Abstract: A rotatable member to be indexed in a machine is intermittently rotated through specific angular displacements between predesignated indexing positions by an indexing mechanism driven by a hydraulic motor and is positively locked in each indexing position by a positive clutch coupling comprising a movable plate fixed to the rotatable member and a mating stationary plate fixed to a stationary part of the machine, the clutch coupling being engaged and disengaged by a hydraulic cylinder. The hydraulic motor and hydraulic cylinder are controllably and coordinately operated by a common hydraulic system including pipe and tank lines and at least one electromagnetic changeover valve of the two-position, four-connection type.