I have never seen a piston ring made from grey iron. Most are not grey at all but black (as is the surface color of nodular cast iron). Or when the cast surface is ground off they are shiny steel , without the gray color of grey cast iron. I suppose you could get away with this on a full size engine, but I don't think the rings would stay intact on our models. Pretty sure our rings are made from nodular meehanite. At least that is the material I would specify if I were the designer. It can be made to be maleable or soft (thus called maleable cast iron), but I think it should be hard but obviously flexable enough to install on the ring.