1/2 male npt by 3/8 female npt air compressor in tank check valve, with 1/8 female npt threads on the side for un-loader port. Check valves are used to prevent backflow of air. This style is used to directly screw into the air tank and keep pressure from leaving the tank and putting pressure against the compressor pump once the compressor is shut off. This item is made in the United States by Conrader Specifications: Air inlet threads: 3/8 female npt which measure about 18/32 in diameter Outlet threads: _male npt which measure about 26/32 in diameter these threads are the male threads that screw into the air tank. Side un-loader port: 1/8 female npt threads, which measure about 10/32 in diameter. This port is used for a un-loader line, some compressors do not have this if yours does not you would just plug this port.