Building the
Delong 29 Diesel


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The Delong series of engines made by Super Motors Inc of Cleveland, OH are well known in ignition circles. It's also widely believed that they produced a 5cc diesel, since the engine was announced and reviewed in Model Airplane News in the late 1940's. This appearance was premature. Apart from a prototype that appeared in MAN as if it were a production engine, none ever reached the public.

Some time back, the Motor Boys were lucky enough to be loaned what remains of that aborted project under the provisions that we not make drawings available and if we did make any casting, the current owner of the property be given one, or two. That seemed reasonable, and as Ken Croft had recently been conscripted into our ranks at that time, he generously (stupidly, he says) offered to not only do the CAD, but produce--wait for it--a die cast case!

The project to build that permanent gravity die hounded Ken's nightmares for over a year. This was nowhere near his first die job. His previous work is of the type that drives historians mad--"Look at this! It's obviously commercial, but who made it?!" But as his inaugural Motor Boys effort, he wanted it to set a new high water mark (it has!), so the dies were made, remade, polished, pondered, new kilns bought, and finally, metal poured, and poured, and...

The Empire Motor Boy says he learnt a lot about patience, perseverence, strong prayer and self-criticism during that ordeal. Several trips back and forth to friend Eric Offen were helping, but still no banana. He could get good bits, but there was always some part of the casting that fell short--often, well short!

Finally we received an email with the subject We have lift off!. The first acceptable (or so it seemed) case had been poured. The trick was to get the die hot--we know this, but what had not been appreciated on a complex shape like this with thin webs and little knobbly bits was the die needed to be very hot! Here's the first case. The only ways in which Ken's cases can be told from originals are: (1) there is no engraving/embossing of Super Motors on the side, and (2) there are no originals!

Ken was elated, but still not quite satisfied; the little bosses for the needle valve had not quite come out right, but he was on the right track. After a *lot* more pourings, Ken finally produced the requisite number of cases, plus one with a tiny defect that would be a spare, although that status did not last long since. With Gordon Burford being promoted to "Fellow" of the MBI, the ranks of mere worker drones could expand and we inducted a new member, Andrew Coholic of Canada. Following long tradition, as the new boy, Andrew gets all our reject bits.

Now comes the really hard part; deciding how people scattered all over the world can make interchangeable parts. Using the theory that such outstanding, show-off craftsmanship should not go unpunished, Ken was quickly elected project manager (or "Chief Cat Wrangler" as we say in software development speak). In a fit of madness, I put myself down for finishing and standardizing the CAD, plus machining the cylinder heads and making compression screws.

This put me on the critical path as we decided that everyone would machine their own cylinder, though GMA would do the honing. To ensure the threaded cylinders would fit the heads, everybody needs their head before they can start their cylinder, so I set up to make chips from a 1-1/2" diameter bar of 2024-T3 aluminum. To reduce waste of this expensive stuff, I sawed blanks to within 1mm of finished length and made a stub that would be both my "standard" thread as well as the mandrel used to hold the blanks during taper turning and fin cutting.

This would let all operations proceed in batch: 1) rough drill, 2) bore, thread and face to correct depth, 3) taper turn, 4) cut fins and drill/tap for comp screw. Notice the "nut" on the mandrel in the first photo. This acts like a locknut so I can remove the blanks easily after turning operations. The little tommy bar is because after it was done, I found I had no spanner big enough to fit my carefully made nut! The finished head in the foreground was made to iron out any bugs in my plan.

The first shot here shows all blanks bored, faced, threaded and tapered, ready for fin cutting. The second shot shows the bucket and tray load of chips after all the heads were finished. Note the Leaning Tower of Pizza on the compound slide--9 heads; 8 perfect and one for Andrew--he'll see why .

Feeling arty, I've posed the heads with my case for the camera. The case is as received from the UK. It is completely machined except to backplate and cylinder mounting holes, purposely omitted so they can be spotted from individual backplates as they are made. These we each make from bar stock. Ken figured they are too simple to bother die casting--not to mention that he does not want to see another die any time real soon.

It is now 10 months later, or thereabouts. In Friday's snail mail is a beautifully made crankshaft with prop driver and washer, plus a finished needle valve assembly. This gift drove me to the lathe on Saturday morning to make some swarf, starting with the backplate. This is a monster that starts life as a slice of 1-1/2" diameter aluminum bar. The part that fits inside the case was machined first, then reversed and gripped on the finished surface (with a shim of ubiquitous beer-can as protection) to machine out the rear cavity with the tank mounting post--a tedious trepanning job, but easy enough.

Finally, the mounting flange needs to be drilled and profiled. This is done using a rotary table and a 1/8" end mill. To determine the precise starting and ending angles, a short stub of 1/4" diameter stock, drilled to clear 4-40 is used to establish the width of the lugs. The flange is them milled away in an in-rotate-out sequence to within .005 of finish diameter. Because 1/8 cutters will deflect under load, two passes are made. In this shot, one segment has been removed and the first cut made on the second. A final pass removes the 5 thou and cleans up the cut.

To complete the lugs, the 1/4" diameter stub is used as a filing button. Normally, you'd harden these things, but for three lugs, filing aluminum, simple care will suffice. In this photo, you can see a finished lug and a roughed one with the button in place, ready for profiling. The holes have been drilled on 120 degree radials with the correct pitch circle diameter. The will be spotted onto the case, but how to hold the case for drilling?

