New 2.5 cc Model Diesel design and build.

[edholly]

This is the process I use to cut the "drive" into the prop driver. Very simple, and works a treat.

Using the cross cutter move it about 3 thou into the work. I have divided my chuck into 20 divisions with a simple texta mark. Lining up the mark with anything thin and available I move the chuck to the next division and move the toolpost into and out of the prop driver. Having done that at about 3 thou I then make another cut at 5 thou, and this is the result.
you will see there is a prop washer - but I won't be doing a drawing for that - something to be designed and made yourself - or just use a 1/4 - 6mm propriety one.

 

[edholly]

Here is the muff, once again when Steve finishes the CAD plans, you will see how this one was made. Muff's are so distinctive and a small change in design can totally alter the look of the engine, so if you decide to build this engine, once again think of redesigning it to your own preferences, maybe a ball, maybe a taper to it, alter the thickness and spacing of the fins ... etc, the parameters you work to are, 28 PCD for the bolt holes and enough meat above the top of the cylinder to bolt them through and also enough meat for the tommy bar to not over-stress the threads.

Remember to keep the internal fit snug, it needs to be to transfer heat efficiently from the cylinder wall, you can use a bit of heat sink paste if it isn't snug though, and it needs to be just a tad short of touching the lower ring of the cylinder, you want the hold-down bolts to hold the top of the muff to the cylinder otherwise A) the cylinder will float in the slackness and B) the expansion of the alloy will be greater than the cylinder which will also cause slackness and stress the bolts at the same time.

Speaking of the tommy bar, David Owen taught me to always drill a hole in the centre of it where it contacts the contra, that helps centre it and avoids stressing the threads by it wanting to wander side to side. The tommy bar shown here is just a 4mm cap-head, it will be a proper one in time.

Getting closer now - piston, rod, backplate venturi and tommy bar to go...

 

[edholly]

Venturi done - found it needed to be a little beefier than the design, so Steve will fix that in the plans. Also found Steve's interpretation of my hole in the crankshaft under the venturi was awry - so Steve will fix that too. Finding out now that mud map drawings are all fine and dandy for the one who drew them - but not so easy for the one trying to make sense of it all !

Have to spare a thought for all those like Steve trying to interpret other's thoughts and notations.

Here are a couple of photos. including one little bloke who continues to think he can fly - 4 times now I have put him back in the tree ! His parents go berserk when I pick him up - if only I could talk to the animals and tell them I am trying to help him out. There are a couple of cats that frequent the backyard so I hope he gets his golden wings soon. By the way he is an Australian Butcher Bird - they have a beautiful song but can get a bit aggressive, have had my ear ripped open a couple of times by them - not these ones - they are pretty gentle despite thinking I am doing him no favours !

P1 - milling the flats for the needle valve - note the %C collets to hold it
P2 - boring the venturi hole
P3 - In place ready to scribe the inlet hole in the crankshaft with a sharp needle
P4- setting the crankpin at 50 degrees BTDC
P5 - venturo in place, actually stuffed up slightly and a bit of a sloppy fit. Araldite or locktite gets a better grip this way !
P6 - the one who thinks he can fly

 

[edholly]

Couple of jobs done today.

First the marking out of the crankshaft venturi hole at 50° BTDC. As can be seen a sharp needle will just mark the steel enough to notate where the opening is. Then with a centre drill just nick the metal till you are sure it is centred then put a lead in with it. Then use the 5.5mm drill to go through. I use a wide flat very fine diamond file to get rid of the burr on the outside and a drill on the inside. It is important not to remove any parent metal as the better the seal around the venturi hole in the crankshaft to the crankcase, the better the suction of fuel/air mixture into the engine.

 

[edholly]

Next was making the conrod. I don't think anyone really enjoys this task, I certainly don't relish the thought of it. Anyways it seems to have come out ok.

I think it was Ron Chernich who came up with this idea initially and this is a bit of a modification on that. I mount a drill the exact size of the pin - vertical in the mill right on the edge (and I also use a drill the same size the other end of the jaws so as not to cock the jaws unnecessarily). I then use an end milling cutter in the chuck.

Now this is where you have to be super cautious as your fingers are pretty (very) close to the spinning cutter and you mill the ends of the conrod as per the photos. Once done you then mount the rod and mill the rise in between the rounded ends. Makes for a nice rod, but you certainly have to bear in mind how close your fingers are to the cutter, and the fact that the cutter wants to pull the work around. I only cut about 6 to 7 thou at a time to manage this best.

Of course you can always use a linisher or a file, I have done so plently of times, rarely anyone except you gets to see the rod, so as long as it is strong it doesn't matter too much if it isn't particularly pretty!

