Model Forum / Radio Controlled / Air Models / April 2004

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.

--
Paul McIntosh
http://www.rc-bearings.com

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 Hampton
Adelaide, South Oz

   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