Scuba Tank Bushings. . .

[sea.explorer]

I am looking for a stock bushing that will reduce from a 3/4" thread to a 1/2 tapered thread. Here is what I think I understand

The 1/2 inch thread is 1/2 NPT
The 3/4 inch thread is 3/4 NPTF

With tapered threads is the female thread tapered as well?
The reason I ask is that I found this bushing that is said to be compatible with both?

Here are the specs from McMAster-Carr:

www.mcmaster.com

Part Number: 50925K358 $2.60 Each

Bushing Type
Hex Bushing (Male x Female)
Pipe Size
3/4" reduced to 1/2"
Pipe to Pipe Connection
NPTF x NPTF
Finish
Galvanized Steel
Zinc Chromate-Plated Steel
Maximum Pressure (psi) 4000
Specifications Met American Society for Testing and Materials (ASTM)
ASTM Specification ASTM A108, Type 12L14
Note: Compatible with NPT Threaded fittings

Will this work? Thanks in advance for the information.
-Ryan

[Captain]

I don't believe you will find an off the shelf bushing that will work. The 1/2" is tapered but the 3/4" needs to be straight thread with a shoulder for the O ring to seal on and a pipe bushing is tapered at both threads and the 3/4 will never seal. I have had them make using a stainless steel 1/2" to 1" pipe bushing. The 1" needs to be turn down and threaded to the 3/4 straight. A fairly simple job for anyone with a metal lathe. Just match the dimensions from a 3/4" tank valve.

www.cutting-tool-supply.com/TechTips/Tapping/NPTvsNPTF/NPTVsNPTF.htm

[sea.explorer]

Thanks -Ryan

[nemrod]

Tom sent me three of these and I have them installed on my Nemrod triple manifold rig which is a work in progress. In any case, they had been turned down as he described and fit well. I used a pipe 1/2 tape to make all three equal so that the manifold set evenly on them. Since the tanks can be removed on the 3/4 thread for VIP there is no need to remove the 1/2 pipe--ever.

James

[Captain]

When are we going to see those triples. I thought I would see them at Portage.

[nemrod]

Well, uh, I got sidetracked. There are numerous isssues with triples, the main one is a lack of suitable tanks. The other is the potential compatibility problem of mixing low and high pressure components.

You know, I have asked about, even asked on the forums and have not been able to determine exactly what tanks were used with these manifolds to begin with? What tanks did they use with them? The tanks will have to be on 7 inch centers so they did not use triple 6.9 inch diameter tanks.

James

[duckbill]

Aren't the USD 30s commonly used on triples narrower than 72s?

[broxton]

I was looking through a couple of nemrod catalogs from the early 1960's and they were selling a triple set up for 72's. But in the section for their valves, they mention that they have interchangeable arms in 5 different sizes for their triple manifold.

[nemrod]

Ohhhhhhhhhhhh--interchangeable arms. So that is why the Nemrod valve is designed modular unlike the USD triple. Well, mine are on 7 inch centers, I guess I could machine different lengths as well. I beleive my valve was intended for a 30 to 40 cf tanks of approx 5 inch diameter. Modern 30s (Luxfer) would work but I would downrate the pressure to 2500 psi thus resulting in a 75 cf rig. Not a problemreally since small doubles and triples were not to carry more air but instead to achieve better balance and bouyancy. The 30 cf Luxfers have perfect bouyancy for no BC diving in a small double or triple. But what I really want are steel 1800/2000 psi 30s. I found some but unfortuantely they were terribly rusted on the inside. They were 5.1 inch diameter, the Luxfers are about 4.9 I think and shorter by an inch or so.

James

[nemrod]

Note the "modular" arms, they can be full removed from both ends as they are O ring sealed unlike the USD triple. I can post pics of the bushings if you like? Note also the 1/2 pipe stems--cool.

James

[duckbill]

Quick question, James-
What keeps the connecting tube from pulling through the threaded cap/bushing?

[nemrod]

Look at the open end just above the threaded part and the O-ring. See that hex nut. That nut can be threaded in and out to adjust the C/C spacing and snug the assembly down once set. The "arm" is threaded of course and that threads into the body, the aforementioned nut then locks the adjustment and snugs up the assembly. In the pic above I have the lock nut threaded all the way in exposing the threaded arm fully. The outboard is different, there the hex nut is actually barrel nut that threads into the body and the arm has a flange on it. The outboard ends do not seem to be readily adjustable in the same manner as the centers. This makes sense becaue they are not trying to provide a large adjustment range for different tank sizes but instead to allow perfect center/center adjustments for the minor variations in tank sizes and alignment.

James

[duckbill]

O.K. Thankyou. I thought the hex and the threads were all one unit. Now I get it.

[pescador775]

I made a set of triples using the Nemrod manifold and triple 35 aluminum with 1 inch NPS thread. I machined the bushings from brass. The manifold arms were shortened. The brackets were custom made. You might still be able to see pics on Scubaboard, however, I sold the triple set a few months ago. The Nemrod manifold is very rugged and can easily handle 3000 psi. The correct disk should be installed. There is one issue which users should be aware of. The manifold will leak or blow air from the regulator hook up unless the correct O rings are used. The correct ring for the arms is 7.0 X 2.0 mm, 70 duro. The correct O ring for the face is 11.5 X 2.5 mm, 90 duro. In a pinch, the latter O ring for the face can be obtained from a Poseidon dealer, part # 1007.

[nemrod]

You probably had the arms that were for 6.9 inch tanks, mine is way to narrow for 6.9s so I will not have to shorten the arms. The O ring info is usefull thanks, you mean to tell me my Harbor Freight O ring kit won't do?

James