A roll over valve fitted in a fuel tank’s vent line. A tank must breathe, or it collapses as fuel is drawn out and pressurises as it warms, but an open vent becomes a fuel outlet the instant the car ends up on its roof. This valve stays open in normal attitudes and closes off if the vehicle inverts, so the vent does its job without becoming a hazard. It is standard fitment on race and rally cars and is specified in the technical regulations of many series, and it is sensible on any road build with a fabricated or relocated tank. Mount it in the correct orientation, as the valve depends on gravity to function.