1/2" to 3/4" Adapter Question's

[captain]

I is against the DOT regulation to modify a tank and rethreading to a different thread is modifying it.

 

[widget]

He got the valve off and tried to thread a scuba valve in to see if it would fit, but it was a 3/4 tapered thread, not a straight thread. His question to me was, if he has it re-threaded to accept 3/4 straight thread can he use it for scuba?
SO I guess my next question is then, would that be safe to do? having the tank re-threaded I mean for use with a scuba valve? Obviously a new hydro/vis would be in order as well as a cleaning.

It depends, if the cylinder is designed for O RING sealing or as in most cases with taper valves is sealed with some kind of "Thread Tape".

Straight 3/4" SCUBA valves all seal with an O-Ring which requires the cylinder to be machined to accept one, cylinders that are thread tape sealed do not have a machined recess for the O Ring on a straight valve so you would certainly end up with leaks.

If the taper valve was designed to seal with an O RING (and some are - although NOT ALL) re threading the cylinder may be feasible providing there is sufficient material in the cylinder neck - the Legality of the operation is another issue entirely.!

Its also unlikely that you could machine a recess into a cylinder that was not originally manufactured for one as the neck is designed differantly and usually there is insufficient material to machine the recess.

It souns like a lot of work to me, why not just get a 3/4" scuba valve with a taper thread?
They are certainly available from any valve manufacturer (note here - there is a world of differance between a valve manufacturer and a valve brand) valve brands usually are only available in threads commonto the cylinders they sell, a valve manufacturer will make valves in any thread the customer requires.

Quite honestly I think it is impractical and could be dangerous to remachine cylinders, rather source a scuba valve in the correct valve thread to fit the cylinder and sleep easy.

 

[pescador775]

Widget is right, the idea is nuts. Having said that, years ago, we used to cut off the necks of Navy life raft CO2 bottles which had an external thread. Then a 1/2 inch tap was run into the bottle. One of these failed hydro when the weakened neck began spitting water. We used the 40 cf tanks to fabricated triple sets. I still have a few of the modified tanks but decided it was not worth the risk anymore. Back in the day, I dove those things up and down the Keys and parts east.

 

[Paintballpsyco2369]

Well, that friend of mine found a hell of a deal on a hp 120, then got 2 more hp 120's and is using those 2 as doubles, then 2 days ago at breakfast he was telling me he got ANOTHER 120, so he has decided to scrap the whole idea and that tank now sits in his scrap pile. HOWEVER, I may take over the project. After reading the posts here, and doing some looking on my own I agree completely that the tank should not be modified in any way. Having just said what I said now, what I am about to ask seems somewhat hypocritical. Assuming the tank does have a o-ring seat; OH when he tried to thread a normal valve into the tank it made it halfway down.; anyways, with the seat, and that the valve threads halfway without modification, would it be feasible to "re-tap" the threads to a straight 3/4?

I know a few of you would still say no mods period, but could that even work? If not, is there such a thing as a 3/4 tapered scuba valve that could be used with an o-ring, or one for use without an o-ring?

One more thing here, how feasible would it be to modify the valve to a 3/4 tapered?

I know a lot of the things I ask here seem like a lot of semi pointless hard work kinds of things, but with school, and the crappy hours I am getting at work, I have a lot of free time to do these little projects and all. Thanks again for all of your input, I hope to hear back soon.

Paint.

 

[captain]

What you probably have is a tank with M25 x2.0 metric threads, when you screw in the 3/4" inch valve it goes part way until the the thread mismatch stops it. The answer to both question is no you can't re-thread the tank or valve.

 

[pescador775]

It is not metric. M25 is larger than a 3/4 taper or straight thread, O ring. This subject is tricky and potentially dangerous to entertain. Paintball is posing questions which are hypothetical and disorganized. A straight threaded valve does not use the same thread profile as a gas thread, it is a machine thread and cannot be modified to taper fit. However, it is possible to fit a 3/4 valve to a SMALLER tank neck like 1/2 inch taper or 7/8 machine, O ring. It requires the intervention of a SKILLED machinist. The cylinder neck should not be modified under any circumstance short of life or death.

 

[captain]

I think we need to know the tank manufacturer and pressure rating to figure out what he has. M25 is .94465" thread but there is a M28 x 2.0 that 3/4 straight might screw in part way. 3/4 straight is 1.032.

http://www.ring-plug-thread-gages.com/PDChart/metric_plugs.htm

 

[pescador775]

Thanks, I got it backwards. 3/4 pipe is 26.7 mm dia.

 

[Paintballpsyco2369]

I still need to get the tank from the guy, but when I do get it, I will post the info and maybe some pics. AS I did say, this is all hypothetical at this point. To see if there even would be anything I can do with the tank. I am hoping that I can find a scuba valve that will work in it so that I would not have had to modify it, if it was even feasible to do so. I will try to get the tank this week and go from there. Thanks again all.

Paint

 

[james croft]

There are hundreds of 1/2 tapered pipe thread steel 72 cylinders on ebay. Buy a couple. I have bought them as cheap as five bucks a tank. They have good bouyancy characteristics and make nice vintage doubles. Don't make this thing harder than it is.