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.