Source: http://www.divegearexpress.com/libra....shtmlCylinder
Neck Threads and SCUBA Valves
Valves attach to the neck of the SCUBA cylinder using one of two types of straight screw threads. Screw threads date back to the third century. For a few centuries everyone did their own thing regarding screw threads. As you might imagine, by the nineteenth century, interchangeability was non-existent. With the first World War, the American National thread form was established so that war materials could be more easily manufactured. With the second World War, the American National thread form had interchangeability problems with the Whitworth form of Great Britain. Thus the United States and Great Britain (and Canada) agreed upon a Unified thread form that would allow better interchangeability. The point of this history lesson is there are TWO different thread form standards.
Still, the most common thread form used with SCUBA cylinders and valves is the American National Standard Free-Fitting Straight Mechanical Pipe Thread also known as NPS. The specific thread form used with most SCUBA cylinders is named 0.750-14 NPS, often referred to as "3/4 NPS."
When Pressed Steel Tank Company began manufacturing the modern 3500 psi service pressure SCUBA cylinder, they elected to use 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 slang 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. 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 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 elected to revert 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.
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.
One important caution regarding thread compatibility. Outside the United States, particularly in Europe, a widely used SCUBA Tank neck thread form is known as M25. The 3/4 NPS and M25 threads are very similar but not identical; it's possible to inadvertently mate cylinders and valves with these two different forms. In an incident in Australia, when the cylinder and valve with mismatched threads was filled to working pressure, the different threads did not hold, causing them to separate with dangerously explosive force.
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Originally Posted by Slüdge
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