ML Midge Build

[jack620]

Thanks Ramon.

Today I finished the CPs (I think) and started on the wrist pins. No pics yet sorry.

I am a bit worried about the fit of CPs (maybe too tight). I used the method described on Ron Chernich's website. I super glued the CP to the piston lapping mandrel, set the topslide over a bit less than 0.5 degree and turned the CP with a slight taper. When the CP would just enter the cylinder liner I stopped turning and polished the CP with wet & dry paper and oil. I did this until the CP was a press fit in the top of the liner. I am able to push the CP into the liner with my thumb, but to get it out again I need to tap it out with a piece of drill rod and hammer. I doubt the CP would return itself to the top of the cylinder under compression. However, I figured I would leave the CPs as they are and see how they go. If they stick in the liner in use I will remove them and polish them some more to loosen the fit.

Half a degree doesn't sound like much taper, but I reckon it's too much. Ron says if the walls of the CP are less than 0.5mm thick, they will compress in the liner and form a nice snug fit. My CPs have walls 0.5mm thick but I don't think they are compressing at all.

Should I try hand-lapping the CP into the top of the liner with some fine diamond paste (say 1200 grit) and oil?

The wrist pins are coming along well. They have been turned over-length, polished to a snug fit in the pistons and parted off. Tomorrow I will put them in a drill chuck and turn them to length. Then I'll heat treat them and lightly polish again (with the Dremel I think).

A question on motor assembly- Are the piston, conrod and wrist pin assembled before slipping the conrod big end onto the crankpin, or after?

 

[Ramon]

Hi Chris, I have tried this taper method myself and have to say did not have much success. Like you I considered the taper too much. That said there are others who have had success with this method but I think the wall thickness has to be pretty thin before it flexes. It's surprising how much force the compression will exert but if it is too tight this places a strain on the conrod and crankpin if the engine is flooded - which it will be at some stage no doubt.

I would say you're not far off if you can push it in with your thumb indeed that might be just a tad to loose so I'd stick with it for now. I certainly wouldn't lap it to the bore - that will be way too loose - but if you need to, just keep stoning it (on the mandrel) until you judge the fit acceptable. Once assembled you should be able to move the CP down the bore with the compscrew without 'forcing' the screw but there should however be some resistance to it. If on the other hand there's little resistance to the screw then the CP is definitely too loose. Unfortunately theres no real 'scientific' way of describing how to get the required fit - it's very much down to feel and experience with the little beggars.
BTW Having to tap it out is not a problem but don't use anything other than a brass, ali or wood rod in the liner - you could easily score it with something like drill rod.


Usually I assemble the piston to the rod and drop the big end over the crankpin with the crankpin at TDC however some engines just don't have sufficient clearance on the piston skirt to allow this in which case the rod is popped over the c'pin, (screw the backplate in to keep it on the c'pin) then mount the piston on the rod. If you have to (gently) force the rod home mill a half round slot in a piece of wood to support the piston first.

Put the CP in the liner but no further than with the top edges level with the top of the liner then, making sure it's really clean but with a little light oil, pop the liner over the piston and push it home in the crankcase.

It's getting close to trying it - have you thought about fuel yet? Are you able to get fresh fuel? Also what about propellors - do have anything suitable or in mind?

Regards for now Ramon

 

[jack620]

Thanks again Ramon. Dunno how I would build this thing without your advice.

I will leave the CPs and see how they go.

Fuel- I haven't tried sourcing fuel yet, but I was planning to make it from Aerostart which I believe is 24% ether. Here's a recipe from the M.E.N. website for running-in and general purpose use:

PARAFFINIC BASE FUEL 45-60%
LUBRlCANT 20-30%
DOPE 1-2.5%
ETHER 20-25%

Props- I believe a largish wooden prop is best for running in a CI engine. Ron C used a 8x4 wooden prop to run-in a 0.8cc Midge he built. I suppose my 1cc version would want a bigger prop. Any suggestions?

Chris

 

[jack620]

I finished the wrist pins today. I heat treated them then polished the ends with a felt pad in the Dremel and 800 diamond paste.

