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Boiler build abandoned !

Chasing leaks round the boiler

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chris organ27/09/2019 13:55:29
16 forum posts

I have given up on my 3.5" Rob Roy boiler build even though it is outwardly completed. I will order one from someone with greater silver soldering skills than me. My priority is a finished engine I can trust and I feel I can get more from working out what went wrong with this boiler than chasing around more leaks. The point of this thread is to invite suggestions as to what went wrong as I may have another go on another locomotive sometime. I will also show the boiler to the inspectors in my club for their opinion.

This was my first attempt at boiler construction. I have used propane with bigger torches as I've gone along to get the heat into the boiler. I was seemingly on the right track right up to the last fittings ! After I fitted the backplate and foundation ring I did a 20 psi hydraulic test, no problems.

That just left the stays, blowdown valve and top fitting for the water gauge. I silver soldered all of these, using a long neck torch to get heat inside the firebox. All seemed to go reasonably well.

When I repeated the hydraulic test I had water running out at atmospheric pressure from the foundation ring ! I fixed that by a little hammer tapping, a PB screw and then re-soldering. At the next test I had a just weep or two at 20 psi inside the firebox at the seam near the tube ends. I re-soldered that. Now I have weeping at very low pressure somewhere from the tube ends. If the leak was anywhere else I would have another go but I feel every time I put more heat in there I create a new problem.

I have been using SS 55%, pickling and cleaning between every soldering operation and refluxing nearby existing joints when going back in to add the last fittings. Whatever I solder seems to work nicely, it's the surrounding areas that seem to be failing.

Thoughts ?

not done it yet27/09/2019 14:10:00
6733 forum posts
20 photos

I’m not a boiler builder but I would use the highest melting point solder for initial joints and reduce the melting points, of later solder joints, in that vicinity.

Rockingdodge27/09/2019 14:11:49
301 forum posts
60 photos

I would also be refluxing the joints already soldered at each stage as well

DMB27/09/2019 14:23:39
1299 forum posts
1 photos

Hi Chris,

If you try making a new boiler as you said, choose a small one to save money on cost of (a) copper, (b)silver solder, bearing in mind that the bigger the boiler so the longer each joint will be.

On a more positive note, it sounds like you're nearly there with the current one. Take on board comments by others above and next attempt, enlist supervision by a known boiler maker in your club.

Good luck and hope you are successful very soon.You are gaining knowledge and experience even with your troubles.


Phil H127/09/2019 14:39:29
458 forum posts
60 photos


I would do as you have suggested and go and see your inspector/other club members before you scrap it. They might agree that you have a real problem but I have read quite a few articles on this site and others where first time boiler builders have a similar leak chasing problem. The few that I have read usually succeed in the end with a bit of help and encouragement.

Also, perhaps put the boiler down for a day or two and think about all the work you have put in so far before you sign your cheque (probably for about £1000ish++++?). A bit of help, propane and plenty of flux might fix it.

I have a similar Rob Roy boiler issue (see my pictures). My snag is that I built mine 20 odd years ago to the early book and drawing set and some features have changed. I need to fit some extra bushes etc - so that might be fun.

Phil H

Graham Stoppani27/09/2019 14:40:42
120 forum posts
26 photos

Hi Chris,

I am no expert but at last year's Midlands Model Engineering Show I attended a lecture by Keith Hale from CuP Alloys that was all about silver soldering and brazing techniques. He has published a book called A Guide to Brazing and Soldering - everything you wanted to know about silver soldering but were afraid to ask! I have found it useful.

J Hancock27/09/2019 16:11:44
832 forum posts

Did you read and follow the step by step construction as shown in Alec Farmers book ?

Do anything in the wrong order and you are in deep trouble.

Nigel Bennett27/09/2019 17:20:26
456 forum posts
20 photos

Sorry to hear of your woes!

If you can find a fellow club member who is well versed in boiler-building, and who has access to oxy-acetylene or oxy-propane, the boiler may be rescuable. See what your boiler inspector says!

One thought; you've not been dumping the boiler when it's still very hot into the pickle, are you? That can easily cause massive thermal stresses which result in leaks.

Also, if the boiler is generally sound but just has the odd weep, then don't discount Comsol to caulk it up - but you won't be able to do any silver-soldering on it afterwards!

chris organ27/09/2019 18:05:59
16 forum posts

..... Nigel ..."you've not been dumping the boiler when it's still very hot into the pickle ... ?"

Oh oh. Schoolboy error ? Some guilt here. Not glowing hot but hot enough to cause some fizzing.

Is there a fix ? perhaps a reheat of the whole area (with plenty of flux on all seams) and a gentle cool down before any pickling.

vintage engineer27/09/2019 20:01:50
256 forum posts
1 photos

I use oxy propane for silver soldering and never have a problem. You can even unsolder joints. High localised heat is the trick to silver soldering.

Keith Hale28/09/2019 09:31:43
333 forum posts
1 photos

The "trick" to all successful soldering, including silver soldering is capillary flow. This is facilitated by joint design, fluxing and heating technique. It matters not about heat source, alloy or manufacturers.

This statement is built on an honours degree in metallurgy and 50 years in the brazing and soldering business as a salesman, technical consultant and manufacturer of brazing alloys.

For more information see the book available from CuP Alloys. Holiday time does not allow the time on this occasion for a fuller answer.

The plot is simple, there are few characters, it won't win a Booker Prize, but it's a good read. My grandson is starting to produce sound joints. He might not understand the science but he does follow the basic principles.

Martin Johnson 128/09/2019 09:39:50
145 forum posts
1 photos


I know how much scrapping a boiler hurts, as I got myself into exactly the same situation with a Rob Roy boiler, more years ago than I care to remember. Just like you, the shell went quite well as did the tubes. But the stays.................

Well, I pickled, chased the leaks around using an oxy acetylene set at work to get inside the box. Created more leaks in diffferent placed, pickled, chased the leaks around a bit more ............. and so on ad infinitum. Until in the end, I was concerned about the safety of it all just like you.

I just hope knowing you are not alone will set things in perspective. You are not the first and I very much doubt you will be the last bulder to get into a similar situation.

The fact is, that commercial boilers represent a bargain when you look at cost of materials and the amount of fiddly work involved.

Best Wishes,


chris organ28/09/2019 14:58:22
16 forum posts

Thank you to everyone for their replies. On balance, I suspect I have been a little heavy handed and clumsy with a job that needs care and patience to complete successfully. I am going to order a made up and certified boiler (engine is the priority) but as a side-line will have a couple more goes at sealing the homemade one up. If that works then at least I can honestly say I built a whole locomotive, not necessarily assembled in the same locomotive but there you are. Definitely I will a copy of the recommended reading above before starting anything else.


Richard Nixon06/10/2019 19:14:27
1 forum posts

I had this problem with some pin-hole 'welsh vegetables' on a 5" gauge 'Enterprise', LNER 2-6-2 tank loco boiler. As a last resort I used some stuff called 'Water Glass', which is sodium silicate. Pour it in the boiler so it lays over where the leaks are and apply low air pressure (say, 20 psi) to force it into the pinholes for 5 minutes or so. Re-position the boiler if necessary so that it penetrates leaks in areas not covered at the first go. Decant it back into the container and leave the boiler for a day or two to Harden off. My boiler has subsequently successfully undergone a hydraulic test of 200 psi. This 'Water Glass' can be obtained on Ebay, a 500 ml bottle cost around £12 including postage, from memory.

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