Where is the most worn part of a piston?

[PatrickDella]

The most worn part of a piston is the wear on the piston ring groove. Here is the relevant introduction about piston ring grooves: 1. End gap: Refers to placing the piston ring in a standard ring gauge, or in the unworn section of the cylinder bore, and measuring and checking the gap between the two ends of the piston ring. 2. Side gap: Refers to the gap between the side of the piston ring and the ring land when the piston ring is placed in the piston ring groove. If the side gap is too large, it will affect the sealing of the piston. If the side gap is too small, the ring may get stuck.