I've run into a problem. One of the wrist pins doesn't fit into its conrod hole. That's because the wrist pin hole in one of the pistons ended up oversize because I drilled with a 2.4mm drill bit, when I probably should have used a 2.3 or even 2.2mm drill. End result was that the 2.5mm reamer didn't really 'bite' into the piston as it passed through. The wrist pin for that piston measures 2.58mm and the conrod is reamed to 2.5mm. I need to open up the hole in the conrod, but I'm not sure how.

Appreciate any advice.

Chris

This pic shows the piston sitting in an MDF 'cradle' to protect it while the wrist pin is being tapped home with a pin punch. Just clamp two pieces of wood together in a vice and drill a hole with a brad-point drill bit centred on the join.

 

[jack620]

I partially assembled one of the Midges and quickly discovered that I'm going to have to relieve the sides of the crankcase. I think RKLopp said he had to do this to his Midge too. I guess the Dremel will be getting some more work unless anybody knows a more elegant way of doing it?

 

[Ramon]

Hi Chris, this is all part of it at times

You might be able to scrape enough clearance (you only need ''just enough') using a half round needle file just ground on the end to act as a scraper - just be careful not to get it too hot and take the temper out. I'd be reluctant to use the Dremel in such a small space but if you do be careful not to remove anything in the top of the case bore. You could however set the case up at an angle in your vise, liner bore toward the chuck, and bring in a longish end mill/slot drill - if you haven't one and can make a D-bit driving it from the chuck it will act like a boring tool - much more control that way.

Re the con-rod - if you have a round fine needle file (I wouldn't use a diamond one) try gently inserting it until it stops on the taper then just as gently rotate it 'backwards' to the cut ('forwards' it will just pull itself in) trying to keep it central to the bore and doing it from both sides. Finish off by rolling some 600/800 wet and dry around something smaller - piece of piano wire or similar and tease it with that until the pin will fit.

Re fitting the wrist pins - if you have to tap them in just make sure the piston hasn't swelled in the area of the pin before assembling the piston to the liner.

I thought you might have difficulty with fuel - I haven't had to use any form of starting fluid so can't really advise you - we are still fortunate over here that it's still available but even so from very limited sources.

What I would say however is having mixed your fuel do you have or know anyone who has a commercial, well running engine with which you can try it on. If it works on such then you can concentrate on your engine if it's lax to start but if you don't know the fuel is okay you could be spending a lot of time chasing something that isn't there. Quality of fuel in this case is important.

Prop size 8x4 wood is just fine - a 6" pitch to start with will load it - it's best to let it rev but not to let it get hot.

Glad to be of help Chris - I love messing with these little things - just want to see you have success with yours Thm:

Regards - Ramon

 

[jack620]

Thanks Ramon, I hadn't considered scraping the crankcase for clearance. Since posting I've been thinking of milling the area with either a 1/2" x 1/8" woodruff cutter, or a small boring bar held in my collet chuck. I would enter through the rear of the crankcase and remove a small crescent shaped area on each side. However, there is potential for this to go wrong with disastrous results. Scraping would be a safer way to do it. At least I have 2 crankcases to play with!

I think my wrist pin did swell the piston because it jammed half way into the liner when I tried to fit it. I tapped the piston out and wrung it back in with loads of oil. I hope I haven't ruined the fit.

I'll have a go at the conrod with a round needle file and some W&D. I was going to make a 2.58mm D-bit, but holding the conrod securely while I ream it might be difficult.

I don't know anyone with a diesel engine who I can test my fuel on. I have found a supplier near me who sells diethyl ether. I just need to find out if they will sell to me without a special permit/licence.

Chris

 

[jack620]

I tried grinding a small scraper out of a piece of round HSS (I didn't want to sacrifice one of my good needle files). Either I'm hopeless at grinding scrapers or hopeless at using them (or both), but I couldn't remove any metal with it. I tried filing with a needle file but that was taking forever. I relented and tried the Dremel with a small toothed cutter. It was very aggressive, so I had to make like passes with a good grip on the Dremel, but it did the job. The finish wasn't pretty, but I was able to smooth it a little with a small wire brush tool in the Dremel. It would have been nice if the clearance had been shown on the plans so it could have been machined while the crankcase was in the lathe milling attachment vice.

The parts are assembled now (minus the CP) and everything moves smoothly. I'll fit the CP next and see how the compression is.

I will have a go at opening up the undersize conrod over the weekend. Then it's on to the prop driver and spinner.

