Abstract: A plug, such as a vent plug for mounting in a hole in the wall of a machine element, e.g. the end wall of the rotor casing of a pump comprising a canned motor, comprising an outer cup provided with a support flange and an inner cup pressed into the outer cup and comprising a locking device. The locking device is preferably formed as a locking flange, e.g. in the form of part of a thread serving to interlock the plug and the wall. The distance between the locking device and a sealing face of the support flange or of a sealing element connected to the support flange is smaller than the axial displacement of the plug in connection with the engagement of the locking device with the wall.