Problem solved; use the crankshaft bush to support the case vertically, just before it is parted off from the stub it was turned from. As usual, I use the drill chuck to support the tap initially, turning it by hand. The tapping is finished with a small tap handle to give maximum "feel" (broken tap in hole == very bad vibes). Stop for coffee and to admire result. It's going so well, I may as well make the cylinder too.

This is a 5cc engine and the cylinder is a bit of a monster (bore is .680" nominal). My sequence will be:

  1. Rough out to within 30 thou of finished size, with a decent chucking stub on the top of the blank.
  2. Drill 5/16, then 5/8" and bore to finished nominal bore. The bore will be blind (finishing in the chucking stub) to prevent chuck clamping forces from distorting the cylinder.
  3. Without disturbing from boring operation, finish bottom of cylinder and flange to close sliding fit in crankcase.
  4. Again without disturbing, finish top to thread OD diameter and correct flange thickness.
  5. Transfer chuck and job to rotary table on mill and cut exhaust and transfer ports, measuring from bottom of mounting flange.
  6. Remount rotary table with work vertical and drill mounting holes in correct relation to ports.
  7. Back to lathe to part off and remove inside parting off burr at top of cylinder
  8. Grip lightly by lower portion (and beer can shim) and cut cylinder muff thread
Only part that I'm not super keen on is the the last step, but as long as the 3JSC chuck is not over tightened, and the thread cut in a number of .001" passes, all will be well. If the thread was cut while the job was still attached to the chucking piece (ie cutting from the flange side, to the cylinder top), it would not be possible to trial-fit the muff.

This shot shows the blank being roughed out. All operations use a live center in the tailstock for support. The roughing cuts were 0.060" per pass, with the tool sharply angled back as seen here. This trick was told to me by an old-hand at the local tech-college. It makes the chip longer and thinner, but the volume of metal removed is the same, so the cutting forces are no different. The advantage lies in what might happen due to drag on the tool. Let's assume we are using a standard "knife" tool with the point angled towards the headstock in the usual way. Drag on the cutting edge is pushing the tool back towards the tailstock. But the tool is like an arm, pivoted at the toolpost mounting point, so if it deflects back under cutting drag (or the tool holder rotates), it will bite deeper which increases the drag. We are in a "positive feed-back" situation and things can get quickly out of hand. With the tool raked back as seen here, any rotation due to drag will reduce the depth of cut, reducing the drag (negative feed-back). Of course, the finish obtained is rather RS, but who cares for roughing-out? Finally, the material in the slope is quickly removed with some deft hand-wheel and cross-slide cuts and who cares if the finish is a bit stepped? -- the first finishing cut (with normal knife tool orientation) will remove all that. This process is effectively fail-safe. It costs you nothing and may save a work piece during heavy roughing-out cuts; well worth doing, I say.

Here we are at step 5 in the sequence. The Delong ports are formed with simple lateral cuts. The port timing sets the sizes to be a few thou larger than standard cutter widths; eg, the transfer port is .130" high, so after cutting the full depth with a 1/8" cutter, two climbing-mill cuts are made moving 2.5 thou laterally. This nicly cleans up the edges and compensates for any minor deflection of the tool during initial cuts.

The hole pattern for the mounting flange is not symmetrical, so the cylinder is centered under the mill using an edge-finder and the holes coordinate drilled using the DRO. The camera focused better on the DRO than the job, so sorry about the blurred foreground. To drill and tap the cylinder mounting holes, the case is setup on parallels and the holes spotted through the flange using a trial-and-error jiggle of the hand-wheels. If I could have got the case aligned dead on the mill axis, I would have drilled these to coordinates too, but that was too hard. Also, my "parallels" may a thou or two off, but that does not matter for this operation.

Lest you think I'm some kinda super-machinist who never stuffs up, here is cylinder attempt #1. All going well until I started to rough out the bore to 5/8" and noticed that the cylinder walls were getting awfully thin. Guess who had reduced the OD to the ID (plus .030 for finishing)? Ah well, with luck this blank will make an AHC cylinder, or two. The other shot shows the Delong on Sunday night, with all the bits from Friday's post installed, plus the bits I'd made over the weekend: backplate, crank bushing, cylinder liner and comp screw. All that's needed now is all the bits that go up and down (plus honing and lapping). Motors-by-Mail is easy!






Even though it will be months, or years even before the Motor Boys finish their engines, I could not resist this preview of Ken's prototype in action. The first shot was taken with no flash, so we see the exhaust plume and the prop all a-blur. In the second shot, Ken's flash not only stopped the prop, but also caught the engine just as the exhaust had opened. Kens testing shows the Delong 29 will turn large diameter props at respectable speeds, as shown in the following table.

Top Flite 14x6 wood 4700 rpm
Top Flite 13x6 wood 5700 rpm
Zinger 12x5 wood 6400 rpm
Superthrust 12x4 wood 6600 rpm
Zinger 11x5 wood 7000 rpm
Zinger 10x6 wood 7600 rpm

Now be warned, I come from the "Anti-Flaming Logo" school of web design. Web sites should not flash, rotate, or make unexpected noises at people. That aside, Ken prepared a sound file of the Delong running on the above props, in ascending sequence of RPM and ending with one that captures starting and tuning. It's actually a quite atmospheric and may start me off on a totally new way to waste my valuable time, so if you are not offended by noisy web sites, press the button!




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