Gone for diving
Contributor
I assumed he wants to tap the neck of the tank and chase the theads
Not totally sure what the din threads are but maybe that's what was meant.. I assume they are metric though...
Today, the most common thread form used in the US with SCUBA cylinders and valves is National Pipe Straight also known as NPS. The specific thread form used with most SCUBA cylinders in the US is named 0.750-14 NPSM, often referred to as "3/4 NPS." This thread form is seen on both steel and aluminum modern SCUBA cylinders with service pressures of 2400+ (2640), 3000, 3180+ (3498), 3300 and 3442 psi, and is sealed with a size 214 O-ring. However, when the now defunct Pressed Steel Tank Company began manufacturing their high-density steel 3500 psi service pressure SCUBA cylinder in 1987 (sometimes referred to as "Sherwood Genesis" cylinders... the brandname they were originally sold under) , they used a 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" and is sealed with a size 116 O-ring. Some similar 3500 psi steel tanks manufactured by Asahi were also imported to the US from Japan.
In 2003, PST introduced their modern 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 use, the long discontinued 3500 psi cylinders with 7/8 UNF threaded opening and the current 3442 psi cylinders with 3/4 NPS threaded opening.
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. 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.
So how do you figure out what high pressure cylinder 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-Xxxxx-3442 it's a cylinder with the common 3/4 NPS threaded opening. If it reads DOT-Xxxxx-3500 it's a cylinder with the relatively rare 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.) If it's neither 3442 or 3500, then it's something other than a high pressure steel scuba cylinder. For more information see our detailed article titled
