A threaded cap for sealing an automotive vehicle fuel tank is provided with a unitary molded central plastic core and a bonnet rotatably mounted on the plastic core. A pair of concentric races are defined on the plastic core. The inner race carries a unitary molded plastic frame which has a pair of resilient, closely spaced transverse cantilevered arms defining a gap therebetween. The frame is also provided with a resilient catch. A unitary molded plastic bolt is reciprocally movable along the gap to engage and disengage the core for rotation with the bonnet. The bolt has two detents which coact with the catch on the frame to hold the bolt at selected unlocking and locking dispositions of movement. The bolt includes a stud which projects into the gap between the arms so that the arms spring bias the bolt toward the locking disposition. A lock mechanism is used to shift the bolt to an unlocking disposition so that the fuel cap be disengaged from the fuel tank inlet pipe. Counter-rotation of the bonnet returns the bolt to a locking position.