A water ring vacuum pump having a body, end members on the body and an impeller. Each port plate is moveable relative to its adjacent end member by screws or the like passing through the end members. The screws abut the face of the port plate and can act to cause its location against the impeller. Locking means pass through the end members and into threads in the port plate whereby the port plate can be moved away from the impeller. The locking means, when the impeller is correctly positioned, act against the screws or the like thus serving to lock the port plate in position. |