1372626 Pistons ROBERT BOSCH GmbH 14 Jan 1972 [27 Jan 1971] 1901/72 Heading F2T The piston of a piston and cylinder unit is built up of a fixed part 4 (or 5) and a radially movable part 9 on a rod 3, a seal 17, 18 (or 19, 20) being provided between one end of the movable part and the fixed part to form a zone to which the fluid pressure acting on the other end of the movable part can be conducted. As shown, the packing 10, 11 is carried by a single part 9 which is free to move radially and sealed to end plates 4, 5 by pairs of 0-rings 17, 18 and 19, 20. The cylinder pressure is applied to the space between each pair of seals through ducts 23, 24 to equalize the end pressures on the radially movable part 9. A guide ring 13 is also provided on the part 9. In another embodiment, Fig. 3 (not shown) the guide ring is carried by a separate central plate and the packing rings are carried one on each side of this plate in L- section radially movable parts (30, 31), each part being sealed to the plate by an 0-ring, the space inwardly of the 0-ring being in communication with the cylinder pressure through a port (41, 42) in the adjacent end plate.