Closing plug

A lockable gas cap assembly includes an inner hollow shell adapted to be threadedly connected to the filler tube of a vehicle gas tank and an outer shell which coaxially surrounds the inner shell for removably retaining it for normal relative rotation with respect thereto. A two-part, key-operated locking member is likewise releasably secured within the outer sleeve, a front hollow portion of the locking member exhibiting a radial slot that is aligned with an inwardly cusped periphery of an aperture in the inner sleeve when such inner sleeve is likewise retained within the outer sleeve. A rear tumbler portion of the locking member has a forward projection with a recess or flat thereon, the recess being moved into registration with the slot in the hollow body when the key is operated. A follower member slidably carried in the radial slot is engaged with the unrecessed portion of the periphery of the tumbler projection when the key is operated, so that the outer end of such follower engages the cusped surface of the overlying inner sleeve. The restraining effect of the cusp on the follower effectively locks the inner and outer sleeves together for joint rotation. As a result, manual rotation of the outer sleeve will cause the gas cap assembly to be unscrewed from the gas tank filler pipe.