TIP: Always install piston rings carefully. Piston rings made of cast material are only elastic to a limited extent and break relatively quickly.
Some pistons have trapezoidally profiled piston rings (so-called keystoner rings), these only fit in one installation position. If the ring does not move freely, please loosen it carefully and reinstall it turned 'upside down'.
Very helpful tools for working on the cylinder and/or piston are: