The CTB has a female inlet and a male outlet. Inlets range from 1/4" to 1-1/2"(FPT). Outlets range from 1/2" to 1-1/2" (MPT). Has optional 1/8" unloader port on hex section. Check valves control the flow of air into the receiver tank. When the compressor reaches unload pressure the check valves closes, preventing the backflow of air from the receiver tank into the discharge supply line. To figure out what size you have, measure the OD of the fitting that fits into the check valve. If it measures 3/4", then you need a 1/2" FPT check valve. (pipe sizes always measure 1/4" over the actual size) Then, measure the OD of the threads on the bottom of the check valve that fits into your tank. Calculate the size the same way subtracting 1/4" from the measurement to determine the correct MPT size.