Valve thread sizes

[kombiguy]

I picked up a 4 steel 72s the other day for a pittance. In the process of cleaning them, I found out that one had previously failed hydro, and the serial number was Xed out. Still a bargain for the rest. My question is really this:

If a 3/4 straight thread valve bottoms out in the neck with about a 1/4 inch or so remaining to be screwed in, is it a safe assumption that the neck of the tank is a 1/2 NPT?

 

[flots am]

I picked up a 4 steel 72s the other day for a pittance. In the process of cleaning them, I found out that one had previously failed hydro, and the serial number was Xed out. Still a bargain for the rest. My question is really this:

If a 3/4 straight thread valve bottoms out in the neck with about a 1/4 inch or so remaining to be screwed in, is it a safe assumption that the neck of the tank is a 1/2 NPT?

There's no such thing as a safe assumption across the Internet.

Take them to a SCUBA shop that has valves and find out.

flots.

 

[Akimbo]

… If a 3/4 straight thread valve bottoms out in the neck with about a 1/4 inch or so remaining to be screwed in, is it a safe assumption that the neck of the tank is a 1/2 NPT?

I’m confused. A current US-standard valve with a 3/4" straight pipe thread and O-ring seal will not even begin to start inside a 1/2" CGA (similar to 1/2" NPT) thread. Likewise, a 1/2" CGA will just drop into a 3/4" straight pipe thread.

You might have one of the relatively rare 7/8" straight thread tanks???

 

[kombiguy]

I’m confused. A current US-standard valve with a 3/4" straight pipe thread and O-ring seal will not even begin to start inside a 1/2" CGA (similar to 1/2" NPT) thread. Likewise, a 1/2" CGA will just drop into a 3/4" straight pipe thread.

You might have one of the relatively rare 7/8" straight thread tanks???

OK, an option I hadn't considered. Since the tank is going to be used as a display piece, it's not a big deal.

 

[kwinter]

The 7/8 neck is not even close to 3/4. It is much smaller so a 3/4 valve wouldn't get started. But European metric M25 cylinder threads are very close to 3/4 and have caused disastrous problems because of the similarity. I'm just not sure there were any old 72s made with M25 threads.


iPhone. iTypo. iApologize.

 

[Akimbo]

… I'm just not sure there were any old 72s made with M25 threads…

Now that you mention it, I’m not sure that any 2250 PSI 72s were made with 7/8" straight threads. There was another post earlier this year that mentioned that Sportsways made a 3/4" straight thread that was not 3/4" straight pipe.

Is one of the tanks or valves a Sportsways brand?

 

[davetowz]

Sounds like thread damage, Take the tank to a shop and have them look at it IMHO.

 

[kombiguy]

I can't make out the manufacturer. The tank has been painted so many times I can't make it out. Since it's already failed hydro, I'm just going to chase the threads, and see if that help. If not, I'll just tap it for the 3/4 valve.
Thanks everyone!

Edit: I just used a wrench and it snugged on down. The internal threads on the neck were pretty rusty, and I think that was the problem. Also, there was no relief for an O-ring that I could see, just sort of a slope down to where the threads started.

Now it's off to my study to hang my Nemrod double hose on!

 

[flots am]

I can't make out the manufacturer. The tank has been painted so many times I can't make it out. Since it's already failed hydro, I'm just going to chase the threads, and see if that help. If not, I'll just tap it for the 3/4 valve.
Thanks everyone!

Seriously?

It failed hydro. It's time to start it's new life as a wind chime, or a ride to the scrap yard and a free McDouble.

 

[KevinNM]

Seriously?

It failed hydro. It's time to start it's new life as a wind chime, or a ride to the scrap yard and a free McDouble.

That's his plan.

Since the tank is going to be used as a display piece, it's not a big deal.