Boiler test

[Michael Brett 1Participant @michaelbrett1]

Hi all

Need help in identifying boiler threads prior to sealing and testing for leaks. I have three holes to seal , inlet and outlet , and one for pressure gauge.

Inlet and outlet are male threaded , the closest I have tried is 3/8 x 32 tpi. This size union nut starts to go on then jams. Looking at thread I would say this should be a 3/8 buy 28 tpi as the thread is a little courser. I have tried the internet but cannot find this size , is it USA size.

The third inlet is for the pressure gauge and is female. Again the closest I can find to fit is a m6 tap but this soon jams after a couple of turns.

This is an old boiler but very well made so would like to eventually get it u and running.

Thanks in advance Mike

[Michael Brett 1Participant @michaelbrett1]

Sealing boiler inlets prior to test

[Jon LawesParticipant @jonlawes51698]

**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.

[noel shelleyParticipant @noelshelley55608]

A likely contender is 1/8" BSP at 28tpi, 0.383" od, could be 3/8" 26tpi. For the near 6mm try 0Ba.  Noel

Edited By noel shelley on 11/08/2022 17:43:29

[JasonBModerator @jasonb]

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

[Harry WilkesParticipant @harrywilkes58467]

Check out some of the ME thread sizes, ME very popular in model engineering

H

[Michael Brett 1Participant @michaelbrett1]

I tried my 1/8 npt 27 die and it screwed on all the way , bit sloppy but I assume because of the taper

Do they do a 1/8 bsp in 27 or must it be 28 or 26.

Mike

[Michael Brett 1Participant @michaelbrett1]

Found and ordered BSP nuts for 1/8 x 3/16 od tube. Seems about the right size , at only 99p each worth a try

Just the smaller one to sort out now, will try 1/4 x 26.

Many thanks mike

[Speedy Builder5Participant @speedybuilder5]

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

[Howard LewisParticipant @howardlewis46836]

1/8 BSP is 28 tpi WHIT form. (55 degree thread form )

If it is 1/8 NPT x 27 tpi it will be a 60 degree thread.

So check the thread with the correct thread gauge.

Getting bit wrong, and forcing on an incorrect fitting could result in a damaged boiler.

At best,with incorrect fittings, it will probably fail because of leakage at the fittings..

Howard

[noel shelleyParticipant @noelshelley55608]

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.

[Paul LousickParticipant @paullousick59116]

As listed by Jason, 3/8" x 26 tpi (British Standard Brass thread) is a common size used for fittings for model boilers and engines. (not to be confused with American bicycle thread which has a 60 degree thread angle instead of 55 degrees)

Edited By Paul Lousick on 12/08/2022 00:36:38

[HopperParticipant @hopper]

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

[noel shelleyParticipant @noelshelley55608]

Hopper is spot on, but there are so many different thread rates that on a short thread piece, maybe less than .5", that if you have 2 rates only 2 tpi apart and may be a metric option as well it may prove hard to decide which is the right one. Then there's the thread angle, again on small sizes ti is almost immposible to spot 55* and 60*. Manufacturing tolerances will then mislead you as to the size OD. IF it feels right, is neither tight or loose and runs on nicely, pitch gauge and OD tally, it will be near enough ! Good luck. Noel.

[CirclipParticipant @circlip]

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.

[HopperParticipant @hopper]

Add a magnifying glass — one with the high power inset lense — to the thread pitch gauge shopping list.

[Nigel Graham 2Participant @nigelgraham2]

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!)

[Michael Brett 1Participant @michaelbrett1]

Many thanks for all your advice. I do intend to make some test pieces using brass hex bar. I think I am getting close to finding the correct size , having found out what does not fit. Job is on hold at the present as my workshop is just to hot to work in, I would rather take my time so as not to damage anything.

Cheers Mike

[Harry WilkesParticipant @harrywilkes58467]

Posted by Michael Brett 1 on 12/08/2022 17:32:07:

Many thanks for all your advice. I do intend to make some test pieces using brass hex bar. I think I am getting close to finding the correct size , having found out what does not fit. Job is on hold at the present as my workshop is just to hot to work in, I would rather take my time so as not to damage anything.

Cheers Mike

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

H

[Paul LousickParticipant @paullousick59116]

A simple method for measuring the pitch of an internal thread is to screw in a piece of wooden dowel and measure the the number of grooves/inch which are left on the wood.

[Michael Brett 1Participant @michaelbrett1]

Progress at last

The two male threads turned out to be 1/8 bsp for 3/16 od pipe. I found a bolt that fits the female pressure gauge union perfectly. Tried to measure it with my thread gauge and it comes up as 1.25 not sure what that means.

Anyway sealed boiler and did a hydraulic test . Only got to 70 psi and there was leakage from bottom tap on gauge glass, need to sort that out. Plus a water leak where elbow screwed into top of boiler , again got to get some suitable loctite on that. Will do another test later in week.

Happy days Mike

[Harry WilkesParticipant @harrywilkes58467]

glad you sorted out the threads good look with the leak

H

[Jon LawesParticipant @jonlawes51698]

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

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