Note the 1mm hole drilled in the bigend for better oil supply and the slight chamfer on the crankshaft side of the rod for the slight radius on the crankpin to crankdisc.

Oh the Butcher Bird got his wings today - whew ...

 

[edholly]

The next step is to lap the cylinder with not just a fine finish, but also a taper from the exhaust port to the top of about 0.1 thou. Yes, this is pretty fine tolerances, but I have achieved that using the lap that has to be made.

The first thing to do is to clean up the bore with a reamer and get it as near as straight or open a bit at the bottom end by a thous or so.

Then make the lap, this is about a 10 minute to make affair, simply machine a piece of ally rod to the inside diameter of the bore less about 2 thou. for about 20 mm, drill and tap with an ordinary tap anything you have on hand (I used a 1/4 UNF) with the most tapered one. Taps are of course taper, medium and end, we want a taper one. Then hacksaw a slit all the way through for about 24 mm. The overall length of the lap needs to be about 60 mm or more so it can be held in the lathe chuck. - see photos.

Cover the bed of the lathe with some protective material, you don't want lapping paste getting on the beds etc. I use a diamond paste to do the lapping with.

Now put a grub screw, or even an ordinary cap head bolt in the lap and force it open with the grub screw and load the end with a bit of paste and I use 1 drop of oil too. I load the cylinder with the conrod end towards the end away from the chuck, as the lap will give a natural taper opening out in the direction which is what we want for the internal bore. You will need to keep reloading the paste and a tiny bit of oil. This process can take a hour or more so be patient.

Then with an in / out motion bring the bore gradually down to a fine finish. You will need to keep checking the taper, which means scrupulously cleaning everything. I have successfully used the lap to measure the bore, but have recently bought a bore gauge which reads in 1/10th thou increments.

The last photo shows the finished article now with that taper.

The reason for the taper is because the top of the bore is hotter than the bottom and expands a tiny bit more, and this allows for that. In the perfect world of model diesel engines, the piston should be a squeaky tight fit over top dead centre, but enter the bottom of the cylinder around the exhaust with little friction. If you achieve this you will have a beautiful running engine.

Next is the piston. Using the lap you can get an idea of the size required, and we are talking to 1 or 2 /10ths thou. A lot of people make a lap also for the piston. but I have found that this gives less a result than the process I will explain next post.

Just to recap - I can't emphasize enough how important a slight taper is, and it is worth the time and effort to get this.

 

[edholly]

Here is how to work out the internal diameter with the bore gauge. Yes 1 (ie 1 to 2) division on the dial is 1 thou how accurate is that !

I take a number of readings as the gauge is more for comparison than measuring. I got 570.5 - 570.8 - 570.2 - 570.5 and the calipers said 570.5 .... so wil make the piston 571 to start with and take it from there with the process I use.

You can still get almost the same accuaracy using the lap by winding the grub screw in and out ... ever so slightly ..

 

[edholly]

After making quite a few of these engines, you can still work out ways to improve what you do.

Contra pistons have to be a tight fit in the bore, and I know from experience that a tight fit (contra) to a nice fit (piston) is usually .2 thou - so it will be interesting to see when we make it.. So methinks why not make the contra first and use that as a sort of template to make the piston ? This is the first time ever I have made a contra first, usually I treat them as a bit of an afterthought - but really it might be a much better way !

So that is what I have done here. Starting off at 573 thou made the contra with its 4mm deptth and 2mm relief. Now it would only just enter the bore at the bottom. Taking it down by about .1 thou at a time I got it to enter to the exhaust ring at about 572.4 another .1 off and I could force it to about 1/2 way up the bore. another .1 off and I could force it to the top of the bore, still very tight had to tap it with a piece of dowel, but that is what is required of a contra if it is not to leak. This also proved the taper in the bore is what was aimed for.

This then gives me the size to aim for for the piston, I'll make it 572.1 to start then take it down if necessary from there. The last photo is of the finished contra needing the pip removed.

 

[Mechanicboy]

To create tapered cylinder: Piston create tapered piston when the piston is lapped into the cylinder at final stage.
To test fit is correct: Dry piston/cylinder ---> tight to enter into cylinder. Oiled piston/ cylinder ---> The piston is loose fit in BDC cylinder and tight in TDC.


