CGA Fitting Thread Issues

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Tom McCarthy
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CGA Fitting Thread Issues

Post by Tom McCarthy »

A while back I got a Victoreen VTS 452 D regulator, with a CGA 350 connection. Discussed on a thread here.

http://fusor.net/board/viewtopic.php?f=6&t=10965

Following the advice in above thread, I bought a CGA 180 nipple, the website says it has a 1/8" NPT thread (http://westernenterprises.com/wp-conten ... 80-200.pdf). As the regulator has a 1/4" female NPT connection to take the CGA 350 nipple, I also got a 1/4" male to 1/8" female adapter to enable me to use the CGA 180 nipple.

The 1/4" to 1/8" adapter screws nicely into the regulator, which hints the adaptor is NPT (got it from a local supplier, and I'm in Ireland, so not always sure) as the regulator thread is NPT. The CGA 180 nipple will not screw into the 1/8" adaptor though. I thought perhaps the nipple is left-handed, but the threads look like they're going right, and a CGA 180 nut is right-handed too - see part 180-2

http://westernenterprises.com/wp-conten ... 80-200.pdf).

Does anybody have an idea why the nipple won't screw in?

Also, is there anything special about CGA nuts? On the link above, it's designated as 5/8" - 18 UNF 2B RH. Will any nut with those specs do, or is there something special about CGA nuts in particular? They seem to close off at the end, so as to compress the nipple against the gas cylinder.

Tom
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Rich Feldman
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Re: CGA Fitting Thread Issues

Post by Rich Feldman »

Tom, can you be more specific about _how_ your purported 1/8"-NPT threads fail to engage? If you have another 1/8"-27 NPT threaded part, or a thread pitch gauge, or a measuring caliper, they might help you determine which part is wrong.

In North America, a partly compatible 27-per-inch threading is extremely common in electric lamps and lighting fixtures. The nominal diameter is sometimes given as 3/8" actual, instead of 1/8" pipe. Like Schrader valve threading, it's worked since the late 19th century. Facilitated the conversion of existing buildings and/or luminaires from gas to electric lighting. Not intended to make leakproof joints, so the threads are usually straight (untapered) and often formed by rolling instead of cutting.

Left-handed pipe threads are uncommon, and not part of any compressed-gas connection keying that I know of. One application is to put together railings and bannisters without welding. Today I learned of another: https://www.plumbingsupply.com/leftright.html

You have the CGA nut figured out. The UNF callout controls the internal thread dimensions, but the thread obviously does not run all the way through the nut. With ordinary handyman tools, one could adapt a regular nut into something that could safely do the job. Hardly worth the trouble, if you can afford the time to procure a proper CGA nipple nut. Doesn't the lecture bottle connection also need a special nonmetallic washer or gasket for a face-to-face seal?
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Jerry Biehler
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Re: CGA Fitting Thread Issues

Post by Jerry Biehler »

Maybe you have a 1/8" BSP to 1/4 adapter instead. Simplest thing is take a 1/8"NPT tap and open it up till you can screw in the nipple.
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Re: CGA Fitting Thread Issues

Post by Tom McCarthy »

Rich: Specifics on thread failing to engage: I can thread screw the nipple in about 45 degrees of one turn. Then the screwing comes to an immediate stop, very high resistance. It's not a gradual increase in resistance, just a sudden lock. The nipple can't screw in any further. Also,the nipple must be fully unscrewed to remove, it can't be wiggled/tugged out laterally. Both threads are clean and new, no gunk in them and I can't see anything blatantly faulty with either. It seems like the nipple can screw along the entry transition from un threaded to thread, and stops as soon as it's in the actual thread. I'll have a look at matching up to another 1/8" NPT part, not sure if I have anything around though. Can definitely measure up the threads etc with a callipers - I'll report back.

I'll grab a standard CGA nut I reckon, thanks for confirming that.

Jerry: If it doesn't work out in the next week or two I'll go for the tap, good idea.

Tom
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Re: CGA Fitting Thread Issues

Post by Tom McCarthy »

Got the adaptor tapped with an NPT 1/8 thread and the nipple goes right in now, so nipple is 1/8 NPT. Adaptor is most likely BSP. Have to replace the thing anyways with an NPT version because the 1/4" part doesn't screw fully in to the regulator itself. Scenic route I took.

Tom
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Re: CGA Fitting Thread Issues

Post by Jerry Biehler »

I just remachined a KF10 fitting to 1/8" BSP a couple days ago. One thing I didn't know is BSP for 1/8" is 28TPI, NPT for 1/16" and 1/8" is 27TPI. Which is interesting since they sell universal fittings that will fit both.
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Re: CGA Fitting Thread Issues

Post by Tom McCarthy »

On a related note to NPT fittings not engaging all the way down the thread, see below link. Apparently NPTs are not expected to bottom out at all.

https://www.pegasusautoracing.com/docum ... =TECH00097
The tapered thread seals by means of an interference fit between the male and female threads. This means that NPT fittings should never bottom out in the port. The fitting should screw in only partway before jamming. If an NPT fitting screws all the way into a female port without binding, the threads are either mis-tapped or worn out
Tom
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Richard Hull
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Re: CGA Fitting Thread Issues

Post by Richard Hull »

I am surprised others needed to be informed that NPT threading was never meant to bottom out. NPT (National Pipe Thread) is a rather ancient standard and has been part of vacuum system work for years. However the days of such crude sealing efforts in vacuum systems seems to be going the way of the carrier pigeon in modern professional systems and now finds use only in foreline, rough vacuum metering, for the most part.

Richard Hull
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