Abstract: A stop system intended to engage with a tool for heating by direct contact for a machine for plastic welding of a component over an opening in a wall of a motor vehicle tank. The system contains a cam mechanism which includes a straight cam with a longitudinal axis and a follower coupled to the straight cam for converting a translational movement along the longitudinal axis of the follower into a straight or quasi-straight movement of a point of the follower in a direction non-parallel to the longitudinal axis. The cam mechanism of the follower is capable of moving from a rest position, where the stop system is configured to pass freely through an opening in a wall of a motor vehicle tank, to a working position. The stop system is configured to bear against an inner surface of the wall of the tank through which the straight cam passes.