Flywheels, used in vehicles equipped with standard transmissions, serve three purposes. They have teeth on the outside ring of the wheel that the starter uses to crank the engine. They help balance the engine so it runs smoothly and keeps vibrations to a minimum. They also connect the engine to the transmission when the clutch is engaged. There are lots of styles and designs of flywheels, but all of them need to pass the same tests before they are used.
Inspect the teeth of the flywheel. Normal wear is indicated by shiny markings where the starter drive engages with the teeth. This is acceptable wear. Broken, cracked or chipped teeth are not acceptable. Replace the flywheel if this is the case.
Resurface the flywheel if normal wear is indicated on the clutch mating surface. Clutch manufacturers will not honour any warranty if this is not done. A slight deviation from perfectly flat, as little as .005 inches, is enough to cause chattering and jumping when the clutch is engaged.
Discard the flywheel if abnormal wear is indicated. This includes gouging grooves that are worn into the face of the flywheel. Too much metal needs to be removed to bring the flywheel to perfectly flat. Grinding the flywheel flat and reusing it will result in the clutch pedal being too low. Also replace the flywheel if it is peppered with blue streaks. These are hot spots that, even though they are ground away during resurfacing, will only return when the new clutch is put into service. They create uneven grip points and are not worth the risk.
Discard the flywheel if there are cracks around the mounting bolt holes, or surface cracks that spider-web the clutch surface area and appear to be more than surface deep. These cracks are signs of metal fatigue and are not recommended for reuse in performance or truck applications.