Abstract: A fixed displacement fluid motor or pump of the ball or roller piston type in which a plurality of axially or radially extending reciprocating pistons co-act with a cam track to produce output motion or to generate pressure fluid output, respectively. Permanent magnets are secured either to the pistons or in or adjacent the cylinders for holding the pistons in fixed retracted positions beyond one end of the normal stroke of the pistons under certain conditions of relatively low fluid operating pressure in the cylinders which occur in modes of operation during which no motor or pump output is desired.

Abstract: A fixed displacement fluid motor or pump of the ball or roller piston type in which a plurality of axially or radially extending reciprocating pistons coact with a cam track to produce output motion or to generate pressure fluid output, respectively. A mechanical locking device, preferably in the form of an expansible retaining ring, is secured to each piston, being adapted to lock the piston in a fixed retracted position and out of contact with the cam in a tapered portion of the cylinder bore in modes of operation during which no motor or pump output is required.