Weird Metric Thread on Air Compressor Switch

[SteveHGraham]

I have a little Chinese (Eaton) compressor I used for cleaning and tire inflation. It has a really cheap vinyl-looking hose that goes from the pressure switch to the panel where the valves are. This hose is total crap, and to make it worse, it's held on by compression fittings which seem to guarantee that the end will split every few years.

I have some spare 3/8" hose, and I was going to replace the factory garbage with it. Problem: the valve/regulator manifold has regular threads that work with 1/4" NPT (ID) stuff. The valve on the pressure switch has a thread that looks the same, but it's actually about ten thousandths smaller in diameter (around 0.510" on the outside of the male fitting), and an American fitting won't go in.

Does anyone have any idea what size the thread is? It must be some crazy metric thing. I put the old hose back in today, but I want to order a fitting and put the other hose in it.

[warmstrong1955]

1/4" BSP maybe?

How many threads per inch?? Is it tapered, or parallel?

1/4" BSPT or BSPP is 19 TPI, and .52" nominal in dia on the male threads.

Bill

[SteveHGraham]

I think it's metric because it's Chinese. It's so close to the threads on a 1/4" ID barb, I'm pretty sure it can't be imperial (NPT, whatever the term is). I don't think anyone would bother making a thread virtually identical to a standard imperial size but slightly off.

I measured the threads on an imperial part which is nearly the same size, and I got 18 per inch. That gives about 1.4 mm per thread. I took the metric fitting out, and I was not able to measure it for lack of a gauge that fit, but it's between 1.5 mm and 1.25 mm. There must be a standard metric pipe thread somewhere in that range. I can't make a fitting with that thread. Maybe I can order one.

The outside of the thread is close to 13mm, so maybe it's M13 x 1.4?

[SteveHGraham]

I guess I could drill the manifold out and put an imperial fitting in there. I would need a tap.

[SteveHGraham]

Well, this is annoying. A 1/4"-18 gauge seems to fit it, but an American fitting with that thread won't quite go in. I am thinking maybe I can catch the thread on the lathe and shave a few thousandths off the new part to make it fit, but I just put my 3-ton 4-jaw chuck on the lathe, and it won't hold the part.

Obviously, the first job to come up after switching chucks will have to be one that requires the 3-jaw.

[Russ Hanscom]

Sounds like a normal day in the typical home shop!

[SteveHGraham]

This is almost interesting.

I don't know much about NPT threads. I know a tiny bit about regular imperial threads, the proper name of which I am too lazy to look up. When you make a hole for a normal thread, the name of the thread gives you a pretty good idea of the size of the hole. A 1/2" thread takes a hole 27/64" in diameter, which is close to half an inch. But with NPT threads, the nominal size appears to be the size of the air opening, not the hole. So a 1/4-18 NPT thread requires a hole 7/16" in diameter, or nearly twice 1/4". Annoying.

I suspect that if I get a 1/4-18 NPT tap and run it through the hole in the compressor, it will clear out enough crap to make plain old Home Depot fittings fit perfectly. But it would not be a huge shock if I were wrong.

[John Evans]

Metric pipe is pretty much the same as imperial I do believe although German 1/8 pipe is 28 threads PI and Imperial is 27. Your HD fittings maybe on the high end of size. give it a kiss with a 1/4 pipe tap. You could have my day replacing a 40 gallon under counter waterheater in a hole with a inch or so of clearance and a sill at the front. Bah

[SteveHGraham]

Anything is better than ironing.

[John Evans]

Anything is better than ironing.

What is this "ironing" which you speak?? Wash and wear is my line !

[hammermill]

Steve you need to get some standard pipe taps and drills. And does. Then this will not a problem be.

I would drill,tap,and go for it. I buy lots of air fittings from hf.

[warmstrong1955]

Everything you wanted to know about identifying port & connector threads:

A-Quip Thread ID.pdf

(314.24 KiB) Downloaded 1707 times

All's ya need is a caliper and a thread pitch gage or two.

Bill