A filler neck an actuation ring (16) having a slot (18) and an entrance portion which is conically narrowed towards the tank, the most narrow cross-section of the entrance portion being smaller than the diameter of a diesel-nozzle and larger than that of an otto-nozzle the actuation ring (16) has an actuation portion at the end facing the tank at least on one side of the slot a closing mechanism is associated with the end of the actuation ring facing the tank and designed such that in closing position thereof it stops the otto nozzle inserted into the actuation ring and the actuation portion of the actuation ring engages the closing mechanism, whereby the closing mechanism is moved from the closing position into an opened position by the actuation portion if the actuation ring is radially expanded by the diesel-nozzle inserted into the actuation ring.