Flywheels could be made by several materials, including cast iron (gray iron and nodular iron), billet steel, billet aluminum and chromoly.
Flywheel materials vary by both cost and strength. Your specific vehicle and application should dictate which you choose.
A flywheel has several functions:
1. It mounts directly to the engine's crankshaft and has a machined surface to engage with the clutch disc which is sandwiched between the flywheel and clutch pressure plate, providing the critical element for transmitting power to a manual transmission.
2. The flywheel also transfers inertia to the drivetrain, which helps get the vehicle moving from a stop as the clutch is engaged.
3. Additionally, the flywheel has a ring gear around its perimeter, which engages the starter drive gear, and on externally balanced engines, the flywheel contributes to engine rotating assembly balance.
The flywheel experts recommend replacing your flywheel fasteners when installing a new flywheel because your bolt strength should match or exceed the material strength of the flywheel.