Abstract: A hydraulic actuator including a piston rod having a centering mechanism associated therewith which, upon actuation, will return the piston rod to a predetermined position even against loads and lock it in that position and hold it there even without the presence of hydraulic power. The centering and locking mechanism includes a drive mechanism and cam surfaces on the piston rod which are engaged by drive surfaces on the driving mechanism. The drive and cam surfaces are opposed ramps with one of the ramps on the drive mechanism always disposed opposite one of the ramps on the actuator, such that at any position of the stroke of the actuator piston rod the centering and locking mechanism may be actuated and is operable to return the actuator to the predetermined position and lock it in place. The cam surfaces are infinitely variable as to their position on the piston rod within the limits of the dimensions of the drive mechanism.