Starter ring gear

A starter ring gear is a part attached to an internal combustion engine that is part of the assembly which transfers the torque from the starter motor to the engine's crankshaft, in order to start the engine. The starter ring gear is usually made from medium carbon steel. 
Starter ring gears are attached to either the flywheel or the flexplate of an engine. The teeth of the ring gear are driven by the smaller gear (known as the pinion) of the starter motor. The pinion engages the starter ring only during starting and once the engine is running the pinion withdraws.
Manufacture.
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. This ring is then machined to create flat surfaces of the required inner and outer diameters. The gear teeth are then created using a hobbing tool, followed by processes to chamfer, deburr and clean the gear teeth. Induction hardening is used to increase the surface hardness of the teeth for abrasion resistance and also increase the core strength of the tooth to resist the bending forces applied to the tooth during starting. This heat-treatment is usually followed by tempering to reduce internal stresses and increase the toughness of the teeth. The hardness at pitch circle diameter is typically in the range 45-55 HRc.
Attachment.
In cars with a manual transmission, the starter ring gear is fitted to the outer diameter of the flywheel. The ring gear is usually fixed to the flywheel through use of an interference fit, which is achieved by heating the ring gear and so that thermal expansion allows it to be placed around the flywheel.
In cars with an automatic transmission, the starter ring gear is usually welded to the outside of the flexplate.