In a fluid pump or motor, particularly one of the radial piston type, a main casing 21 is screwed to an end cover 22 by a mating helical screw formation using a taper plane 23 which is at an angle α of less than 45° to the axis m. An external thread in the boss 26 of the cover 22 is screwed into the internal thread of the part 21 a until surfaces 21 b and 25 are engaged to determine the axial position.