The confusion about large conical threads, small conical threads, DIN, ISO and Euronorm EN-144 and why you should know that …

With the plan to buy the last part of a complete equipment: a double 7 cylinder, I had to realize how confusing the specifications are.

Actually, as a “normal” scuba diver, you don’t have to deal with the cylinder threads, but in the last time, the accidents during the filling of diving cylinders with small conical threads increased, so that now many diving stores refuse to fill these further. For them, there is danger to life and limb when filling.

Here I will now try to describe the different cylinder threads and terms around it:

The cylinder thread M25x2 ISO
The bottle tapers at the upper part of the pressure cylinder. This is the part into which the valve is screwed. As a rule, and this is now the most common format, there is an M25x2 ISO thread right here.

Decisive now, whether it is a DIN 477-6 version 1 or DIN 477-6 version 2 or even the new EN 144-1 are the following differences:

DIN 477-6 version 1: chamfer in the upper part of the cylinder thread and a slightly chamfered valve
DIN 477-6 version 2: chamfer in the upper part of the cylinder thread and a flat valve
EN-144-1: no more chamfer in the upper part of the bottle thread, but a groove for the O-ring and a flat valve.
The standards are not enough, because the EN-144-3 for Nitrox cylinders is completely new. On the one hand, it prescribes a higher material purity, and on the other hand, it brings with it an M26x2 thread. Both DIN standards are still approved until 2010, the current standard is EN-144 (Euronorm).

The conical bottle thread
Basically, conical threads do not have a constant diameter. In the case of cylinder valves, they become narrower from top to bottom. Due to this effect, it would be possible to screw a conical bottle valve into a standard bottle thread and to seal it with Teflon tape. However, we strongly advise against this. On the one hand, the thread can be damaged or even burst if the valve is screwed in too tightly, and on the other hand, the thread only fits over a very short area. A distinction is made between 2 different formats:

For small conical 19.8×1/14 bottle threads.
For large conical 28.8×1/14 cylinder threads
Other terms that often appear in diving cylinders and cause confusion are related to the part of the cylinder valve to which the first stage is connected:

The thread to which the first stage of a DIN automatic is connected is G5/8″ (5/8 inch). In German-speaking countries in most parts of Europe, the DIN first stage connection is the most common. INT first stage connections come from the USA. The new bottles today almost all have the DIN connection as basic equipment, so with an adapter that is turned into the DIN connection, any bottle can be converted to INT in seconds. There is also a corresponding adapter for the DIN 1st stage.

So if you want your own diving bottles to be filled by the diving stores in the future, you should pay attention to M25x2 bottle threads with non-conical valves that comply with the DIN or Euronorm.

Translated with www.DeepL.com/Translator (free version)