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I'm working on a DIY system at home that involves parts from different locations and industries, which means I'm running into many thread adaptor-related issues.

I have a gas cylinder that I'm trying to attach something to (I have the cylinder as well as the male threads for it that I'm trying to replace). I took the male part into ACE Hardware and someone helped me out by identifying that it fit well into a 3/4" NPT coupler I used to test. So I figured "Well then, the 3/4" NPT male threads will fit in the cylinder itself" and went ahead buying those. When I got home I found that it was just barely large enough that it won't fit in the cylinder. Turns out they're 0.44 mm (0.017") too wide to fit.

After doing some more research, I determined that the gas cylinder has 1-14 UNF threads, which I found by looking up the part number of the gas cylinder which lead me to Catalina Cylinders' Gas Cylinder Specs document (notice all of the threads say 1"-14UNF) Then using my calipers to carefully measure them and they perfectly match the specs for the 1-14 Thread Designation on a Unified National Thread Chart.

So it seems obvious that I'm looking for a (M)1-14 UNF to (M)1/2 NPT (or BSP) joint/adapter. But I can't seem to find one on Ebay, Amazon or some of the other websites I go to. I have found a few that are unreasonably priced.

Are the UNF threads not as common as one would expect? Or am I just looking for something improperly? I keep finding that UNF is a popular thread type, but it seems very difficult to find anyone selling anything that would fit these.

Edit: Adding a few photos of the tanks. These components came out of a Parker CN-6NA (which is very similar to a DN-6NA). Here's the tanks as they were when they arrived in the mail (with the fittings installed): enter image description here enter image description here

Measuring the threads on the attachment. They're fine parallel threads, 1" in diameter. enter image description here

More photos here.

J H
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    If your usual websites don't include McMaster-Carr, (just a happy customer, not always cheapest but has more different weird hardware things than the cheapest places do) they should, but this still seems pretty odd, mixing straight UNF threads with pipe threads. – Ecnerwal May 27 '23 at 00:57

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I have found a few that are unreasonably priced.

You have a weird, oddball adapter application. There's no "millions of parts volume" to make for a low discount price as seen on common adapters. It's very likely that for the production & sales volume for such parts the prices are perfectly reasonable, given the sales volume.

Get a bid to have the part custom-made, if you like - could be one of the online clearinghouses for such things would lead you to the guy who'll do it cheaper since it's just his hobby business, and likely the parts you can find are done in such small batches they are basically custom-made..

If you want a part, suck it up and pay the going rate. If you want a new hobby and not to have to pay for such parts, buy a small or medium (or larger if you find a deal and have a place to put it) metalworking thread cutting lathe and make your own. Then keep track of your hours and see if your "wage" would be reasonable at a price you'd pay for the part you make. Especially if you had rent and power bills on the space you were working from, rather than those being "free" at your hobby location...

Or you may working on adapting to the wrong piece. If "the male part" is a cylinder valve, connectors to those for various pipes are quite common.

Ecnerwal
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  • Thanks for the reply. I'll resort back to just using the attachment that came with the tanks (original post updated to include photos). The reason I wanted to replace them was so I could attach a regulator to the tanks directly, which would allow me to close off a tank and bring it elsewhere if needed. But I suppose if it's _that_ important, I'll cough up the $60 for a fitting, or purchase more appropriate tanks. – J H May 27 '23 at 22:39
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    If, as your Catalina link suggests, that is default tank threading, cylinder valves (that thread right into it) are very common. Your local scuba shop might even sell you one cheap from a tank that failed hydro-testing, since they usually get scrapped valve and all. A welding supply might do the same, or either would have new-and-expensive ones. – Ecnerwal May 27 '23 at 22:51
  • I think you're right... I couldn't find the threads very easily when I looked for "1-14 UNF" or "1-1/4 UNF", but I just looked around more on scuba tanks, and they use "3/4" x 14 NPSM" - Which looks nearly identical to the threads I have. See, I knew it was just a matter of knowing what to Google, lol. – J H May 28 '23 at 08:33