1/2" to 3/4" flare connection for tubing and 1/2" to 3/4" male threads (MPT) for the tank. Has optional 1/8" hole on hex section for connecting to unloader valve on pressure switch. 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 determine the correct size, measure the outside diameter of the top of the check valve threads and subtract 1/4". (pipe sizes always measure 1/4" over the actual size). To determine the correct size that fits into your tank, measure the outside diameter of the threads and subtract 1/4" again. For example, if both threads measure 3/4", then you need a 1/2" x 1/2" check valve or a CTF1212.