Abstract: This invention relates to a fluid operated, positioning apparatus for a workholder (43, 144, 146, 148, 150) for maintaining the locating surfaces (22 and 24) of a workpiece, such as a crosshead (2), in contact with fixed abutments (62 and 68) of a workholder (43, 144, 146, 148, 150) designed to receive workpieces (2) subject to substantial manufacturing tolerances in which forces are imparted to each workpiece (2) during machining operations which tend to move the locating surfaces (22 and 24) of the workpiece (2) away from the fixed abutments (62 and 64) of the workholder. The positioning apparatus (44) includes an L-shaped member (46) in which are mounted a movable pin (70) operating in an advanced position to clamp a crosshead against fixed abutments (62 and 64) of the workholder (43, 144, 146, 148, 150) and a retracted position in which the crossheads (2) may be inserted and removed from the workholder (43, 144, 146, 148, 150).