All Brass Replacement In Tank Check Valve for Quincy 111491 3/4" Compression x 3/4 MNPT With Unloader Port. Air inlet threads: 3/4" Compression Which Measures 16/32" in Linear Inches. Outlet threads: 3/4” male NPT which measure about 1 1/32” in diameter. Side Unloader Port: 1/8” Female NPT Threads Which Measures 5/16” in Linear Inches. 3/4" Male NPT by 3/4” Compression Thread Air Compressor In Tank Check Valve, with 1/8” Female NPT Threads On the Side for Unloader 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.