamond ends up there anyway. 






Clean the lap with mineral spirits or other solvent and wipe dry with a paper towel. Clean the parallel and tool bit and then THROW the paper towel away. You don't want it around where you might wipe down a machine tool surface with it. I seldom use rags in the shop as they can be a safety problem around power tools, and I NEVER use rags when working with abrasives.

If you look at the lap at this point, it will not look any different than when you started. Mark the lap by engraving the shank or some other method so you will know what grit the lap is charged with.

Now repeat the process with the other compound, again making sure to mark the lap with the grit used. 

You can NOT clean a lap that has been charged with coarse grit and then use a fine grit. Once charged with a grit, a lap must always be used with that grit.

This photo shows a portion of a lap charged with 600 mesh diamond as seen under a 200 power microscope. The dark spots are diamond. This lap has seen quite a bit of use. The scratches in the brass are probably caused by particles of the metal being lapped. Remember the dark spots are only about 0.001 diameter so the scratches are not very deep. 

Next up will be the lapping of the cylinder.






Gail in NM,USA


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## GailInNM (May 10, 2009)

Before lapping we have to define the bore we want to lap. For a small compression ignition (CI)engine, such as the IMP, it needs to be a little bit different than many other engines. On larger bore, low compression ratio, and engines with rings the bore is most often just required to be round and parallel. Because of the high compression ratio on CI engines the cylinder pressure needs to be about 180 PSI or so before the engine will fire. This means that the piston/cylinder fit must be very good. As the diameter of the bore decreases this becomes more of a problem as the volume is proportional to the cube of the diameter and the sealing area is pproportional to the square of the diameter. To achive the tightness of the seal required, it is more or less standard practice to taper the cylinder bore a very small amount so the piston becomes a tight fit at top dead center. Different people do this in different ways. This is what works for me.

When working with any form of abrasive in the lathe, I move the carriage toward the tailstock end of the bed and then cover the bed with a plastic sheet. Then I put a paper towel over the plastic sheet. Never use a rag for two reasons. One: A rag can get caught on the rotating part or chuck jaws and be a safety hazard. Two: When I am done, I throw the paper towel and plastic sheet away so there is no posibility that I will wipe or other wise contaminate a precision surface with anything that may contain any abrasive.

First off I start with the lap that is charged with 600 mesh diamond. Apply lots of very light weight mineral oil to botht eh lap and the inside of the cylinder. The lap diameter is adjusted to just have a light drag on the cylinder. The lap is chucked in the lathe and the lathe is set to run about 300 RPM. Start the lap from the bottom end of the cylinder. The cylinder is slid back and forth over the lap and will free up very quickly. Adjust the lap larger and repeat. I move the cylinder about two times per second. The lap should not be so tight that you can not hold the cylinder with your fingers grasping firmly. Each time, or at least every other time, that I adjust the lap, I rinse the part in a container of mineral spirits to remove any slurry of metal particles. I use a tube brush to clean it out. Wipe the lap with a paper towel moistened in mineral spirits. The put a fresh coating of oil on both the cylinder and the lap. I repeat this until all the machining marks in the cylinder are removed and the bore is uniform from end to end. I can feel if there are any tight or loose spots on the bore with the lap.

Clean the cylinder very well with mineral spirits and a tube brush. At this point there is a distinct crosshatch pattern in the cylinder. The grooves created by the lap may be up to about half of the diameter of the diamond grit, or about 0.0005 deep.

Clean the lap and tag it with the grit or put it in a marked bag so it is ready for the next use.   

Next I move on to the lap that is charged with the 1800 mesh diamond. The same procedure is used as was used for the previous lap. As I dropped two grades in the size of the diamond, I do not try to remove all the grooves created by the previous lap. I like to do the equivelant of what is known in automotive circles as "plateau honing". There are peaks and valleys left by the previous lapping, and I want to remove about half of the peaks and leave the remaining valleys to retain lubrication. This means only increasing the diameter by about 0.0005 inch or so. None of it is very critical, but it makes for a shorter break in on the engine and a longer life. These little engines are not known for a long life time anyway.

Finally, I put a taper in the bore from the bottom end of the cylinder up to about the top of the exhaust ports. The taper only needs to be about 0.0003 or 0.0004 inch on the diameter. I just expand the lap and then run the cylinder on the lap part way. I just approximate the distance from where I feel the lap start to cut.

Clean everything !! Throw away the paper towels and plastic covering the lathe bed. As a last cleaning operation on the cylinder, I scrub it with hot water and detergent, making sure I get everything out of the various ports. Then I oil the cylinder with light oil to prevent rust.

It probably took you longer to read the last few posts than it takes to lap a cylinder. It certainly takes less time than it took me to write it. Depending on how good a job id do on boring a cylinder and the resultant finish, it takes me from 15 to 20 minutes to lap a cylinder from set up to tear down.
Gail in NM,USA


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## GailInNM (May 10, 2009)

The cylinder on the IMP is spaced up from the crankcase with a spacer. The only unusual thing about this is that the original IMP was supplied with three spacers rings. The normal one was installed on the assembled engine. The two additional ones were slightly longer and shorter that the normal one, and could be changed to vary the timing of the engine. Not a big advantage as the timing on everything changed. Intake, exhaust, and transfer all shifted. For completeness I made all three, but doubt that I will ever try the two additional spacers. They are a simple turning and parting off job. As they need to be gas tight, each one received a rub on both faces on 600 grit abrasive paper.


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## GailInNM (May 20, 2009)

With the cylinder finished, the piston and contra piston are next. Both are carefully fitted to the cylinder, but in a different manner. Both are made of cast iron. I started with the piston. 

The goal on fitting the piston is for it to almost jam in the cylinder when it reaches what would be top dead center (TDC) in operation. I try to get it so by the time it reaches the point where the skirt of the piston reaches the top of the transfer port opening on the cylinder it takes about one pound force or a bit more. At this point the piston is beyond the taper portion of the cylinder and should be able to be forced through the cylinder if one really wanted to try. All this is necessary to make sure that a near perfect seal happens near TDC. If it won't hold the 200 psi necessary for ignition to occur here then the engine will not fire.

