The piston carriers are in the form of flat discs (7,8) to which the pistons are attached or which are integral with the pistons. The circumferential area (16) of the discs are shaped to match the toroidal shape of the cylinder and form part of the cylinder wall, a part from the area over which the pistons are attached to the discs. The piston carrier discs are sealed against one another and against the cylinder housing by sliding sealing rings (26) or by annular ridges which run in grooves and form labyrinth seals. One piston carrier disc (8) is mounted on a shaft (21), the other disc (7) is mounted on a hollow shaft (18) which fits over the shaft (21) of the other disc.
