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Stainless steel fittings on copper boiler

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bogus07/05/2020 21:52:09
31 forum posts
54 photos

Dear All,

as in topic: i'm doing my firste ever overhaul of live steam loco. So far i managed to diassemble the engine and took the boiler off the loco to clean it and inspect it. Unfortunatelly some of the fittings broke during the attemp to unscrew it from the boiler body

Some question on the materials: i found the lower water gauge fitting is made out of brass however the part that goes into the boiler looks like copper. Is it normal practice?

One of the boiler water feed fitting (check valve) is broken too. Have no taps/dies in imperial system so i'll have to make new threads in metric system - metric fine threads, main pitch i plan to use is 0.5mm (ca 50 TPI) or 0.7mm (ca 36 TPI).

Is it good idea to make the fittings from stainless steel instead of phosphorous bronze/brass/copper?

How about sealing - i found the fitting were probably set with some kind of sealant/glue(?). Is it good practice to use thread locker like Locktite #xxx (xxx is for the correct number - i can't remember now the working temperatures).

After paint stripping i found there are some places covered with soft solder - now i'm worried if these are leaks repairs or what??

Have no idea who built this loco, it is 3.5 gauge Atlantic ca 40-50 years old.

In my country there is literally zero live steam models clubs/inspectors/associations - the only knowledge is from books, forums and internet in general.

I'm happy to hear advices from you, experienced, fellows

Thank you.

Paul Kemp07/05/2020 22:08:29
477 forum posts
18 photos

No to stainless steel boiler fittings, there are many grades of stainless and some are subject to corrosion at elevated temps in combination with certain contaminants (chemicals) in water.

Common on boilers of that age for soft solder caulking to be used. Pressure test it, if it doesn't leak don't worry about it.


Nigel Graham 208/05/2020 00:06:39
632 forum posts
12 photos

Br\ass is used for fittings but bronze or gunmetal is preferable.

The bushes that form part of the boiler should be bronze not brass, and the lower water-gauge fitting you say resembles copper may well be phosphor-bronze (similar colour).

If you can obtain the appropriate taps and dies where you live, you would be better using compatible threads for the new fittings, at least where they screw into existing boiler-bushes. The problem with re-cutting the bush threads from their probable ME or Brass thread originals is that you'd need a larger diameter thread to obtain a full thread-form, risking even the nearest size up over-thinning the bush walls.

I would not use a liquid sealant on boiler fittings, though I am sure others will say they do. It would need be one that does not set hard, and withstands steam and water at the boiler's full working temperature (I don't know your loco's working pressure, but at the 90 psi / 6 Bar common on many miniatures, it is nearly 200ºC). The sealant manufacturers' or stockists' own web-sites may contain the relevant information.

I have also been advised on very good authority not to use PTFE tape on the threads of fittings. That can put shreds of tape into the boiler, risking them blocking fittings like the water-gauge. Instead, use a soft copper or fibre washer.


[Incidentally I asked a friend in the building-trade about PTFE tape. As a plumber and Gas-Safe registered gas-appliance fitter and maintainer, he tells me he uses less than one of those small reels of PTFE tape a year.]

JasonB08/05/2020 06:56:47
18081 forum posts
1988 photos
1 articles
Posted by bogus on 07/05/2020 21:52:09:

er water gauge fitting is made out of brass however the part that goes into the boiler looks like copper. Is it normal practice?

This is a classic example of why brass should not be used where it comes into contact with boiler water, due to chemical action the zinc is removed from the brass leaving a honeycomb of copper which is weak. This is why you are seeing a bronze (copper) colour on the part

Use bronze or gunmetal for the replacements.

bogus08/05/2020 10:23:41
31 forum posts
54 photos

Thank you Guys,

So i'll try to get bronze for fittings. The grades available around are B101 and RG7 - both are good for bushings, highly stressed parts and both are corrosion proof.

As per repair if i find leaks - i've read here on forum that pinholes can be repaired by squeezing nearby material by punching, though it is not forever.

That soft solder repairs - if still leaking i'll remove it and find as high temperature soft solder as possible. I found the highest temperature still considered 'soft soldering' is 450 deg C.

It will take time to identify and collect all the taps&dies to make the plugs to prepare boiler for hydrostatic pressure test. The original pressure gauge mounted on the boiler has a marking on 80 psi - i assume it is original working pressure. Max gauge reading is 120 psi. Anyway i planned to make hydrostatic test at 1.5 times the working pressure - is it ok?

i'll post some photos later for reference.

Thank you Guys!

Oldiron08/05/2020 10:52:43
442 forum posts
22 photos
Posted by Nigel Graham 2 on 08/05/2020 00:06:39:

'[Incidentally I asked a friend in the building-trade about PTFE tape. As a plumber and Gas-Safe registered gas-appliance fitter and maintainer, he tells me he uses less than one of those small reels of PTFE tape a year.]

Most plumbers/pipe fitters use liquid PTFE these days. There are offerings from many manufacturers available. Very little is needed on the threads. I only use PTFE tape now on very rare occasions.