I started with a length of 5/8 diameter continuous cast iron rod. Most of the distributors don't have cast iron below 1 inch. Speedy Metals is one of the few places I have found that has it down to 5/8 inch. When I get a new length of cast iron I clean up the whole length to the largest size that I have collet that will fit. The cast iron rod is always oversize enough to at least clean up to the nominal size. In this case, the rod cleaned up to 0.671 (43/64). I do this so the rod will clamp firmly with no danger of rocking on a high spot while machining. With an inch or so of the rod extending from the collet I turn it to a few thousands over the measured size of the bottom of the cylinder. Then i carefully start dusting off the diameter until the piston will just start to enter the taper of the cylinder.








Photo 1

I then polish the piston until it will enter about half way in the cylinder using 600 grit abrasive paper with a little bit of light and backed with a ground piece of steel that is wider than the piston. I use a parallel for the milling machine vice.

The piston is center drilled, drilled and bored to form a flat bottom recess in the piston.








Photo 3

With a 60 degree counter sink I chamfered skirt of the piston to reduce the wall to about 1/2 of it's thickness and then cut the piston off leaving about 0.005 to 0.010 extra length to clean up.








Photo 5

Since the piston fit in a standard collet, I cleaned it up to length by chucking in a collet and facing the end until it was the correct length. I could have been done on the mandrel in the next step also if I did not have the correct size collet.

An expanding mandrel was made from a short length of 5/16 steel, the same nominal size that the piston is. I turned the end of the mandrel to fit the piston internal bore and reduced the diameter of the shank to a little less than the diameter of piston for an additional 1/2 inch. The mandrel was drilled for about 3/4 of it's length with a clearance drill for a 4-40 bolt and then drill through with a tapping drill size. The end was counter sunk to match a flat head 4-40 bolt. The mandrel was reversed in the collet and the other end tapped 4-40. In the milling machine, I slit the mandrel with a 1/32 slitting saw. I turned the head of a long flat head 4-40 bolt down so it would fit inside the piston and then screwed it into the mandrel. At he far end I secured a nut on the thread with a dot of silver solder with the nut spaced out from the end of the mandrel an 1/8 inch or so.








Photo 7

The piston was slid on the mandrel and secured by expanding the mandrel using the nut on the far end. I gripped the mandrel at an angle in the corner of my milling vice to tighten the nut with both piston and the nut outside the vice jaws.

Back to the lathe I polished the piston with 800 grit and oil in the same manner as shown in photo 3 using a steel backing plate. The piston was cleaned with a paper towel and mineral spirits before test fitting in the bottom of the cylinder. When I got near the fit I wanted, I switched to 1200 grit to finish to the desired 1 pound force when the piston skirt was at the transfer port.

The piston was moved to the milling machine and the wrist pin hole was center drilled, drilled and reamed. The hole was deburred with a bit of 1200 grit abrasive paper. Cast iron does not leave much of a burr if the tools are sharp.








Photo 9

I gripped the piston in a collet in a square collet block. By inserting a rod in the wrist pin hole and using a spacer block while tightening the collet, the wrist pin hole was aligned to the collet block. 







With collet block in the milling vice, I milled the transfer notch into the side of crown of the piston. The photo shows the notch being cut for V3 of the engine, where the notch is cut on side of the piston that has the wrist pin hole. For versions V1 and V2, the block was rotated 90 degrees so the notch was parallel to the wrist pin. The edges of the notch were deburred with 1200 grit abrasive paper.






Gail in NM,USA
Edited to add slitting mandrel and correct spelling.


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## kvom (May 20, 2009)

Gail,

Nice thread. What"s a transfer notch?  ???


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## GailInNM (May 20, 2009)

kvom,
A transfer notch is cut into the side of the piston to deflect the incoming fuel/air charge toward the top of the cylinder to help force the combustion products out of the cylinder while keeping the new charge in the cylinder. As the notch uncovers the transfer port in the cylinder the charge from the crankcase comes up, bends 90 degrees to pass through the port, and then bends back 90 degrees upward. The "Z" bend was not the most efficient way of doing things but it worked and was quite popular on small compression ignition engines designed in the 1940's into the 1960's. 

On glow and spark ignition engines of the same era, it was more common to machine a baffle, or in some cases bolt one on, into the top of the piston. A recess for this baffle was then machined into the cylinder head to keep it from striking the cylinder head. This was not practical on a compression ignition engine as the top of the cylinder was closed with a movable contra piston to adjust the compression and it would be difficult to keep it aligned with the piston. Some CI engines used a domed piston with a matching recess in the contra piston. This did not work very well as the incoming charge was spread to the sides about as much as it was deflected upwards.

A good solution was developed and has become known as "Oliver" porting and is in wide use today. With it the transfer ports are at an angle to the cylinder so the fuel charge is brought into the cylinder at an upward angle so no baffle is necessary. This is what Bob (Maryak) did on his Maryak 10 engine and is illustrated in his build photos at:

http://www.homemodelenginemachinist.com/index.php?topic=3712.msg40335#msg40335

and the cylinder plans at:

http://www.homemodelenginemachinist.com/index.php?action=dlattach;topic=3629.0;attach=3193

Gail in NM,USA


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## Maryak (May 20, 2009)

Gail,

Coming along great guns. :bow:

As I read it you are using a similar technique for fitting the main piston to the bore as used to make the contra piston, (Dave Owen method). Much easier than a couple of hours lapping. Thanks for the tip :bow:

Best Regards
Bob


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## GailInNM (May 20, 2009)

Thanks Bob,

It's not the same as the Dave Owen contra piston method in that the piston not tapered but the lower portion of the cylinder is tapered. There is no compression of the piston skirt. I get away with using abrasive paper in that the piston is turned to less than 0.0005 of the finished diameter.  Most of the diameter reduction is just removing tooling marks from the turning. I have used both this method and lapping in the past and can't tell any difference. I know that the abrasive paper will knock it out of round a little bit (much less than I can measure), but lapping a short length will often cause a barrel shape. So if figure that if I am going to have an error either way I will take the lazy man's way out.

