Kyosho IS118 10T Hard Drive Bevel Gear ST-RR Evo

A 10 tooth hard specification drive bevel gear for Kyosho's Inferno ST-RR Evo truggy differentials.

This is the small pinion bevel that drives the large ring gear inside the diff, and together the pair transfer power from the centre driveline out to that axle. Being the smaller of the two, it turns faster and its teeth engage more often per revolution of the axle, which means it generally wears before the ring gear does.

The hard specification is a heat treated grade, and on a truggy it earns its place. These cars are heavier than buggies and carry more rotating mass in their wheels, so the shock loading through the diff gears when a tire regains traction after a jump is substantially higher.

Wear is progressive rather than sudden. The tooth faces polish, then pit, then round off, and the diff gets noisier under load until teeth strip. Because the pair wear into each other, replacing this against a worn ring gear just destroys the new part.

It mates with the 43 tooth ring gear for a 4.3:1 ratio. Mesh is set with shims and matters as much as the gears themselves.