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 valve from Mocal has a large flow capacity for cars with a high fuel consumption rate. The valve is designed to fit onto the tank itself and can either be bolted from inside the tank or screwed into a fitting which requires welding onto the tank.
The outlet of the valve has a -6 JIC male fitting to allow fitment of a vent hose, the tank connection is AN8 ORB (3/4 UNF)