Next up will be the contra piston, on which I do use a variation on Dave Owen's method. It is by far the easiest method to get a good contra piston seal with an o-ring seal, which I have also used, running second. Some people have worried about the deterioration the O-ring when using them, but I still have a OK Cub 0.074 cid of about 1955 vintage that I bought new and used a lot and the O-ring in it is still fine. Still, I like the Dave Owen method better for it's simplicity and it is easier to fit.

Gail in NM,USA


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## radfordc (May 20, 2009)

rklopp  said:
			
		

> Gail,
> My Little Dragon fired up on the second or third finger flick!! Subsequent starts required the electric motor, until the thrust from the motor chewed up the bearing behind he prop driver. Like you, I've moved on, and the Little Dragon is now a "shelf queen."
> RKlopp



The solution is to use a thin shim stock steel washer between the aluminum parts. This was done in plain bearing production engines.

Charlie


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## GrahamC (May 21, 2009)

radfordc  said:
			
		

> The solution is to use a thin shim stock steel washer between the aluminum parts. This was done in plain bearing production engines.
> 
> Charlie



A steel washer works well. However, so does brass or even phenolic; as long as it is smooth on both sides, doesn't cause any binding and is a different material than the thrust washer and crankcase. My preference is to use brass and polish up both sides although I have been meaning to try one made of hardened or case hardened and polished steel to compare long term. And, before running the engine put a few drops of fuel or just plain oil on the crankcase/washer/thrust washer.

Cool threads and builds. Very inspirational. One of my long term goals is to build a small diesel (CI) to put on a Tomboy or Buzzard Bombshell (reduced size). I am making sketches and keeping notes - soon I keep telling myself, all I have to do is clear out some of the projects first.

cheers, Graham in Ottawa Canada.


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## GailInNM (May 29, 2009)

With the piston and cylinder complete, only the contra piston remains of the trilogy of parts that must fit together properly. If any of these are not fitted properly a small compression ignition engine will not run. 

For the readers who are not familiar with compression ignition model engines, the contra piston fits into the cylinder from the top end and is adjustable to adjust the compression ratio of the engine. Unlike a diesel engine where the timing is adjusted by when the fuel is injected into the cylinder, a model comprssion ignition engine's timing is dependent on three things. 

First is the fuel components. About a third of most CI fuel is ether, and it evaporates easily. So A fresh batch of fuel will react differently than one that has the can opened a number of times.

Second is the fuel-air mixture, which on a small engine like the IMP is adjusted by a conventional needle valve operating in a spraybar in the intake air stream.

Third is the compression ratio. This is what the contra piston changes.

The contra piston must be a tight fit in the cylinder. Any leakage around the contra piston and a small engine will not run. On the IMP, I tried to get a fit such that it took about 5 pounds force to press the contra piston into it's normal operating position. With the high compression ratio, about 15 pounds force will be applied to the contra piston as the piston passes top dead center if there is no leakage. This pressure goes much higher when the engine fires.

With that bit of theory out of the way it is time to make the contra piston.

At the time the IMP was produced, the standard way to make a contra piston was to just make a short solid piston that was a tight fit in the cylinder. It was not uncommon for home builders to make several before they got the right fit. A few years ago David Owen developed a method that made a perfect fit almost automatic. He made a hollow piston that had thin tapered walls that would compress as the piston was pressed into place. For more on his method see:
http://modelenginenews.org/faq/index.html#qa7
http://modelenginenews.org/weaver/Weaver_pg6.html#DCO_CONTRA

To taper the contra piston, I first set the compound to 1/4 degree. On my lathe this is easy to do as the side of the compound slide is accurately ground parallel to the dovetail. I have a dial indicator that I can clamp in the headstock that I use for this. A magnetic mount on the lathe ways would also work. Since the tangent of 0.25 degrees is 0.0044, the compound is set so the indicator moves 0.0044 when the carriage is moved 1 inch.






Next the contra piston was turned about 0.003 over the measured size of the cylinder bore at the top of the cylinder using the carriage feed. It was turned from the same cast iron that the piston was made from. I turned to a length of 0.325 which allows for the 0.25 inch finished length plus enough for my cut off tool plus an extra 0.01 for cleanup later.

With the carriage locked in position I started making taper cuts on it until it would just start to enter the cylinder about 0.03 inch. Then I started reducing the taper section with 800 grit abrasive paper using a steel backing the same as I used for the piston. I reduced it down until the contra piston would enter about 1/2 way into the cylinder and then cut it off from the stock.






Changing collets in the lathe, the piston was reversed and faced to 0.25 length. Then it was counter bored to a depth of 0.187 and a diameter to leave a wall thickness of 0.015.






Gripping on about 1/3 of the length, the skirt was polished with 1200 grit abrasive paper, again using a steel backing. I would polish a little bit, and then remove from the lathe a test fit it in the cylinder until it was reduced to a point where I could press it into the cylinder with about 5 pounds force. 

If you try this procedure, be sure that the piston in NOT in the cylinder before you start test fitting the contra piston. With the contra piston in one end of the cylinder and the piston in the other it is most difficult to remove either one of them. ;D ;D

Gail in NM,USA


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## Maryak (May 29, 2009)

Gail,

Soon we will hear the lovely purr from another of your fantastic engines. :bow: :bow:

Video is mandatory,  : well please make one.  

Best Regards
Bob


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## ariz (May 29, 2009)

another great craftsman here!

chapeau :bow:


great documentation too


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## GailInNM (Jun 2, 2009)

Thanks Bob and Ariz.

Another little part. The wrist pin. No photos of it, but it will show up in later photos.

The wrist pin is just a length of steel rod to connect the connecting rod to the piston. On the IMP it is 0.093 diameter and about 5/16 long. It needs to be a tough steel as it has a lot of stress on it. I have broken two on small compression ignition engines over the years. One that I had made and not drawn the temper far enough so it was too brittle, and one commercial one on an Allbon Dart (0.5 cc I think). The Dart was broken many years ago and I replaced it with a section of music wire and it is still going strong. So I have used music wire where I could ever since.

