Workpiece holding device

A piston holding device in which a conical register (16) is provided inside the skirt of a piston (2) and is engaged by a conical locating surface (15) on a tailstock (3). The tailstock (3) may also include a ram (17) which is adapted to push on the inside of a piston crown (11) and force the piston (2) against a headstock (1). The holding device is used during piston machining.