As far back as 10 tears ago when I had a new boiler fitted PTFE liquid was used on the tapered fittings.


Paul Kemp08/05/2020 12:42:18
477 forum posts
18 photos

80 psi would have been the working pressure. With old boilers, it it ain't broke don't try to fix it! Trying to get things clean enough on an old boiler to make a new soldered repair that flows where it needs too is very very hard. There are still early rivetted and soft solder chalked boilers in service! Brass was often used for boiler fittings and shouldn't be for the reasons above.

For 3 1/2" gauge Atlantic try searching for Maisie by LBSC.


bogus09/05/2020 12:06:31
31 forum posts
54 photos

ok. So far i've identified the threads around the boiler, well most of them. Turned out they are Model Engineering threads - ME1/4-40, ME1/4-32, ME9/32-40, ME7/32-40, ME5/16-40 - now i'll collect taps&dies to make a plugs, repair/make fittings, etc.

Got problem to identify safety valve thread - O.D. is 12.5mm - apparently 1/2 inch however the pitch is 25 or 26 tpi... I can risk 26 tpi is the true. Hard to say as there is only 5 turns. Anyway, i can't find the thread 1/2 in in diameter with 25 or 26 tpi. UNEF is the closest one but mine is not 28 tpi for sure. Funny, as on the second loco safety valve have 11.0 mm in diameter so it look like 7/16 and the same pitch - 26 tpi. Confused:/

As to the boiler - as you guys suggest - if it turn out not leaking i'll not touch it...

Maise LBSC - thank you Paul, yes, i know it. Already tried to find as many info as possible. Unfortunately not much on the internet.


Paul Kemp09/05/2020 12:18:10
477 forum posts
18 photos

Given its age and the fact all other threads are ME series the safety valves are are almost certain to be 26tpi. 26 was commonly specified by LBSC for safety valves. Remember there are are two standards for 26tpi, brass and cycle, former is 55 degrees, latter 60 degrees so if buying taps and dies be sure to get the right one. Brass will screw into cycle but cycle won't screw into brass!


bogus09/05/2020 12:56:54
31 forum posts
54 photos

Wow, thank you Paul. Never heard of brass and cycle threads...

My thread leaf gage is Whitworth 55 deg. and when checking the thread pointing it to the light i can see uniform distances between gauge thread form and safety valve thread form along the thread tooth. This suggest it is 55 deg. However, with such a small pitch, it is really hard to tell the difference:/

Ihave Royal Scot from LBSC drawings - safety valve thread is denoted 3/8" - 26tpi. No info weather 55 or 60 deg.

Do you think these should be ok?

thank you,


Paul Kemp09/05/2020 13:21:36
477 forum posts
18 photos

That should do the trick, most likely will be 55 deg. Strange as you say that the two are different sizes, I would guess that originally they were both 7/16" and at some point one of the threads in the boiler bushes was either stripped or damaged so was opened up to 1/2" to recover it and a larger safety valve body made. A lot of old machinery usually has some interesting tales to tell!


bogus09/05/2020 13:52:04
31 forum posts
54 photos

Maybe i confused you. By telling about 7/16 was reffering to safety valve on different loco i have. Different diameter, the same pitch 26 tpi.

DMB09/05/2020 14:06:42
999 forum posts


Do not use/reuse brass fittings. As above, zinc dissolves due to electrolytic action, leaving copper in a spongy form, very weak. Failure likely, accident waiting to happen and a danger to you.

Normal to use ME32 and ME40 and Brass thread 26tpi. All3 are whitworth form with 55° flank angle. Do not use Cycle or any UNEF/UNF/UNC as they are all 60° flank angle and incompatible with whitworth form.

Don't use stainless, you dont know composition of the alloy and could fail in a similar fashion to brass. Blocks of zinc are actually used as sacrificial anodes on boats to prevent corrosion damage to fittings. However, dont even think about doing that for a boiler. Just stick to phos. bronze to be sure of being safe. You may see a yellow metal Bush especially large diameter dome bushes and the like. These are gunmetal, not brass. GM is Copper and Tin and is OK for use in boiler construction. Brass screws and union nuts can be used external only, like injectors, pumps and their pipe runs. Dont use steel screws, not even to say secure a dome to its GM Bush.

I have Maisie book somewhere and Model Engineers going back donkeys years so if you need info on subject, I'll see what I can do.

Good luck,


bogus09/05/2020 21:43:56
31 forum posts
54 photos

you convinced me - even if plan to fire my engine twice a year i'll use proper materials. I'll go to local foundry. They are doing things out of alu, brass and bronze - mainly B101 (CuSn10P). I'll ask to make some B101 stock for my small lathe.

John, regarding the Maisie - have you got some instuction manuals, characteristics/technical data of the model, drawings? Actually i need every kind of information. This is my first time i have my hands on coal fired steam engine. Already did some mistakes during stipping the loco and i'd love to avoid more mistakes:/

I uploaded the latest photos i took last days showing current state of the boiler. Comments much appreciated.


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