The music wire I use is the straightened type found in model airplane hobby shops in 3 foot lengths. It is hard drawn and has very hard skin on it with a tough core. It is fairly round in most cases, but I take a micrometer with me when I go shopping and select looking both for roundness and oversize. 

Since I am building 3 engines, I made 4 pins - one to lose. I started with a long length of music wire chucked in the lathe. I had previously measured it and it was just a little oversize, and after deburring the end this was verified by trying it in a connecting rod. Running the lathe at about 2000 RPM I polished the wire with some 1200 grit abrasive paper and oil while backing it with a smooth piece of steel. It only took a minute to polish about 1.5 inches untill it was a smooth fit in the connecting rod.

After removal from the lathe, I used a Dremel tool with a thin cut off blade to cut off 4 lengths about 0.025 long. To clean up the ends, I chucked each piece in a battery powered electric drill and ran it against a fine, freshly dressed, grinding wheel in the bench grinder. After cleaning up each end, I continued to grind the pin until it was about 0.002 shorter than the cylinder bore. Then I polished the ends using a non woven abrasive wheel in the grinder, again using the drill. By rocking the drill, I put a slight radius on each end to roughly match the bore of the cylinder. This completes the wrist pin.

On the IMP, the wrist pin floats, that is there is no means of retaining the pin to keep it from touching the cylinder and no room for end pads. For that reason that the ends must be polished to keep from damaging the cylinder. 

Referring back to the cylinder, each port consists of two holes with a bridge between them. When the engine is assembled, the cylinder is aligned so the wrist pin rides on this bridge so it can not get caught in a port. 

Gail in NM,USA


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## GailInNM (Jun 2, 2009)

With the wrist pin done, all the parts necessary to relieve the inside of the crankcase are complete. 

The crankcase assembly is inserted into the crankcase. As it is a close fit, it was not necessary to use any screws to hold it in place. The piston is connected to the connecting rod with the wrist pin. The connecting rod is slipped onto the crankpin on the crankshaft. The cylinder is slid on to the piston and into the crankcase while rotating the crankshaft so the crankpin is at bottom dead center. By rotating the crankshaft, I could see where the connecting rod would hit the crankcase. The whole mess was disassembled so the crankcase could be relieved and the process repeated until the connecting rod cleared the crankcase with a little extra clearance. 

For photo and details on reliving the crankcase see:
http://www.homemodelenginemachinist.com/index.php?topic=4422.msg47296#msg47296

Gail in NM,USA


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## GailInNM (Jun 19, 2009)

Sorry for the break in this WIP. I had a visiting model engineer here for a week and we were working on other projects. Then I had a bit of surgery and that slowed me up for a few days.

The next part will make the IMP start to look like a real engine. The finned cylinder head is really a fairly easy part. There are no close tolerances and it is large enough to be easy to handle.

Starting with 7/8 diameter aluminum bar stock in the lathe, the end is faced off, then drilled to to a depth to allow boring to size. The bar is bored to a depth of 0.318 and a diameter about 0.001 over the size of the top of the cylinder. Small compression ignition engines do not generate a lot of heat, so a slip fit between the cylinder and the head is adequate to dissipate the heat generated when running.






Next the four grooves forming the five fins are cut using a 1 mm parting tool. A slow feed and a sharp tool are necessary as well a a little bit of cutting fluid. I use Tapmatic Edge and apply it with an acid brush where the bristles are pushed into the groove as it is being cut. 






Using a metal rule and a bit of 400 grit abrasive paper the inside fo the grooves are lightly polished and the edges of the fins are lightly beveled just enough to break the sharp edges as I like the square edge look on the fins. When using the rule in the groove, it is better to run the lathe in reverse so the friction is pulling on the rule rather than pushing it. That way if it binds a little from being out of line it does not bend and bind more.






I parted the head off a little over length and reversed it in the collet. After cleaning it up to length I turned a section to 1/2 inch diameter I rounded the edge of the top using a corner rounding end mill. It was then polished before removing from the lathe. 






The remaining lathe operation is to center drill, drill, and tap the top for the 10-32 compression adjustment screw. 






Moving the head to the milling machine, head was centered using a dial test indicator. For the V1 head, two holes were center drilled and drilled as clearance for 4-40 bolts. For the the V2 and V3 head four holes were drilled for 2-56 clearance. The holes were located by coordinates using the X and Y handwheels. I used a very sharp drill with cutting fluid and slow feed to keep burrs between the fins to a minimum. 






Gail in NM,USA


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## Maryak (Jun 19, 2009)

Gail,

Beautiful work. :bow:

This will, I'm sure be the most impish of Imps. ;D

Best Regards
Bob


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## GailInNM (Jun 19, 2009)

Thanks for the kind comments Bob.

At the very top of the engine is the compression adjustment screw. It is used to adjust the compression ration by positioning the contra piston in the cylinder. It only needs to push as the compression and firing of the engine will keep the contra piston in contact with the screw. 

While a commercial screw stuck in the head and adjusted with a tool will work, a modified or purpose made screw will work much better.

I started off with a 10-32 socket head cap screw. I first made a split collet out of alumninum that was a little shorter than the desired finished length of the screw. The small diameter needs to be smaller than the head on the screw, and a single slit can be a hacksaw or band saw cut. The center hole is drilled for the major diameter of the screw. It is shown in the first photo along with a nearly finished screw. 






I first turned down the OD of the head to get rid of the serations on the head that would make cross drilling difficult and then removed the hex inside with a small boring tool. This was so the drill would not be deflected by the flat sides of the hex. As the head was taller than I wanted I cut some off the top. 






Moving to the mill, I held the split collet in a small vee block and center drilled the screw head following by cross drilling with a 1/16 drill bit.











Back to the lathe, the screw was reversed in the split collet and the end faced off and center drilled. I also shortened it as it was longer than I wanted. The edges of the thread were beveled to keep them from touching the contra piston. 

