- Sealing the combustion chamber to maintain good compression.
- Supporting heat transfer from the piston to the cylinder wall.
See: Ring gap for replacing piston rings
Some moped pistons use two rings, while others use one ring. Two rings are for longevity and to help prevent piston rocking, to improve oil control, etc. One ring is for performance (less friction). A Dykes (L-shaped) ring is usually used in one-ring pistons. This serves several purposes: pushing out for good compression (upstroke), relaxed for oil control (down stroke), and extra stability from being wider than normal (prevents rocking).
Chrome rings (usually the top one only or a Dykes) provide longevity, but take longer to break in.