**LINK**

I googled 3/8 x 28. I would suggest you get a thread gauge to be sure before spending money though.

Can I respectfully suggest you pause before going too far; jamming in an M6 tap to an old boiler sounds like a disaster waiting to happen. If its an old british boiler I suspect M6 won't be an option and you could be causing damage by trying.

Some people recommend using a soft wood or even wax on internal threads, then withdraw the wood and measure the TPI on that.

3/8" x 26 sounds likely for the larger ones and the smaller could well be 1/4" x 26

Michael, please be aware that even manufactured nuts etc are made to tolerance, however I am not so sure how well the finished product is inspected. I bought some union nuts from one of our suppliers which would not fit other manufactured fittings before I ran a plug tap through them. I found it quicker to correct a dozen nuts than send back etc.

Bob

If the boiler was made in the uk then it can be assumed to be BSP as your guess at 28 correct and sloppy because you were using both wrong tpi and thread angle plus a short thread. Measure the od ! if it is .383" it's pipe thread ! Under 375" could be 3/8" 26. Noel.

A good thread pitch gauge is a small but valuable investment. It's the only way you can be sure of what you are working with. Screwing on commercially made fittings in trial and error can give a false result due to manufacturing tolerances and could damage the boiler fitting.

Edited By Hopper on 12/08/2022 05:02:52

It could be helpful in cases like this that the O/P gives their Universal location. This COULD give an indication of what may have been the threading system used in that location.

Regards Ian.

I was faced with a somewhat similar problem recently, with a thread on an engraver spindle.

As well as using thread-gauges and magnifying-glass, I made some brass test-threads in bits of bar, to what seemed the measured size, and tried them – very cautiously.

If they start to stick within 2 or 3 turns, they are obviously wrong; don't try to force them further.

Note the threads must be scrupulously clean so you don't risk a bit of swarf jamming or tearing the threads, and I lubricated them with a little spot of WD-40 or white spirit.

.

(It actually turned out to be a 30tpi Unified Special, of all things!)

If you do sort it please let us know what the thread in question is

H

I wonder how much people read the previous posts before answering. Would save a lot of duplication.