If the end touching the contra piston does not have the center relieved, the screw will "walk" when the engine is running. It is frustrating to have the engine adjusting it's self. 






Finally, and with no photo, a piece of 1/16 music wire 3/4 inch long was kinked slightly in the middle and pressed into the cross hole. The ends were buffed so they would not be sharp before pressing it in place. I mixed a small amount of epoxy glue and added a bit of black coloring to it and ran it in the head around the music wire. I made a little mound on top and turned it off after it had set.

Gail in NM,USA


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## GailInNM (Jul 2, 2009)

A couple of quickie things and the main assembly can be done.

First a couple of gaskets. The gaskets were made from 0.006 thick vegtable fiber gasket paper. I cheated and cut them on a laser engraver that I have available, but they are easy enough to cut by hand. The square one is for the V1 model and the round one for V2 and V3. V2 and V3 take two each, one for the front bearing assembly and one for the rear cover.






Gail in NM,USA


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## GailInNM (Jul 11, 2009)

Although it is not part of the engine, a knob to help adjust things is worth while. Just something to turn the crankshaft while fitting the connecting rod to it, and later to adjust the position of the contra piston. A propeller can be used, but it always seems to get in the way. Of course a few washers stacked up will also work, but a knob is so quick and easy that it hardly seems worth while to improvise.


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## GailInNM (Jul 11, 2009)

All the parts necessary to assemble the engine are now made except for the exhaust pipe, fuel assembly, and for a couple of things that are nice, but not necessary to make the engine run.

The assembly starts with the piston, connecting rod, wrist pin and cylinder. I wipe a thin film of oil on all the parts as I assemble them except I put a good drop on the connecting rod at the wrist pin. I just use 20 or 30 weight shop oil.






With the piston, wrist pin and connecting rod assembled, the piston is inserted into the cylinder. The transfer notch on the piston has to face transfer passage on the cylinder.






The cylinder spacer is added to the bottom end of the cylinder and the cylinder is inserted into the crankcase and aligned so the intake and exhaust ports on the cylinder line up with the intake and exhaust openings on the crankcase.

The contra piston is inserted part way into the cylinder and the head bolted down to hold it all in place. 

A gasket is placed on the crankshaft/front bearing assembly and it is installed in the front of the crankcase while guiding the connecting rod onto to the crank pin. A good drop of oil here is also a good thing. On V1, this assembly is glued in place with epoxy, so the crankcase and front bearing are cleaned before assembly and epoxy used to secure it all, and no gasket is used. On V2 and V3, assembly is secured with six 0-80 screws.






Gail in NM,USA


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## GailInNM (Jul 15, 2009)

With the head bolted on, the compression adjustment screw is run into the head until it touches the contra piston. The piston was adjusted to top dead center by rotating the crankshaft and feeling where top dead center was. Then the compression adjustment screw was tightened to move the contra piston down until it touched the piston. Again by feel and by rocking the crankshaft a little bit it was easy to tell when they touched and the screw was backed off until they did not touch.

The space between the top of the cylinder head and the bottom of the head on the compression adjustment screw was measured and a spacer made to fit on the screw was made. The length was made equal to the measured distance plus 0.010 inches. With the spacer in place, the screw can not press the contra piston closer than 0.010 to the piston. 

The nominal operating compression ratio of 18:1 had been calculated to be with the contra piston to piston distance of about 0.025 inch. Since the compression screw has a 32 pitch thread, the operating point for the engine should be about 1/2 turn up from bottoming on the spacer. 

Gail in NM,USA


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## arnoldb (Jul 15, 2009)

Very nice work GailInNM :bow:
The last couple of close-up pictures nearly makes one forget the cylinder is about your thumb's thickness!

Regards, Arnold


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## cobra428 (Jul 15, 2009)

Very Nice GailinNM :bow: Can't wait for the run video!!
Tony


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## GailInNM (Jul 15, 2009)

Thanks Arnold. You got me curious with the thumb comment. I keep forgetting to put a size reference in the photos. 
I ran a caliper over my thumb, and it measured 0.95 inch at the joint. The IMP cylinder is 0.87 inch diameter. Overall height is the length of my thumb, close enough. At 5'6" (almost) I am not a very big man so most thumbs will be larger. ;D

Tony,
This WIP is a compilation of three similar engines. It is being done after the fact except for a bit of tinkering and a few parts being made where I was not happy with the first ones.

The first one was run in March and there is a linked video of it at:
http://www.homemodelenginemachinist.com/index.php?topic=4338.0
The other two been run also, but no video shot. I was not planning to post video of V1 and V3 as they are essentially the same engine with different external features and run the same.

Gail in NM,USA


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## GailInNM (Jul 26, 2009)

In post 62 I detailed how to make the compression limiting sleeve, but I did not give all the reasons for using one. After all, most of the commercial compression ignition engines do not have any means to limit the contra piston position. The only really early popular engines that I remember that did so were the OK Cub engines. 

If you read the early operating instructions of the commercial engines, most caution against using an electrical starter. That is fine advice, but if you watched the IMP video you know that I used a starting motor. About a dozen years ago I found that I could either hand prop engines or the next day I could raise my arm enough to do machining in the shop. NOT both. 

When hand starting a small engine you can feel if a hydraulic lock is developing or if the compression is set too high. You lose this feel using an starting motor. 

On V3 of the IMP, I fitted the piston a little bit loose, but thought I would try running it anyway. Not really loose, just that the pinch point on the piston position was about a tenth of an inch higher than it should be. So the piston was really less than 0.0001 inch too small. Thats too much for small CI engines like the IMP.

Since I was just trying it, I did not make up the compression limiting sleeve. I lost track of the position of the contra piston and ran the compression screw down a little too far while tinkering with start up. Since the con rod is the weakest link in these engines, the photo shows the result.  :-[

New piston, properly fitted, and a new con ron and all was well.  ;D

Gail in NM,USA


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## arnoldb (Jul 26, 2009)

OUCH - At least you could re-make it!
Regards, Arnold


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## Maryak (Jul 26, 2009)

Gail,

Bad luck  but I agree about the options of a start of the engine v continued use of your arms and back.

Best Regards
Bob


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## GailInNM (Jul 26, 2009)

Arnold:
It was not as big an OUCH as one might expect. If you noticed when I was making the connecting rods I was making one on each end of some bar stock so I had something to chuck on. So, I had made 4 and only needed 3 and that left me a finished up spare. When I blanked the pistons, I cut an extra couple of blanks that I left slightly oversize and did not put the transfer notch in as the transfer notch is different on V1/2 and V3. So just had to finish to size (the right size) and cut the notch. All told it only took about an hour from broken rod to running. 

Bob:
It really was not "Bad Luck". Just me being my optimistic self with a "maybe it will run OK" when in my heart I knew that I would have to rework it at some time to make it run right. Most of us get a little lazy sometimes and try to cut corners. I am one of the worst offenders, but don't mind remaking parts when necessary. 

The main reason for including this in this WIP was to explain why I did something the way I did. I normally don't mention the stupid mistakes I make. Those are just part of building toy engines. When I mess up a part beyond recovery I make a new part and get on with it. But, I get irritated when instructions say "don't EVER do this" and then don't say why. Also, beginners to engine building need to know that no matter how many engines you build, you are still going to make mistakes. Making parts over is a life long part of building engines.

Gail in NM,USA


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## GailInNM (Jul 26, 2009)

Getting close now. Only the fuel system for all three versions and exhaust stack for V2 and V3.

The fuel system is the same for all three versions. The photo shows all the parts to be made except the 1/4 jam nut used to lock in into position when screwed into the crankcase. 

No detail on the spring will follow, It is just 10 turns of 0.018 stainless steel spring wire wound on a 1/8 inch diameter mandrel. More than 10 turns have to be wound as it will spring open when the winding tension is removed. When it springs open, the ID opens up to a little over 0.140 diameter so it will slide on the smaller diameter of the needle barrel shown on the center right of the photo. I close wound the spring and then spread it so 10 turns covered 1/2 inch, more or less. It's purpose is to keep the needle valve from changing position from vibration when the engine is running.

Gail in NM,USA


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## GailInNM (Jul 26, 2009)

I did the jam nuts first. Mostly because I wanted a jam nut to help hold the spraybar later for cross drilling, but also because they are easy and I wanted to get them out of the way. I needed 5 jam nuts 5/16 AF (Across Flats) with 1/4-40 threads and 3 nuts 3/16 AF with 5-40 threads. Thickness is not critical so I made them 0.080 thick because I have a 0.020 parting tool ground up. 0.080 + 0.020 = 0.100 and that is a nice increment for slicing them off.

Same method is used for both nuts. Photos show the 1/4-40 nuts being made.

Stock is center drilled, drilled and tapped as deep as the tap would go. Then sliced off. Step over and slice off another one. A thin rod was held in the tail stock drill chuck so the nuts would not fall into the chip tray. Then the faces of each nut was cleaned up with a fine file to remove the cutoff burr. It is a big burr because of the thread.

Gail in NM,USA


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## Maryak (Jul 27, 2009)

GailInNM  said:
			
		

> Most of us get a little lazy sometimes and try to cut corners. I am one of the worst offenders, but don't mind remaking parts when necessary.
> 
> The main reason for including this in this WIP was to explain why I did something the way I did. I normally don't mention the stupid mistakes I make. Those are just part of building toy engines. When I mess up a part beyond recovery I make a new part and get on with it. But, I get irritated when instructions say "don't EVER do this" and then don't say why. Also, beginners to engine building need to know that no matter how many engines you build, you are still going to make mistakes. Making parts over is a life long part of building engines.



Gail,

More power to you for your words of wisdom and how you obtained the wisdom. :bow: :bow: :bow:

Best Regards
bob


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## GailInNM (Jul 29, 2009)

Next up is the spray bar. It is at least more exciting than the nuts.

The spray bar has a lot of operations on it, and the most intimidating one for most people, the cross hole, is by necessity the last operation. 

I started with 3/16 AF hex brass stock. Round stock could have been used, but the hex makes it easy to align and tighten the spray bar when it is installed in the venturi. I turned it to 1/8 inch diameter for 5/8 inch and then threaded that 5-40 to a point that is about the middle what will be the reduced section. 

It was carefully center drilled and then drilled 1/16 diameter to a depth about a 1/16 inch beyond where the cross hole will be. Then the hole was continued with a 0.040 drill to the finished length of the spray bar. For both of these drilling operations it is essential that the drilling only go about 1-1/2 times the diameter of the drill and then with draw the drill to clear the chips. These holes must be straight and if the drill is pushed too hard it will wander.

















Using a 1/16 wide parting tool, the section that will be in the venturi hole is reduced to 0.093 and the radius left by the turning tool is squared up next to the 3/16 hex section. Then the stock is extended from the chuck and the spray bar is cut off to length.

The part is reversed in the lathe, griping on the 1/8 diameter section. and the other end in turned down to 0.093 and a small groove about 0.010 deep is cut to aid in holding the fuel line on. A liberal bevel was filed on the end to make it easier to install the fuel line.









Gail in NM,USA


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## GailInNM (Jul 29, 2009)

The last operation on the spray bar is to drill the cross hole. 

I threaded a nut on the spray bar and then clamped it in a small (1-1/2 inch) drill press vice. As I was clamping I turned the vice upside down on a flat surface. The points on the nut and the hex part of the spray bar then brought the spray bar level with the top of the vice. This positions the hole with the point on the hex and makes alignment easy on final assembly.

Drilling was done with a 0.032 carbide circuit board drill in a small high speed drill press running at 10,000 rpm. A stop had been set so the hole would only go through one side of the spray bar. Positioning is not critical. It just needs to be some where near the center of the reduced section of the spray bar and close to the center line. The circuit board drill has a sharp self centering point and is short and stiff so it does not wander when starting the hole as long as I am close to being on the top center of the bar. I wore a 10 power magnifying hood while positioning the drill and then fed slowly. After drilling I ran a 1/16 inch drill bit in the threaded end of the bar with my fingers to remove any burr that may have developed on the inside.


