A self-tightening retaining system for securing a closure in position closing an access port of a pump housing. The system includes a retaining cover for obstructing removal of the closure from a bore of the access port. The cover has external threads interengageable with screw threads of the bore. A locking device is configured for being secured to the cover such that the locking device rotates together with the cover. The locking device has a fastener receivable in a hole of the closure and has external threads interengageable with threads of the hole. The threads of the cover and the threads of the fastener are spiraled in opposite directions such that when the cover rotates in a loosening direction, the fastener becomes tightened in the hole of the closure thereby stopping the rotation.