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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Restoration of old C/L Stunt ringed motors by honing

By Joe Supercool

My regular readers may recall my obsession with overcoming the engine problems of my delinquent youth. Perhaps I should say, middle age, as these engines were typically produced in the 1970’s. This is a progress report bearing on piston/cylinder fits.

The engines in question are the ST G21/46, Enya 45 I 6001 and Enya 45 II 6002. Diamond lapping the rings in the ST G21/46 and Enya 45 I 6001 produced very good results. The above mentioned Enya 45’s have very similar intake, transfer and exhaust timings to the Super Tigre engine: This has led me to puzzle why the Enya’s as supplied did not run with the superb ST 46 burble.

The folklore goes something like this. The Enya 45 6001 was popular with stunt fliers in the early 70’s: the 45 6002 was not up to scratch for stunt and had therefore been changed to suit R/C fliers, while the ST46 remains the yardstick by which to judge more modern engines. Well, in view of the timings I have measured, none of this really makes sense. Since I also have a dud ST46, there must be some underlying, dare I say intransigent, factor at work. Further, the observation was made that ringed motors are run out as soon as they are run in! (non-Dykes).

As noted above, lapping the rings to improve compression seal was a great step forward: indeed the Enya 6001 now does run with a superb rich burble, thereby validating my hypothesis concerning the way these motors run. The tendency of these motors to go lean and stay lean has been eliminated, the engines breaking back rich after manoeuvres with a satisfying regularity.

Well with all this joy bursting out, it was time to bring out the Enya 45 6002. This engine had been zero-timed, thanks to the unbelievably fortuitous circumstance that Model Flight in South Australia had in stock a new piston/cylinder/ring set, which I hungrily snapped up. This for an engine that came out in the mid-70’s!

I gave it 45 minutes on the running-in stand, put it in the beat-up old Firecracker, whereupon the joy-bubble promptly burst. The run was unbelievably bad: un-tuneable, poor power, sagging power, RPM everywhere. This was essentially a new motor: how could this be happening to me? My previous life must have been sinful indeed.

Resisting the urge to duck out to the shed for a quick diamond lap, the time to set the old mind into gear appeared to have arrived. Nothing much happened, but a chink in the corner of my poor stroke-addled, tinnitus riddled brain was screaming “piston/cylinder/ring fit”. Well, if I were not to do the lap, what was the right course to take? Would it be possible to find the problem by actually measuring the shape of, say, the cylinder? Many years ago I had mounted a Rossi 15 cylinder in a lathe chuck, with a dial gauge mounted on the tool post. This particular dial gauge had a little finger sticking out of it, so it was indeed possible to measure the bore taper in the Rossi.

Well, I have a nice new lathe, but lack the dial gauge and funds to buy one. Was there another way? Would I ask this question if there were not? Dear reader, this in not an attempt to insult your intelligence, just an attempt to indicate the slow and torturous way my brain works. Remember, this is all happening 30 years too late to be of any help to my competition record!!

My CNC milling machine is essentially a giant 3-dimensional micrometer with digital read-outs. The spindle normally holds a cutting tool, but there is no reason why it should not hold a finger, just like the dial gauge. The position resolution of the machine is .005 microns in each of the 3 axes, which is about 0.2 thousandths of an inch. Hopefully, this would be enough: in the event, barely, but better than nothing.

So the Enya 6002 cylinder was mounted vertically, flange down on a piece of perspex, as shown in the photos. The required finger was machined up out of brass, with extra diameter at the measurement point. My multimeter has a setting which goes “beep” whenever the probes are placed across a low ohmic resistance, so one probe went to the finger, the second to the cylinder. By moving the spindle down, the finger could be made to find the bore at any given depth, simply by moving the finger sideways until the meter went “beep”. This procedure worked splendidly, so that after an hour of measuring the bore front to back and transfer to exhaust, I was able to make some observations.


Between top-dead-centre (TDC) and the exhaust ports, the bore was out-of round by 0.8 thou. There was no (detectable) bore taper front to back, but 0.4 thou taper across the ports. Below the exhaust ports, the cylinder was 0.4 thou out of round, with taper front-to back of 0.7 thou and 0.4 again across the ports.

