ir 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.
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.