Abstract: A fluid motor comprising a piston and cylinder combination includes mechanism for locking an output shaft which is helically displaceable along its longitudinal axis upon actuation of the piston. The locking mechanism includes a locking element driven by a fluid operated motor for displacement in a direction transverse to and spaced from the longitudinal axis of the shaft, and a notch in the shaft for receiving the locking element, the notch and locking element have mating surfaces which are inclined with respect to the longitudinal axis of the shaft and transverse to the shaft's helical path of travel. The locking element motor comprises the piston and cylinder chamber defined within a body upon which there is also mounted a cam guide for guiding a cam member which is secured to the shaft on the opposite side of the notch. The locking element is secured to and extends transversely of the locking piston into an opening within the body adjacent the shaft.