An obturating assembly includes a body having an aperture for passage to the pipe of a fuel supply spout of predetermined outer diameter and an obturator moveable with respect to the body, able to assume an obturating position in which it conceals the passage aperture and a freeing position in which it does not conceal it, the obturator being driven towards the freeing position by that spout when it meets the obturator during a thrusting movement of the spout into the head. In accordance with the invention, the passage aperture is provided in an oblique wall or wall portion, with the angular offset between the orientation of said obturator in the obturating position and the transverse orientation being at least equal to 20°.