Abstract: A piston-and-cylinder type actuator, for use particularly in steplessly-variable-ratio transmission units of the toroidal race rolling friction type. In such units the actuator may serve to thrust the input and output members and their intermediate rollers into engagement, so that drive will be transmitted. According to the invention enhanced thrust is obtained by interposing auxiliary pistons (6, 7) in the space between the main piston (1) and the end (4) of the cylinder (2). One of pistons 6, 7 is linked to the main piston 1 and the other to cylinder 2, and they divide the said space into two pressurized zones (16, 18) with a vented zone between them. Where the actuator has to rotate about the cylinder axis, unwanted centrifugal forces generated within zones 16, 18 may be diminished by forming the vented zone so that a quantity of fluid may accumulate in it.