Cylinder Neck Threads and SCUBA Valves
The first SCUBA cylinders originally introduced in the early days of the sport were steel and used a 1/2-inch National Gas Taper Thread form, often referred to as "1/2 NGT." The cylinder neck does not use an o-ring seal with the valve, instead the tapered valve threads are wrapped with Teflon or PTFE pipe tape before insertion. These cylinders typically have relatively low service pressures of 2015, 2216 or 2250 psi. Keep in mind that these very early SCUBA cylinders, typically from the late 1950's and early 1960's (sold under the names US Divers, Voit, and Healthways), are very uncommon.
Today, the most common thread form used with SCUBA cylinders and valves is National Pipe Straight also known as NPS. The specific thread form used with most SCUBA cylinders is named 0.750-14 NPSM, often referred to as "3/4 NPS." This thread form is seen on both steel and aluminum SCUBA cylinders with service pressures of 2400+ (2640), 3000, 3180+ (3498), 3300 and 3442 psi. The current United States Department of Transportation rules prohibit the transport of metal scuba cylinders on public roads with pressures above 3442 psi (230 bar), even if the cylinders and gas delivery systems have been rated for higher pressures.
When Pressed Steel Tank Company began manufacturing their high-density steel 3500 psi service pressure SCUBA cylinder in 1987, they used a more modern thread form for the neck opening known as Unified National Standard Fine. The specific thread form used with the steel 3500 psi SCUBA cylinder is named 0.875-14 UNF, often referred to as "7/8 UNF."
It's very important to understand that the 3/4 and 7/8 trade descriptions do NOT indicate relative sizes of the physical dimensions of the opening! This is because the two thread forms are from two different standards, and the names used to describe them refer to only the trade size and schedule. The 3/4 NPS opening is visibly larger than the 7/8 UNF opening.
Until recently, it worked as follows: "High-Pressure" steel SCUBA cylinders (i.e., 3500 psi, sometimes referred to as "Sherwood Genesis" cylinders... the brandname they were originally sold under) were manufactured with the smaller 7/8 UNF threaded openings, and all other modern steel and aluminum SCUBA Tanks were manufactured with larger 3/4 NPS threaded openings. Then in 2003, Pressed Steel Tank Company introduced their new high-pressure 3442 psi E-series cylinders, and they reverted to the 3/4 NPS threaded opening. This was followed by similar designs from Faber and Worthington. So now there are two different high-pressure steel SCUBA Tanks in wide use, the older 3500 psi cylinders with 7/8 UNF threaded opening and the newer 3442 psi cylinders with 3/4 NPS threaded opening.
Luxfer recently introduced a limited production run of 106 cubic foot capacity SCUBA cylinders that are a composite design: aluminum shell with fiberglass hoop wraps. This cylinder has a service pressure of 4350 psi and 7/8 UNF neck threads.
So how do you figure out what you have? Assuming your cylinder was manufactured exclusively for the North American SCUBA market in the last 50 years or so, look at the SCUBA Tank neck for the DOT stamp. If it reads DOT-3AL or DOT-3AA it almost certainly has a 3/4 NPS threaded neck opening. (3AL means aluminum, 3AA means steel.) If it reads DOT-Xxxxx-3442 it's a steel cylinder, also with 3/4 NPS threaded opening. If it reads DOT-Xxxxx-3500 or DOT-Xxxxx-4350, it's a steel or composite cylinder with a 7/8 UNF threaded opening. (The Xxxxx is a manufacturer specific four or five digit exemption number that starts with the letters E or SP.) 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.
Currently, SCUBA valves with 3/4 NPS threads are widely available from a variety of different manufacturers. However, because most cylinders with 7/8 UNF neck threads or 1/2 NGT neck threads have not been produced in recent years, finding replacement valves for these less common thread sizes can be a challenge. Currently, XS Scuba is still manufacturing some 7/8 UNF valves with 300 BAR only outlets and 1/2 NGT pillar valves with 200 BAR outlets that will accept the "K" insert.
In very rare circumstances, we see very small cylinders intended primarily for the European market but also qualified for the North American market in specialty inflation or rebreather diving applications. If your cylinder is stamped "M18x1.5" or "M25x2" then the threads are metric and not compatible with 7/8 UNF or 3/4 NPS imperial thread valves. By the way, if it's not a SCUBA tank, all bets are off; it will probably have some other neck opening threading which is incompatible with SCUBA valves.
