Abstract: A motor (20) is driven by pressure medium delivered from an external pressure source. A rotor part (28) forms sealing abutment against a local area (2Id) of the motor's (20) cylindrical inner wall (21c). A piston-forming plate part (30) is pivot-mounted at one end to the rotor part (28) and can be pivoted forwards and backwards in the motor casing's working chamber (21) relative to the rotor part (28). At the same time at its opposite end the piston-forming plate part (30) forms sealing abutment along the motor casing's (26) circular inner wall (21c).