Gauge adaptor fittings have 1/4" BSPP female threads to mount a standard Pressure Gauge. Male adaptor has a 5/16" O.D. tubing stub that can be inserted into a compression nut on any standard compression-type fitting. Compression fittings are designed to secure heavy-wall tubing using a bite-type ferrule and compression nut. These fittings accept our standard 5/16" O.D. tubing (cut off straight and deburred) as well as Fixture Hose and Heavy-Duty Hose with compression-type tube ends.