I've gone through about 40 engines in the last 6 months that proves the piston is not the issue, it's the ring. They get hot, they loose their tension, and they stick. Every time. If you're getting the engine so hot that the piston is expanding into the cylinder wall then you've gone far past general over heating or moderately lean running. Sticking pistons is a catastrophic condition that only occurs in extreme situations. Typically extremely lean running and/or lack of lubrication. Many engines can attain temps of 228c (442f) for a singular extended period and not stick a piston if lubrication and mixtures are in line. The rings will be toast but the engine will otherwise be ok.