 

[jack620]

I've searched eBay for an 8x4 wooden prop and I found just about every size but. I did find an 8x4 pusher prop, but that's no use to me. Should I go down in diameter and up in pitch or vice versa? I need a prop suitable for running in the Midge and another for use after it is run in. Although it will never actually fly, so the second prop should be chosen for engine longevity, not performance.

Most of the recommended sizes relate to 2 stroke glow and 4 stroke engines. I gather a CI engine needs a slightly heavier prop to provide a flywheel effect? Any reason why I can't just use a nylon prop?
Chris

 

[Ramon]

Hi Chris, Well at least you've solved the problem. I had the same issue when building the Nova engine and set it up as here to mill out the sides with a long series slot drill...

When I modify a file I only do the very tip - no more than about 8mm long. I use a half round section and just take the teeth off the lower surface holding it on the periphery of the wheel so as to give a small degree of hollow and then just lick the sides to form a cutting edge. The back of the file can still be used as such and the rest of it of course is still okay.

Props - I like to use wood because they are usually much lighter and don't as such load the engine but there is nothing wrong in using nylon. For runnning in though and indeed for general demo running I would stick to 4" pitch - 6" again puts a load on and the engine will soon get (too) hot - it's much better to let these small engines run a bit faster and lightly loaded when running in. If you can't get a 7 x 4 for faster general running then you can always clip the tips of the 8 x 4 but make sure it's still in balance

I have just looked for wooden props here and 8 x 4 is readily available in the UK - 'Zinger' brand. If you really get stuck I can get you a couple and send them if you want.

If you can obtain some of that diethyl ether that will be much better than using starting fluid as you will have a known factor. I don't know what you can use for an ignition improver (dope) Amyl Nitrate is long gone and so is IPN. 'Improver' is availble over here from one supplier but I don't know exactly what it is. You should however be able to get away without using it on these engines but I would keep the oil and ether both at 30% with 40% paraffin. I believe there are additives for (proper) diesel fuel which can be used but have no idea what they are either. There are a lot of Aus control line flyers who use diesels who post on the Barton Control Line Forum -
http://controlline.org.uk/phpBB2/index.php might be worth a post to see what they do.

When you make your prop driver do you have a knurl? or will you have to cut it on the lathe with a side ways mounted tool?

Regards - Ramon

 

[jack620]

Ramon,
Thanks for the offer, but I found a eBay seller in the US with a good selection of Zinger props (but no 8x4s). I will order an 8x4 nylon for running in and a couple of 7x4s and maybe a 6x4 for demo'ing.

Haven't heard back from the ether supplier. Will call them today. Thanks for the link to the control line website. I'll do a search there and see what comes up.

I have a set of knurls that I was given, but I don't know if they are diamond pattern or straight. I'm away at the moment, so it will have to wait till tomorrow. If the are straight pattern I will make a holder and use that. If they are diamond I think I will use a threading tool mounted on its side to carve grooves with the cross-slide. There's a good "how to" on the M.E.N. website explaining both methods. The pic below is from that "how to". It looks like Ron uses a tapered D-bit to cut the taper in the prop driver. I was planning to make a very small boring tool and bore the taper with topslide offset 5 degrees. Not sure what to do now.

Chris

 

[Ramon]

Hi Chris - don't know if you've caught up with the Eta build of late but I've just described making the prop drivers which may be of some help.
As you've probably moved the topslide from doing the crankshaft I would turn a taper in some scrap first and blue it to your shaft adjusting the angle if neccessary to make sure the angle is a good match - once you have that making them will be quite straightforward.

Good to hear you've found some props - good luck with the ether.

I'm starting my crankshafts tomorrow

Regards - Ramon

 

[jack620]

Yes I have moved the topslide since I machined the crankshafts Ramon. I'll do the bluing as you suggest.

I put a post on the control line forum and I have already been offered 250mL of diesel fuel gratis by a member of a local flying club. That has saved me a great deal of trouble. Thanks again for the link.

The Eta build is looking great. Good luck with the crankshafts. Some good info there on the knurling too.

Chris

 

[jack620]

Today I fixed the undersize hole in the conrod top-end. I thought I had a round needle file, but it turns out I don't. So I decided to try making a D-bit the same size as the wrist pin. I'm glad I did- it worked a treat.

