Abstract: A rotor indexing system is provided for indexing the massive high inertia hammer-carrying rotor (5) of a hammer mill (1) or the like. The rotor is mounted on a large main shaft (2) which is rotatably driven by a main drive motor (4). A motively powered indexing apparatus is disposed at and in axial alignment with the rear end of the main shaft, and includes a power transmission or coupling (12) comprising a first coupling member (14,56) coaxially mounted to and for rotation with a rotatable output member (the rear end of the main shaft in this embodiment) and fixed against longitudinal axial movement; as well as a second coupling member (15,58) normally spaced from the first coupling member and mounted for longitudinal movement into and out of full coupling power-transmitting engagement with the first coupling member.