" wrote in
ups.com:

On Oct 20, 4:41 am, "Morgans" wrote:
Yes, some air cooled engines have tapered bores (when cold) to
allow for different rates of expansion in cylinder base-to-head
so that it's perfectly cylindrical when up to temp, but i don't
know of any aircraft engines that have that feature.

Really? I thought that they did have a tighter bore at the top.
I guess I read wrong, or remember wrong.

Maybe. I didn't know that they did. It would make sense if they
did, all right. I don't do machining. I just run my fingers over
the pretty parts when they come out of the machine shop!

This ought to be an easy question for someone in the group. Surely,
there is someone that does do machining on engines that hangs out
here, that would know for sure.

Anyone?
--
Jim in NC

Ok, First off don't call me Surely. GGGG.
The concept is question is called "choke bore" and the idea is the
bottom of the cylinder that's close to the case runs at a much lower
temp since combustion happens at the other end. The top end grows at a
greater rate because of this. In theory when the engine is up to temp
the cylinder ends up being pretty straight. I have the specs around
here somewhere but if I remember correctly the bore when cold is in
the .006-.008 smaller at the top. Boring one is usually done by using
a tool post grinder and performing the taper feature is a calculated
process. Honing one with a flex hone is straight forward and easy,
using a Sunnen CK-10 or a CV-616 takes ALOT more practice...

And I thought it was just my old model airplane engines and bikes that
had that feature. Who would have guessed a Lycoming could clamber to the
height of sophistication reached by Ohlsson and Rice?

Bertie