I started by turning a piece of 6mm silver-steel down to 2.58mm and put a slight 45 degree bevel on the end. Then I milled away just under half the thickness to create the "D". I heat treated it and gave it a quick hone on an oil stone. The resulting surface was still a bit rough, with the milling marks still obvious under a 10x loupe. Despite that it did a beautiful job on the aluminium conrod and the wrist pin now fits the rod perfectly. I am completely sold on D-bits!

After assembling the piston/conrod/wrist pin, the piston jammed in the liner just like the other one did. I worked the piston/liner pretty thoroughly with oil and the piston now slides in the bore, albeit tightly. I have assembled the engine but it will need more work to loosen the piston/liner fit.

Chris

 

[jack620]

Progress has been slow lately. Today I finished the spinners. The prop drivers are finished except for the knurling. I'm awaiting a couple of straight knurls I bought off eBay.

I hope to make the spray-bars this weekend.

Chris

 

[Ramon]

They're looking really good Chris but my that matchbox shows just how miniscule they are. I'm not sure I'd be able to work so small -that must be very testing. Are you planning on fitting a nut to the venturi thread to stop the venturi rotating?

Glad you got the conrod sorted - D-bits can get you out of all sorts of scrapes

Don't know what you are planning on using for needles but I find piano wire is ideal - probably 18swg (.048") would be suitable if not 20 swg. I used 18swg on the Nova engine but all the others use 16 swg. The point itself is not too critical - too coarse though and the settings can be sensitive - too fine and the settings too 'open' it does need to be reasonably concentric though.

BTW the Nova is a great project and easy engine to build

Regards for now - Ramon

 

[jack620]

Hi Ramon, the plans call for a 5 mm brass jam nut to lock the Venturi.

The metric plans call for 1.5mm piano wire while the originals call for 1/16" for the needle. I'll use whatever I can find. Thanks for the tips.

The Nova looks great, but the 0.5mm fins look challenging.
Chris

 

[Ramon]

Hi Chris, That's a surprise that's quite thick wire for such a small engine. Just something to bear in mind re piano wire - well over here that is -possibly not in Aus.?

'Piano' wire is usually in Standard Wire Guage and as such has 'odd' diameters relative to drill sizes 16SWG -.063", 18SWG -048", 20SWG -.036. The wire itself however can be made into a very functional reamer to give a good fit on the wire. (See the Tigre post)

'Music' wire appears to be slightly different - we have over here 'KS Supplies' in most model shops which is sold in 'diameters' and is often sold as 'Piano' wire but the diameters differ - 1/16 is .062 and the wire is not so hard and definitely less springy and easier to bend than true piano wire. It would still be ideal for making needles of course.

Don't be put off too much by these thin fins - like you I did have concern particularly that they might push over so I made mine slightly thicker -.8mm and very slightly tapered - 3deg. inclusive.

Because of those thin fins the material chosen was freecutting En1a and no problems were encountered. However my friend John who had encouraged me to build one made his correct to print and had no problems either. One thing that did concern me was whether the transfer port would silver solder successfully due to the lead content but by keeping the heating to a minimum that caused no issues either.

I'm in occasional contact with another of your fellow countrymen - another Chris, Chris Dunn - who has not only successfully made a spark ignition version but recently made an impressive twin (spark) version as well.

Should you need some drawings I have a copy of the original as published in Model Aircraft magazine in 1946. Though there's a couple of ommissions there are no errors and are very good to work to. Let me know if you'd like to know more

Regards for now - Ramon

 

[jack620]

Actually the plans do call for music wire Ramon- my mistake.

The original plans use a 1/16" needle in a very basic spray bar with a 1/16" bore all the way thru. Ron Chernich's metric plans use a 1.5mm needle in a slightly more elaborate spray bar that has a seat formed by the intersection of a 0.5mm bore and a 1.5mm bore (note the plans show a 1.0mm bore, but I think this must be a mistake- I have emailed Ron for clarification).

I am certainly interested in the Nova plans. I saw a picture of Chris's spark ignition version. If I went down that route I think I would opt for hall effect sensor and electronic ignition rather than points.

Regards,
Chris

 

[jack620]

Not much progress today. I turned the handle of a $3 screwdriver down to 19mm to be made into fuel tanks. I also turned up a small internal threading tool out of silver steel to cut the threads in the lid of the fuel tank.