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## GailInNM (Jul 30, 2009)

Now for a venturi to mount the spray bar in.
Started with 3/8 diameter 6061 alumnium. Turned down a 1/2 inch of it to 1/4 inch diameter and drilled a 0.140 hole 1-1/8 deep. The finished length will be 1.062. Then threaded 1/4 inch of the end 1/4-40 to match the crankcase.









Switched ends of the part and faced off to length. Set the compound on the lathe to 10 degrees and cut the external flare. I used a home made taper cutter with 20 degree included angle to cut the inside taper. Most of the time I use a small boring bar to cut this sort of thing as the compound is already set at the correct angle, but since I had the taper cutter on hand it was quicker and easier than mounting a boring bar.









While I still had the 1/4 inch collet in the lathe a made up the exhaust stacks. They are just a 1/4 inch diameter rod 1/2 inch long and drilled through 3/16 inch diameter. Then threaded for 1/4 inch 1/4-40, the same as the venturi.






I then moved the collet from the lathe to square collet block and put the venturi in the collet block. Clamped the collet block in the mill vice and milled two flats on opposite sides of the venturi, turning the collet block 180 degrees after milling the first side. This was followed by center drilling and drilling 1/8 diameter through for the spray bar.
Gail in NM,USA


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## CrewCab (Jul 30, 2009)

Gail, you do make it all look easy  ............ first class thread and very informative, thank you, I'm looking forward to the rest :bow:

CC


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## arnoldb (Jul 30, 2009)

Very nice going Gail :bow:
Kind regards, Arnold


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## CrewCab (Jul 30, 2009)

Diymania  said:
			
		

> You make everything look so simple.



The Art of a good Tutor do you think Diymania 8)

CC


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## cobra428 (Jul 30, 2009)

Very Nice Carb Gail
Tony


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## GailInNM (Jul 31, 2009)

Thanks for the kind comments CC, Arnold, Diymania and Tony.

Next up is the needle parts. There are two parts for the needle. The needle and the barrel that holds the needle. Lets start with the barrel.

The barrel is just a length of 3/16 brass a little over a half inch long with 5-40 internal threads in most of it, and a 1/16 hole in the rest. About half of it is turned down to 0.14 inch for the spring to slip over. After facing the end of the rod, I turned wond the end for about 0.281.  Center drilled and drilled with at tap drill for the 5-40 to a depth of about 7/16 inch. Then drilled an additional 1/8 inch with a 1/16 diameter drill. Tapped as deep as I could with a plug tap.







Cut the barrel off to length plus a little bit to clean up. Reversed the barrel in the chuck and faced it off and rounded the end a little bit with a file.

The needle is a short length of straight 1/16 music wire with a 1/4 inch long taper ground on one end. When finished it does not need a sharp point, but I find it easier to grind it to a sharp point and then to blunt the point to about 0.015 diameter. Eyeball accuracy is close enough for all these dimensions. I use a Dremel (rotary tool) with a 3/4 inch fine grinding wheel to do the grinding. 

The first step is to dress the grinding wheel if it has never been dressed. A new wheel will have runout and a rough surface. If it is not dressed, the needle will have a rough surface, and probably be egg shaped from the grinding wheel bouncing. I use a single point diamond dresser that I use for my other grinders to dress the wheel. I hand guide the wheel over the dresser, holding the Dremel in one hand and the dresser in the other. To steady my movements, I rest both arms on the workbench or outdoors on the picnic table. Eye/face protection is a must, either safety glasses or preferably a face shield. Don't do this near your machine tools as the dust created is pure abrasive.

I put the music wire in a 1/16 collet on the lathe with about 1/2 inch protruding. Cover the lathe bed with a paper towel. For safety and other reasons, don't use a cloth rag. I run the lathe at about 1200 RPM. It works best if the lathe is run in reverse so the wire and the wheel are turning into each other. It gives a little smother finish, but is not absolutely necessary. I first ground a blunt taper on the end of the wire. If the end of the wire is not uniform, then the shallow taper will not be concentric when it is ground.Then the shallow 1/4 inch long taper is ground. It only takes light pressure and by moving the tool along the axis of the wire as it is being ground a very smooth finish is produced. I grind for about 5 seconds at a time to prevent overheating the wire and destroying the temper. Then let it cool for one or two seconds. It takes less than a minute to grind the taper. I ground to a point and then blunted the point. After extending the wire about an inch from the chuck I polished the ground section with a bit of 800 grit abrasive paper and polished up section of wire above the point taper so it would solder easier during assembly. 

Last step is to throw away the paper towel covering the lathe ways so I don't wipe something down with it by accident.

Gail in NM,USA


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## GailInNM (Jul 31, 2009)

With the needle parts made, they need to be assembled. I use regular electronics grade soft solder with a little bit of extra flux added to the parts. It does not take very much flux or solder.

I started by installing the spray bar in the venturi and held it in place with one of the nuts made earlier. I aligned the spray bar cross hole so it pointed at the side of the venturi. This was easy to do as I had drilled the hole so it was in line with one of the points on the hex section of the spray bar. 

The barrel was screwed onto the spray bar until it bottomed out and then backed off one turn. I clamped the venturi in my small drill press vice on the ends as shown in the photo. I made a ring of solder out of some 0.020 diameter electronics solder by wrapping it around the needle and trimmed it off to make one full turn with a hobby knife.

The needle was inserted in the barrel until it bottomed out in the spray bar, then was lifted up about an 1/8 inch and a small amount of flux was applied to the needle just above the barrel. The needle was slid in and out of the barrel with a twisting motion to distribute the flux in the joint. Finally the needle was bottomed out again and the solder ring slid down to the needle-barrel joint.

