While piston rings can be made from a variety of metals including steel, bronze, and variations of cast iron, the old accepted stand-by is regular old class 30 grey iron.

A friend sent me a Webra Heli engine after he had gotten the ring gap caught in the exhaust port. I lifted the end of the ring with a scribe and pulled the ring out with a pair of needle nose pliers. It came out like it was a paper clip??? Very soft? It didn't appear to have hurt the cylinder or piston so I put a new Bowman ring in it.