Abstract: A fluid pressure actuated system for axially reciprocating an element while the element is being driven in rotation. A piston is mounted within a piston chamber in a rotary shaft. The rod side of the piston head is continuously exposed to fluid under pressure supplied by a pump. A pilot valve is mounted in a chamber within the piston and is axially movable relative to the piston between opposite end limits. The valve cooperates with a ported sleeve which defines the valve chamber to apply pressure from the pump to the valve in a manner which shifts the valve from one of its end limits to the other each time the piston arrives at an end of its stroke. Shifting of the valve in turn controls the supply of fluid under pressure to the piston to cause reversal of the piston stroke. Both piston and valve are biased in one direction by fluid pressure at all times, movement of the piston and valve against the biasing pressure being accomplished by applying an overcoming fluid pressure in the opposite direction.