ORIGINAL: buzzingb
I am recalling what a motorcycle mechanic in Bruce tought me back in the 70s about motorcycle and some automotive engines, and that is that some pistons aren't round. Not round at all, they are oval. The man ask why they put oval pistons in round cylinders, and I said I thought they were round. Then he held one up that he had on the bench and sure enough much space on two sides. This allows for thermal expansion in the aluminum pistons. I must admit I have never checked a glow piston to see if they are round or not. It isn't as critical in small engines and some spall engines have rounder pistons. The bigger the piston the more expansion that occurs. I have a Honda engine (with many hours of use) that smokes profusely on start up but give it about one minute of warmup time and it all stops. This is because of the thermal expansion that seals the piston/ring/cylinder gap.
Mostly of all model engines are piston not oval. They are made tapered, top of piston are smaller than wrist pin zone to allow expanding to near same diameter as bore.
I has model engines who has home made piston with ring, works well. The diameter of top of the piston are calculated by temperature range when the piston are full expanded. See what i maked the parts in my OS 61 :
http://www.rcuniverse.com/forum/m_4929500/tm.htm
If compression are not good enough, it will take a time to run in the engine before the ring are adapted to cylinder wall. It is normal there are a little leakage cause the ring gap let it compression pressure leak out of cylinder. When the engine are running , there are leakage not big enough to make engine trouble..
Jens Eirik