A starter ring gear, sometimes called a starter ring or ring gear, is a medium carbon steel ring with teeth that is fitted on the periphery of a flexplate or flywheel of an internal combustion engine, mostly for automotive applications.
The teeth of the starter ring are driven by the smaller gear (the pinion) of the starter motor. The primary function of the starter ring is to transfer torque from the starter motor pinion to the flywheel or flexplate to rotate the engine to begin the cycle.
The starter ring gear is most commonly made by forming a length of square or rectangular steel bar into a circle and welding the ends together. There then follow various heat-treatment like normalising, tempering and machining operations, including the machining of the teeth and finally a heat-treatment operation. The teeth of the starter ring need to be hardened in order to increase their strength and resist wear. The normal hardness at pitch circle diameter is 45-55 HRC. The hardness at the root of the teeth is about 40 HRC and at 2 mm below root the hardness is 35 Hrc max. The body of the ring is generally left untreated.
Engines with manual transmission usually have a heavy flywheel, typically 5 to 10 kg of cast iron, with the starter ring gear shrunk onto the outside. This is done by heating the ring to around 200 °C to expand the ring which is then placed onto the flywheel, often held in firmly against a location shoulder until cool. The interference fit renders the starter ring firmly attached to the flywheel.
Engines with automatic transmissions instead have a pressed steel plate with the starter ring gear usually welded onto the outside of the plate.