A small propane torch was used to heat the assembly. The flame was directed at the top portion of the barrel at a slight upward angle so the flame would not touch the solder ring. After about 2 seconds of heating the solder ring melted and flowed into the joint. 






After cooling, everything was removed from the vice. The needle was unscrewed from the spraybar and given a little clean up to remove any extra flux. The end of the needle was bent about 45 degrees with two pair of pliers to form a handle for adjusting. The spring was added to the needle and then the needle was reinstalled into the spray bar. A 1/4-40 jam nut was added to the venturi and the fuel assembly was complete and ready to be installed in the crankcase.






Gail in NM,USA


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## cobra428 (Jul 31, 2009)

Gail,
N.....I.....C.....E
Tony


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## arnoldb (Jul 31, 2009)

Very nice looking carburetor Thm: And thanks for the step-by-step description :bow:
Regards, Arnold


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## GailInNM (Aug 1, 2009)

Penultimate post on the IMPs.
All three IMPs are finished and test run. 

The main thing still to be done is to build a display stand to hold all three engines. When that is done I will take a decent photo and post it. It may be a little while as I am still sketching what I want and have some other projects going right now.

At some point in the future I may anodize the V3 version with a red head and black crankcase. Probably not, but I have been thinking about it.

From left to right in the photo below.
V1 which is a fairly close replica of the original IMP with a few minor internal changes to get rid of the insane exhaust timing. 
V2 is the same as V1 internally, but with external cosmetic changes.
V3 Same external as V2 except intake a exhaust interchanges and internals changed to match.

Check the first post of this thread for a link to V2 running.
EDIT: run link video added. 
[ame]http://www.youtube.com/watch?v=8x3PalJlz0w[/ame]

It's been fun trip. Thanks for joining me and thanks for all the comments along the way.
Gail in NM,USA


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## Maryak (Aug 1, 2009)

Gail,

Wonderful work. :bow: :bow: :bow:

Smooth runner.

Best Regards
Bob


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## Deanofid (Aug 1, 2009)

Great build, and a fine build log too, Gail. They look just pretty as can be, and the one in the vid runs as well as any I've seen. Sounded like it was running where it should when you got the comp and mix adjusted. Impressive work!


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## ariz (Aug 2, 2009)

great work Gail, many compliments, very nice running engine :bow: :bow: :bow:

but... did you make 3 engines? in the same time or in different periods?


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## arnoldb (Aug 2, 2009)

Gail, Three stunning little engines in a row! :bow:
Thanks for the write-up and video! - surprising how reactive the engine is to the compression screw changes.
Kind Regards, Arnold


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## CrewCab (Aug 2, 2009)

Very nice work Gail :bow: cracking runner too 

CC


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## rake60 (Aug 2, 2009)

Beautiful runner Gail! :bow:

Rick


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## kustomkb (Aug 2, 2009)

Great work! :bow:

Sounds awesome.


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## GailInNM (Aug 5, 2009)

Thanks to everyone for all the kind comments.

Ariz: All three engines were built at the same time. It took about three weeks to make the parts and assemble the engines. I would guess that it would have taken about 2 weeks to build only one. Most of the parts are either common to all engines, or have a minor variation from one engine to the other. Once a set up is made it only takes a little bit of time to make a second or similar part. In contrast it took me about three months to write everything up for the thread. Of course a lot of that is bacause I would rather machine parts than write about it. 

Arnold: The sensitivity of the compression adjustment is because the thread pitch is rather coarse on the screw. While the thread is not the same as the original production engine, it is quite close. More modern engines often had a finer pitch screw, but I was trying to keep the engines in at least the spirit of the original design. Most small commercial engines of that era had similar sensitivity. Two other factors are that the propeller is quite large for an engine of that displacement and that I live at about 5300 feet (1600 meters) elevation and the thin air affects performance quite a bit. 

Gail in NM,USA


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## arnoldb (Aug 5, 2009)

GailInNM  said:
			
		

> Arnold: The sensitivity of the compression adjustment is because the thread pitch is rather coarse on the screw. While the thread is not the same as the original production engine, it is quite close. More modern engines often had a finer pitch screw, but I was trying to keep the engines in at least the spirit of the original design. Most small commercial engines of that era had similar sensitivity. Two other factors are that the propeller is quite large for an engine of that displacement and that I live at about 5300 feet (1600 meters) elevation and the thin air affects performance quite a bit.



Thanks for explanation Gail  - And it is good to know about the elevation issue for when I build an IC engine one day; Windhoek is also 1600m elevation.
Kind Regards, Arnold


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## GailInNM (Aug 17, 2009)

Final photos of the IMP's. 
They are now in the display case.
For reference, the base of the display is 6 inches (15cm) wide.
The mottled appearance of the bottom photo is a reflection of the wood surface it is sitting on.
Thank you to everyone who joined me for the ride.
Now it is time to move on to the next project. Drawings in progress.

Gail in NM


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## zeeprogrammer (Aug 17, 2009)

Very beautiful work Gail.
Thanks for the thread.


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## arnoldb (Aug 17, 2009)

Once again Gail - WELL DONE!
I wish I can see this display in real-life one day.
Kind Regards, Arnold


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## CrewCab (Aug 17, 2009)

Top class Gail, from start to finish, including this excellent thread, and the final pictures do you proud :bow:

and very enjoyable 8)

CC


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## GailInNM (Sep 1, 2009)

Thank you to all who commented on this build.

For those who might be interested, the IMP build has been included on Ron Chernich's website in the Gallery on pages 12 and 15. He also gave a nice plug for this HMEM thread in the second link.

http://modelenginenews.org/gallery/p12.html
http://modelenginenews.org/gallery/p15.html

Ron did the reverse engineering of the original IMP and produced the plans that started this project off. Thank you Ron.

If any of you have not visited his website, you really should do so if you are interested in building model aircraft type engines. His site has a most comprehensive list of "how to's" on most problems faced when building this type of engine, and a fantastic photo gallery of both homebuilt and commercial model aircraft engines. When I first discovered it I got lost for several days while reading it, and it has a much larger content now. It is updated at first of each month.
Gail in NM


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