Lets take a look at a typical lapping job - that of producing a fine finished bore and piston for an IC engine. In fact, piston and bore are both lapped in separate operations (NOT both together). All of these operations will be carried out in the lathe (and I need hardly mention the importance of keeping lapping compounds off the machine, particularly the chuck and slideways). For the bore an expanding lap is ideal, and this should be some 3-4 times the total length of the bore. The first grade of abrasive would be mixed with light machine oil (10W or lighter) and liberally coated on the inside of the workpiece. Similarly, the slurry would be added to the outside (and inside assuming it is of the ventilated type) of the lap. The lathe would be started at about 300rpm (for a nominal 1" bore) and the lap passed rapidly through the bore, keeping it moving back and forth without it coming out the bore. How to hold the lap? well, perhaps the best way is with a 'floating' tailstock holder, and failing this holding with the hand is a method as good as any. Be careful when holding the lap by hand as it's possible it may jam, hold it lightly and expect the unexpected. Remember also that unless the lap is maintained dead parallel with the bore (an almost impossible task) it will tend to bell-mouth the bore a little - hence the reason for making the work a little longer than finished size and trimming to length later. When the inside of the bore has achieved an all-over grey appearance, with the fine scratches appearing even and criss-crossing both ways, and with no evidence of any deeper scratches (as might be left by the reamer) it's time to move onto the next finer grade. The work will have to be removed from the chuck to clean it properly, and this should be done with clean paraffin oil followed by hot soapy water. The same procedure applies to the lap and all traces of the abrasive must be removed. The process continues until you reach the 'flour' grade of abrasive by which time the finish on the workpiece should be very fine indeed. A final polished finish, should this be deemed necessary, can be achieved using metal polish (diluted Autosol, or some liquid chrome cleaner). The lap should be a separate 'finishing' lap so there is no chance of contamination with the coarser grades of abrasive which might be embedded in the main lap. The piston is treated in a similar way except of course the lap is female. Work will continue with the coarse abrasive until (using the un-trimmed bore as a gauge) the piston will not *quite* enter the bore. At this stage finer grade abrasives are used and work continues until the piston will just enter the bore tightly. At this stage, it is usual to finish mating the two parts by using metal polish and briefly using the piston to lap the bore directly. Great care needs be taken but this method ensures that the fit is good for the entire length of the bore.

 

[petertha]

edholly, my interest always perks up when it comes to cylinder lapping. Did you happen to record your pre-lap & after-lap bore ID dimensions? (ie. how much material was removed during the lapping process).

 

[edholly]

Mechanicboy, thanks for the insight as to how best to make and use a lap. I have found this simpler way for both cylinder and piston that works for me and is not as labour intensive. Obviously the way you describe will give wonderful results, but my way too gets the engine running pretty satisfactorily.

Petertha, After running the ream down the bore a couple of times, it cleaned up very nicely. The actual lap only removed about .3 thou to get it where I was happy. I have found the more you dwell on getting a perfect lap the more risk there is of making it bell shaped inside, I think the lap follows the less material around the exhaust. The most critical thing from what I have found is the slight and evenly made taper from bottom to top. I found with this engine holding the cylinder with the bottom away from the chuck works in your favour as the shape of the lap is biggest part away from the chuck, so you aren't fighting the natural shape of the lap.

 

[edholly]

Here's a photo fo everything washed in the ultra-sonic cleaner with kero and then Nulon Carby cleaner. The ball bearing finally had its protection seals removed and the grease inside washed away. The venturi was Devcon'd into the nose and then whole engine oiled up and assembled.

The backplate was made and attached, the tommy bar was made using the caphead bolt with some ears put through it, and a mount made.

I have put in another couple of photos to show how I accurately get a centre with the slightly bent centre finder. Note the arrow on the top of the belt driven spindle - I turn that arrow to the direction the centre finder is being used - that way all errors are cancelled out and you get a centre within microns of where it is. If I was to find an edge I average the readings with the arrow to and the arrow away from the edge. Simple but it works everytime.

Forget to mention, when I made the piston, I stuffed up a bit and it ended up .4 thou smaller than the contra - I wanted it .2 Nevermind can either make another or cherry bomb this one. It has reasonable compression but not as good as if it was just .2 thou larger. We'll see what happens ...

 

[edholly]

Here is the finished engine, the comp is in fact a little soft as said in the previous post, but that didn't stop it from starting reasonably quickly and running superbly. I ran it for about 3 minutes on a very rich setting and right at the end just tweaked the comp bar a bit and the revs rose beautifully on the 9 x 4 prop. It augers well for when it is run in and can be leaned out and up-comped. Just took this short video of it running, this truly was its first ever run .... Ed

[ame]https://www.youtube.com/watch?v=FkDS0WEEWTQ[/ame]
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I must thank Steve who has worked so hard to follow my rough sketches for the componentry. Steve has almost finished the CAD plans, just the rear cover/engine mount to go and hopefully we can publish them in the next week or so. It is a pretty simple negine to build, and Steve's CAD plans do things a bit different to other plans I have worked from, I find them a lot easier to follow, and a couple of pitfalls I fell into have been changed around so no-one else will make them ...

