This one way valve fits for 6mm inside diameter fuel hose. Suitable for carburetor and low pressure fuel systems. This is designed to stop fuel siphoning back down the pipe and hold the fuel in the pipe. Designed to pump the fuel by hand but also prevents fuel from returning back to the tank and keeps it in the line, so if you have a vehicle that is hard or slow to start because the fuel is running back through the lines this will solve all your problems, Not suitable for Fuel injection. Package included: 1x Fuel Line Hand Primer Bulb(8mm)