Filler cap for a motor-vehicle fuel tank

The locking system locks the filling neck (2) of the fuel tank by means of a locking cap (1). A tube stub projects from the cap into the filling neck. There is a protective tube sector (11) in the filler neck, fixed to it at one end, for the sealing surface of the filling neck. This, together with the filling neck, forma a receiving chamber (13) for the tube stub of the locking cap.