This fitting has an OEM 1/4 inch Schrader connection which is the most common connection size for almost any fuel pressure test kit. - Screw in the adapter fitting and tighten lightly with a wrench to seat the oring - Attach your fuel pressure gauge to the Schrader fitting and finger tighten. - Start the engine and carefully check for leaks - When finished, remove the fuel pressure gauge and adapter fitting and re-install the factory Allen plug.