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G3/4 Valve Thread is not 3/4 NPS

Thu Jan 19, 2023 11:02 am

A recent industry memo told of an accident involving an M25x2 threaded valve installed in a G3/4 threaded cylinder. And it recommended taking G3/4 threaded cylinders out of service. I was confused by the G3/4 terminology because most of our cylinders are 3/4" threads... More accurately they are 3/4" NPS threads... Which are not the same as G3/4.

This needs further clarification - I did some reading at Dive Gear Express and BOTH G3/4 and M25x2 are metric threads of Europe and not common in the USA.

G3/4 - An old metric BSPP 3/4" thread used mainly in Europe
- It' not common over there anymore.
- It's not common at all in the USA

3/4 NPS - This is the common USA scuba thread for old and new scuba tanks.
- It's NOT the same as G3/4 (above)
- It's NOT the thread that this memo recommends taking out of service.

M25x2 - This is a metric/European thread
- It's stamped on the cylinder neck

High Pressure Cylinders 3442 and 3500 psi use different valve threading but you can't mistakenly thread one into the other.
- 3442 psi uses USA 3/4 NPS thread.
- 3500 psi uses USA 7/8 UNF thread.
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Re: G3/4 Valve Thread is not 3/4 NPS

Thu Jan 19, 2023 11:34 am

The 3/4 NPS is a pipe thread standard with straight threads (as opposed to the tapered threads in NPT).

The actual thread size for the 3/4 inch pipe is a bit larger that 1" in size. Therefore, the 3/4 NPS male threads are 1.05 inches in diameter.

The danger with mixing metric valves in the US is still an issue. The 25mm valve will fit into a standard US 3/4 NPS cylinder, but it will start off a bit lose and as you threaded it starts to fill like is fitting tighter, like it should fit. But, that is what makes it so dangerous. It fill almost correct, but it is not.

One inch is 25.4 mm. Therefore, the 25mm is just a tiny bit smaller than the 1.05" threads used in the 3/4 NPS pipe threads. The thread pitch is also just a tiny bit off. That is why a metric valve will fit in a US cylinder and when you threaded down far enough it feels like it fits correctly, but it is not fully engaging the threads and they have been known to blow out when the pressure gets above around 2000 psi.

Pipe Size (inches): 3/4
TPI : 14
Pipe Diameter (inches): 1.050
Female Thread Inside Diameter (inches): 0.97
Tap Drill: 15/16 ... _1520.html ... s-threads/

Note: I am not familiar with the G3/4 valve standard. So I have no comments on that subject.

Hi Eben, are one of these the articles the one you are referring to?

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Re: G3/4 Valve Thread is not 3/4 NPS

Thu Jan 19, 2023 4:39 pm

G3/4 usually refers to O2 fittings in Europe and on NATO bottles on the OUTLET side, the LARV (MOD1) series is a good example, the MOD 2A went to a CGA540 valve at the request of the USN. The standard G3/4 valve fitting INTO a cylinder is all over the place and on smaller O2 bottles is usually M18 x 1,5 although there is a tapered thread which uses either tef tape or a special sealing cap that goes over the threads and seals as it's tightened as well as what looks like a 3/4 neck fitting, it is not however. Interestingly enough, NATO also refers to a standard DIN valve as G5/8, which most know. watersafety online has a bunch of info, tecme did at one time but Martin no longer has valves on his website.

I posted a link a while ago about a tank that exploded in a pool session due to the same situation Luis mentions, it's in the "tanks & valves" section of the board. BE VERY CAREFUL whatca do with valves and tanks, there's the right way and the fast way to the grave, you decided which is best for you.

Here's some info from 2014 that is probably more inline with the question at hand, sorry it's in Dutch but has pictures, make sure and look at the 2nd to last picture to see the consequences of being stoopit ... naald.html

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