A fuel cap is provided for threaded engagement with a vehicle fuel tank inlet in which a locking mechanism employs a key to move a lug from a stable locking position to an unstable unlocking position. The key may be immediately removed and the fuel cap may be threadably disengaged from the inlet and thereafter reengaged therewith once fuel has been introduced into the tank. The key need not be employed to lock the fuel cap, since once manipulation of the fuel cap to threadably engage the cap with the fuel inlet is initiated, the lug is moved from the unstable unlocking position to a stable locking position. The cap may thereafter not be threadably disengaged from the fuel tank inlet unless a key is employed to again shift the lug to the unstable unlocking position. A clutch mechanism is employed to prevent overtightening of the cap and consequent damage to the cap or fuel tank inlet, yet allow sufficient tightening to effectuate a vapor tight seal between the cap and the inlet.