Fuel tank breather valves do as the name suggests, they allow the fuel tank to 'breathe' should it become pressurised or subject to vacuum. Should fuel try to escape in the event of an accident the valve shuts off to prevent spillage.This alloy valve has a medium flow capacity for cars with a moderate fuel consumption rate. The valve has a 7/16 UNF (4 JIC) thread and is designed to fit onto the tank itself, being secured from inside the tank with the supplied lock nut. The outlet of the valve has a barbed male fitting at 90 degrees to the body to allow fitment of a 4.75mm bore vent hose.Please note: Access to the inside of the tank is required for installation. The valve should be positioned as close as possible to the rear of the tank so that under braking the fuel load will move towards the front of the tank and away from the valve.