SCUBA Valves: Regulator Fittings, 200 vs 300 Bar FAQ and Cylinder Neck Threads - Dive Gear Express

SCUBA Valves: Regulator Fittings, 200 vs 300 Bar FAQ and Cylinder Neck Threads - Dive Gear Express: "This article explains some of the terminology regarding the SCUBA valve used in SCUBA cylinders. It reviews how a SCUBA regulator mates with the valve, as well as how the valve mates with the SCUBA cylinder."


So how do you figure out what you have? Look at the SCUBA Tank neck for the DOT stamp. If it reads DOT-3AL or DOT-3AA it has a 3/4 NPS threaded neck opening. (3AL means aluminum, 3AA means steel.) If it reads DOT-Exxxx-3442 it's a steel cylinder, also with 3/4 NPS threaded opening. If it reads DOT-Exxxx-3500, it's a steel cylinder with a 7/8 UNF threaded opening. (The xxxx is a manufacturer specific four or five digit exemption number.) Another test, if the valve is out of the cylinder, is that a U.S. quarter coin will fit through the 3/4 NPS cylinder neck opening but will not fit through the 7/8 UNF opening. By the way, if it's not a SCUBA Tank, all bets are off; it will have some other neck opening threading which is incompatible with 3/4 NPS and 7/8 UNF.