Flywheel Ring Gear

The flywheel ring gear (or starter ring gear) is a band of hardened steel gear teeth shrunk-fit or welded onto the outer circumference of the engine's flywheel (on manual transmission cars) or flexplate (on automatic transmission cars). It provides the mechanical interface for the starter motor to crank and start the engine.

When you turn the key or press the start button, the starter solenoid pushes the starter motor's small pinion gear forward to mesh with the teeth of the flywheel ring gear. The starter motor then spins, rotating the flywheel and cranking the engine. Once the engine starts and runs on its own, the starter pinion retracts to prevent the running engine from over-spinning the starter motor.

Because of the heavy impact forces when the starter pinion engages, the teeth on the ring gear can wear down, chip, or break. Broken teeth usually occur in specific spots because engines tend to stop rotating at the same few positions due to compression cycles. Symptoms of a damaged ring gear include a loud grinding noise when attempting to start the vehicle, or the starter spinning freely without cranking the engine.

Replacing a damaged flywheel ring gear requires removing the transmission to access the flywheel or flexplate. In many cases, the entire flywheel or flexplate assembly is replaced rather than just the ring gear band.