.

 

[petertha]

Looks good, congrats. That brings back memories when I was a kid. Guys screwing the compression tommy bar in & out, open exhaust, engine rpm ebbing & flowing, slimy castor oil soaked hand, usually with an open knuckle wound form recent a prop shaving

 

[Roboguy]

Great looking engine and very informative build thread, thanks so much for sharing! Amazing how quickly you've built this. I'm hoping to get my Boll Aero finished in the next week or so.

Cheers
James Fitzsimons

 

[Herbiev]

Great build and great write up.

 

[Cogsy]

Great engine and built so fast as well!

 

[shodan]

Thank you! Super build, can't wait to try build one myself, as soon as plans are available. Cheers!

 

[edholly]

I was disappointed with the soft compression, entirely my fault, just machined the piston a tad too close to final finish to be able to get a good surface on it at the size I was aiming for and ended up .2 thou smaller than intended. When I measured it after disassembly and after the running it had, it was in fact .5 thou smaller than intended.

So it was either make a new piston or "cherry bomb" this one. Decided on the latter, nothing ventured-nothing gained. First heating no growth. Second heating a delicious 1 thou growth. Made a mandrell and pushed the piston onto it and then linished it as before. Took it down to the 572.2 size finished I was aiming for in the first place. It looked a treat.

Reassembled the engine and as I had hoped, the engine would not turn over but the piston did enter the bore to a bit more than 1/2 stroke. Plenty of light oil and then with a prop on worked it back and forth for about a couple of minutes, each time it went just that bit further. Not too hard as the conrod and gudgeon and crankpin are all being a bit stressed whilst this process goes on.

Eventually the engine went full circle, very tight, but that is exactly what is required. I never cease to be amazed how an engine will start and run with so much tightness .

So on to the test bed, fuel and flicking. A few pops then a few rotations, then a few more then away. Maybe not quite so quickly as this sounds but it didn't take too long.

First run just let it burble away at low comp for about 3 minutes, with a bit of a tweak right at the end. Let cool down 100% then another 3 minutes and a tweak at the end gave 8600 on the 9 x 4 wood Turnigy prop I have always used for comparisons. Totally cooled and 3rd run starting to tweak it often even a little leaner, although that indicated where I had it was about right, this time just got to 9,000 again about 3-4 minutes. 4th and last run tweaked it for max and got 9,060 initially, backed off next tweak 9100, backed off next tweak saw 9250. I then backed off the comp and let it run at about 5500 and then it ran out of fuel. So after about 12 -15 minutes it is showing some wonderful signs.

A) Good power.
B) Getting better with each run.
C) Easy to start, just suck a little fuel through the spraybar and flick.
4) Compression - fantastic - as good as it gets cold or hot.
5) Vibration certainly within the low to average range.
6) Never made any black in the exhaust, so materials must be happy.

So there we have it - an engine that would probably be the equal of a good sport 2.5cc from the 50s or 60s that can be built by the Home Machinist and hoping to inspire budding designers to have a go at their own design.

Time now to put the CAD plans on here.

A bit about me. I think my first flick of a diesel engine was about age 12, it was a Taipan red head possibly a Mk5 if I remember correctly, that was the only diesel I had as a kid. I must profess that I am a new-comer to playing around with building model diesel engines, and have only scratched the surface of what knowledge those involved in the Motor Boys camp have. But I enjoy using my mill and lathe immensely and that is where all this started a few years ago when I built my first engine the BollAero 18. The Holly Buddy is the 15th one built and I guess it won't be the last.

Again I would like to mention Steve Jenkinson who has so kindly worked these CAD plans from my rudimentary drawings into easy to follow and a joy to use plans of the engine.

And as you will see I have dedicated this design and build to the memory of a terrific fellow aeromodeler, David Owen.

 

[edholly]

Here are the CAD plans for the Holly Buddy.

Please read the "Forward words for Buddy Holly" before building the engine there is a wealth of information here.

If you do decide to build it, would very much appreciate a PM to me.

Ed

View attachment Forward words for Holly Buddy.pdf

View attachment 1- Engine.pdf

View attachment 2- Crankcase.pdf

View attachment 3- Cylinder.pdf

View attachment 4- Crankshaft.pdf

View attachment 5- Conrod piston.pdf

View attachment 6- Nose.pdf

View attachment 7- Venturi&Propdriver.pdf

View attachment 8- Cylinder Muff.pdf

View attachment 9- Back plate.pdf