JEGS Universal Crankcase Evacuation/O2 Fitting welds into your header collector. The 18mm x 1.5 nipple fitting threads into the weld bung. This adapter accepts common 1/2" NPT straight thread check valves like JEGS 555-52211 (sold separately) when used as an Evac fitting. The weld bung also accepts an O2 sensor (for fuel injected applications) or a wide band fitting (for dyno testing and tuning). .