Abstract: Closure for openings in the housings or covers of piston pumps or the like to withstand an operating pressure which is in part an alternating load comprising an inner cover, a loadable and releasable pressure-applying member therefor, a loading device for the pressure-applying member, and for the purpose of satisfactorily meeting the requirements of practical operation without critically loading screw-threads, the loading device comprises a compression spring arranged between the pressure-applying member and a backing member, and a biassing device is provided for producing a releasing force acting in the direction opposite to that of the force of the compression spring to initially release the spring force on the pressure-applying member so that the backing member can be finally adjusted into a position to produce the desired compressive force in use.