Ringed Large Engines

I have heard it is due to how the larger size engines change shape when they reach operating temp. Supposedly hard to control how the cylinder head will expand from cold to running temp. I could have heard wrong, of course. Personally, I LIKE ringed engines. I have a GMS .76 ringed and it ran like a champ pretty much out of the box.

ABC engines are based upon the theory of equal expansion of the piston and liner by using materials of the same expansion coef. for both parts. Unfortuniatly, this theory is false as they do NOT have the same operating temperature when hot, the piston normally being several 100 degrees F. hotter than the liner. The amount of expansion is a linear function of size, the larger the bore, the greater the difference in size between parts when hot.

Ray S.

Why? Don't know. But ask yourself this. When was the last time you got the ABC engine option for the family car?

I suspect it has something to do with the ability of larger pistons and cylinders being able to get that right fit and seal with each other. Just an educated guess though.

Mostly it is because it is hard to get controlled expansion rates on the larger parts. An ABC enfgine relies on a very close fit between the cylinder and piston to maintain the oil film for sealing. Small differences in expansion on small parts makes fro very small changes in the fit. The larger you go, the greater the change in fit until you get to the point that you lose sealing and blowby destroys the piston, liner or both.

My second or third hand SC 1.08 (and presumably the ASP and Magnum) is an ABC. There's no nip left on it, it leaks compression past the piston like an old Fox 35 but it still gives me a one flick start and runs flawlessly on zero nitro fuel :)

But like some others have said, I suspect the larger engines revert to rings because any unevenness in temperatures around the piston/liner will exaggerate the clearances and roundness of the liner. The piston will run in to match the (hot) out of round liner but when cold it could appear to be almost worn out. Not good for their image. But given the choice, I'll have a ringed engine any day.

Brian Hampt> Why are most,if not all large engines ringed.Take the 90 size engines.Do

Thermal expansion is more difficult to control on larger engines simply because of the greater mass of the piston and cylinder that have to EXACTLY work with each other during expansion and contraction during heating an cooling. But don't sweat it. Ringed engines give up very little in power, aren't as critical to damage from overheating, and last a lot longer than ABC engines. My favorite .61 size engine right now is the OS FX .61, and I wish to hell OS would offer it in a ringed version.

MJC

MJC- Buy a heli engine. OS has traditionally made ABC airplane engines also available in ringed piston versions for helis, as they are more tolerant of poor cooling.

Abel