1/2 Taper Thread Tanks

[gj1963]

Greetings,
Been reading through old threads trying to find out as much as I can... Looking for a little more info.

The tank that came with my Nemrod Snark III is steel with 1/2 inch taper threads and appears to be part of its original package setup (tank, valve, backpack, regulator). First dive shop said it looked good inside, but no way was it worth bothering with a tank with those threads. I got it hydro tested and it passed, and the tester said it looked good. Second dive shop said there was crud at the bottom and they could not inspect it. It does appear to have a liner, so I am thinking of taking it back to the place that did hydro and having it tumbled or shot peened if possible. The guy at the second shop said it could not be tumbled because of the indentations in the base. What are others thoughts on this? Is it worth bothering with? I really would like to use it as part of the complete setup.




Thanks!

[antique diver]

I recommend that you not give up too easily. That is a desirable Scuba tank.
Have you actually looked into it with a good light yourself to see if there is significant corrosion or pitting apparent? I have seen places fail steel cylinders just because they had a little flash corrosion that amounted to no real problem at all.

I'm not suggesting that anyone be lax on inspections, I just have run into to many knuckleheads that were all too ready to fail any tank that did not look like a shiny aluminum cylinder inside... so beware and get additional opinions before giving up on this. Maybe a return trip to the hydro facility will garner some additional information on what they can do to help if it needs tumbling. I am thinking if it was too bad to use they would not have tested it.

As far as the threads... there is nothing inherently wrong with the NPT threads, other than they are a little more trouble to remove and replace valves, and not as easy to inspect. Some shops don't even have a light that will fit through the smaller opening and can't inspect them properly. Ask your hydro facility how many thousands of portable oxygen cylinders are in use with the same threads! Keep trying!

[Bryan]

It's a little challenging to tumble the bottom of that type of cylinder but it can be done. If you take a piece of nylon that will fit in the center of the base between the points, get it from McMaster Carr, cut it from an old cutting board etc and put it on the tumbler plate and then the cylinder will roll around on the nylon when it's tilted on the base and the points wont hit.

[1969ivan1]

I just received one tank like that myself. I am debating keeping it or offering it up for sale. They are very cool with the dimple bottoms. No need for tank boots on that one, eh.

[gj1963]

Thanks again for the great info. I did look in it all three times, both dive shops and the hydro place with their lights. Also myself. At first there appeared to be just some crud. I hit it with some high psi air out of curiosity and it flaked off chunks of the liner. There is no rust though.

I have no intention of giving up on the tank. I know it will just take time. I am just eager to use it now though. I have 11 month old twins so anything like this sort of stays back burner by default.

[antique diver]

Well, if you ever find yourself coming to Fort Worth/Dallas area of Texas, give me a few days notice and bring the tank with you. We will help you get it back into service!

[gj1963]

Once again, thanks for the info....
Not much development here with it due to time.
Antique Diver:
Don't make it through Dallas too often. But I was thinking... Would you or anyone else here be willing able to tumble/check out the tank if I pay shipping both ways, and whatever other costs for materials/time?
I would very much like to get this unit in nice operable condition.

Thanks!
Geoff

[antique diver]

Once again, thanks for the info....
Not much development here with it due to time.
Antique Diver:
Don't make it through Dallas too often. But I was thinking... Would you or anyone else here be willing able to tumble/check out the tank if I pay shipping both ways, and whatever other costs for materials/time?
I would very much like to get this unit in nice operable condition.

Thanks!
Geoff

There are a couple of nearby vintage-friendly local dive shops that I have a good working relationship with. I will check on the cost of tumbling your tank, and offer to help them with the process of getting it done in hopes of saving your cylinder. I'll send an email tonight and follow up tomorrow.

Please contact me via my email button to remind me if you don't hear from me by late this week.

[antique diver]

I just heard back from IARTD (International Association of Recreational and Technical Divers) about tumbling your tank. He said they would tumble it for $20, which I think is reasonable for that service around here. I am pretty sure that they normally charge more, but I told him I would help with the dirty work of getting the media in and out.

You may still want to check with your hydro facility about that service because it would save you a lot of shipping costs and time, and they are usually pretty reasonable on tumbling costs... but we will be glad to help you if needed.

You can contact them and get their address at their website, www.iartd.com .
I talked to Adam.

[SurfLung]

- I have a little tank with those bumps on the bottom and the tapered thread. It was a 1950s CO2 fire extinguisher at my folks house since I was a kid. In the 1980's I converted it to Scuba with a Sportsways K-valve. I've inspected it inside with a penlight several times over the years and it has always been pristine... even in the dimple/bumps at the bottom. Recently had it hydro'd and inspected and it even came back with the "+" sign stamped on it.
- Even so, it was leaking thru the threads when I got it back. A VDH forum thread said to use 6 wraps of teflon tape to seal it. I took off the valve, wrapped six, and screwed it back in. It5 sealed up perfectly and is still a nice little Scuba tank for my grandsons!
- My point is hang in there. That tank of your's already passed the hydro. You are almost there! I'd be tempted to stick a powerwasher in there just to see if it would blast out the crud in those dimples. - Good Luck, Eben

[gj1963]

Sorry it took so long to reply - two babies and work keeping me busy. I greatly appreciate the help. I will try to get it by the hydro place to see if they can tumble it, they seemed leery when I talked to them on the phone -- but it cant hurt to take it in. I will post an update. If not will investigate shipping. Thank you!

[swimjim]

When I tumble my tanks I tip up the tumbler for the last part of the process to help scour out the bottom. Sometimes I get some stubborn stuff so I welded a wire brush on the end of some rod stock and chuck it up in a drill. It does a nice job of finishing things off.



Might work on your dimpled bottom tanks.

[gj1963]

Well almost a year later this story comes to a happy end. After hydro and deciding the flaking liner had to come out - it went into limbo land getting it tumbled. It seemed to go here and there for a while and nearly forgotten about. Finally it got the liner removed and tumbled. Re-testing and inspection and a good look with a bore-o-scope and the valve finally went in. Filled and ready for a dive finally.

Thanks for all the advice in this thread that helped to finally put in service.
-I also owe a lot of thanks to local friends that helped.

[eskimo3883]

Hi,

Can you say a few details on taking out the liner? Always good to hear from someone who has had recent sucess.

[swimjim]

I had three 72's this year that I bought off Craigs list. All three of them had the brown teflon type lining in them. First I tried rolling them wet with no success. Then I tried dry rolling them with ceramic media. I checked the first tank after an hour on the roller and it appeared to be working. After three hours the sides were clean. Then I tipped the machine up some so the media would scrub the bottom of the tank. After an hour I removed the media and blew the dust out of the tank. There was just a small amount of lining left in the very bottom of the tank. I welded a wire brush onto a piece of threaded rod stock I had laying around and chucked it up in the 1/2" drill. That took care of the rest. I sent all three tanks in for hydro after that and they all passed! After a 30-40 year hiatus all three are back in service.
Jim