Piston and cylinders are a crucial part of rebuilding an engine. You will need new pistons any time you have the cylinders over bored. FYI: Pistons are a component of a reciprocating engine. The pistons ride up and down in the cylinders and are sealed tight by the piston rings. The purpose of the piston is to transfer the force from the expanding gas in the combustion camber to the crankshaft via the connecting rod. The tolerances for pistons are very precise and pistons should always be carefully inspected and measured when rebuilding an engine.