Hirth joint

Hirth joints consist of radial teeth formed by grooves milled or ground into the end face of a cylindrical shaft.

The centre of a shaft is not toothed, as this would add little torque capacity to the coupling and the increasingly narrow teeth would become impractical to cut.

Hirth joints are designed as mating pairs and, unlike splines, there is no standardised off-the-shelf sizing system for them.

A large number of cylinders and often the need for reduction gears and supercharger drives made it impractical to manufacture a one-piece crankshaft, so Hirth joints were used to couple the sections.

Specialized's "S-Works" top-level racing mountain bike cranksets use a Hirth joint with a center hex-head threaded rod to secure the crank halves together.