What did these numbers mean? These are small dimension errors: could they cause the motor to run badly? One assumes the cylinder was intended to be perfectly round, with no tapers. In this fashion, the ring could be expected to seal, even though it is not pinned and does rotate while the engine is running. Further, with no taper, the ring would not need to expand and compress with every stroke: surely desirable.

Not having a clue myself, it was off to see the Oracle at Delphi, or should I say, the Cestone at Gosnells (aka Charlie Stone, model engineer and aeromodeller extraordinaire!). Charlie has been battling with piston rings himself, as Norm Kirton, like myself, tries to restore the glories of his (Wharfdale) past, viz getting the ETA 29 back up to 120 MPH, as was done so readily 50 years ago. Charlie has some nice machines, including a filing machine, an item I had never heard of before, but which I instantly desired!

His work included trade secrets, which I cannot divulge here. However, he was able to describe to me the means and principles of honing cylinder bores, in sufficient detail to frighten me off from such a demanding task. In my fantasy world, I had planned to machine up a hone on my nice new Taiwanese lathe. Having spent half a day trying to get the swarf out of the geared head oil bath, I really needed a project to justify the frustration plus the $4200 expenditure. However, I was further disillusioned by Ian Thompson, silver medallist in Aussie team place F2C at the recent (2006) Spanish W/C, when he asked me if I had lined up the spindle with the lathe bed. You mean they sell them not in alignment?

This was starting to get embarrassing, when Charlie dropped the name of F2B/F2C guy Richard Morrow.. Very kindly, he invited me around to his factory, wherein were no less than 7 honing machines.

Whistling bravely, with my hands firmly placed inside my pockets, I wandered in. There was this huge hydraulic cylinder, being firmly and repetitively penetrated by what looked like a large sex machine. Evidently this is called “honing”, but I do know other words for it. Dick wandered up to greet me, so I shook his left hand and we got down to business. This was a man who knew honing. He even had tiny little hones for making gudgeon pin holes, so it was time to listen and keep my thoughts to myself.

Producing my miscreant Enya sleeve, I passed it to Dick who proceeded to look it up and down, round and about, in a manner I can only describe as disgusted. Eventually, he went to his Delapina honing machine, installing the appropriate Sunnen hone: with the hone turning at 300 RPM, and the cylinder drenched in what looked like treacle but which I assumed to be honing oil, he proceeded to improve on Mr. Saburo Enyas’ factory product.


To cut a long story short, on re-measurement, the cylinder had indeed changed its dimensions. Above the exhaust, the out-of-round had reduced to 0.2 thou, an improvement by a factor of 4. The taper in both axes was reduced to nothing. Below the exhaust, the out-of-round had increased slightly, while the tapers were basically unchanged.

Hopefully, the “above exhaust” measurements were the important ones.

So I reassembled the motor, without touching the ring. No point changing 2 variables at once: the idea was to see how important the bore was, and by what amount the out-of-round needed to be reduced.

With the engine on the test stand, I was somewhat alarmed to find it had no compression worth mentioning. Hand starting was not going to work, so I hit it with the electric starter. Stopping it after 5 minutes of very rich running, I found good hot compression, which was rewarding. After a further half-hour, with the compression fully restored, into the model and off to Whiteman Park. Sure enough, the motor ran beautifully, including the Super Tigre burble.

Draw your own conclusions on all this. But here are some useful snippets. Dick has a Mitutoyo bore gauge: if I heard him right, it reads down to 1 micron, which is down where you need to be for model engines. The gauge is useful, but I got the impression that honing is largely done by “feel”: that must mean the human hand can detect movements less than 1 micron. Clearly my next step was to buy my own hone. This idea didn’t get far; the starting price for a Sunnen hone assembly is $1000. With ringed motors such as these, the bore is parallel. How you would ever control the honing of tapered bores, I cannot begin to imagine.

So there you have it. Watch this space.


Addendum: Seems I'm not an orphan re piston/cylinder/ring fits. This is what Bill Werwage had to say in Flying Models, January 1980, on engine performance in his "Juno" article.

"Much emphasis has been on trivia. Little on the fact that most of our current motors don't have a proper ring seal. Certain engine cylinders on the market now should be against the law and a reward posted for missing, good rings. Sometimes I feel as though I am more in competition with the machinery and people at the factory than the other competitors.
These motors are fine with their stock bearings, and stock just about everything else too. However, a good ring seal that lasts any length of
time is truly rare." Amen to that.


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