Cast Iron (soft) flywheels need to be ground with harder stones. Experiment to find the best stone. A starting point is the FGW-57, our best all-around stone for Cast Iron.
Cast Steel (medium) flywheels require medium-hard stones. The FGW-65 is our best all-around stone for Cast Steel.
Billet Steel (very hard) flywheels need to be ground with softer stones. Our best all-around stone for Billet Steel is the FGW-50-2. It's our most friable and free-cutting, requiring minimal dressing.