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Boiler cross tube leak.

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mick H19/06/2019 13:49:44
766 forum posts
28 photos

I have just completed a vertical boiler build which has a central flue with cross tubes. Aware of the propensity of this design for leaks, the flue and cross tube assembly was silver soldered with SF24 and the whole assembly subjected to a 120psi hydraulic test for several hours. I then silver soldered the flue assembly into the boiler barrel using SF55. On hydraulically testing the whole boiler I noticed a very small drop in pressure of about 5psi over about 20 minutes and on investigation found the tiniest of weeps from one of the cross tube joints. The received wisdom seems to be, scrap it and start again. I will not do that .....yet.

My first thoughts centred around Comsol. Do I need a special flux?

Second thoughts were around a sealant but have things moved on from horse dung and oatmeal? Would it be very wrong to try say Fernox leak sealer provided the boiler was thoroughly washed out after a seal is effected.

This is the first of 8 boilers that has failed on me and although I accept it was my fault, whilst soldering up the flue assembly, the heavens opened and I made the error of trying to get the job done ASAP instead of concentrating more on the quality of the braze. As they say around these parts, you only get wet once.


John Ockleshaw 120/06/2019 03:25:14
53 forum posts
7 photos

Hello Mick, Tin the inside of your boiler with soft solder. Plug all the openings except the largest one. Pour in about three tablespoons of Bakers Fluid, Heat the boiler to about 90 deg. C., plug the hole ,lift the boiler and swish the flux around vigorously then pour it out. Melt in the equivalent of a whole stick of plumbers solder, do not use resin cored solder. Heat the boiler until the solder is molten. replug the opening. Again pick up the boiler and shake it vigorously, pour out the excess solder, Wash the inside well.

Regards, John

Brian H20/06/2019 07:39:45
2312 forum posts
112 photos

One solution is to use one of the products for stopping car radiator leaks.


Simon Collier20/06/2019 08:11:05
454 forum posts
63 photos

Before you do anything heroic, put some fine white pepper in the boiler and hydro it again. Leave it pressurised for a while. There is a good chance the pepper will calk the weep. No need to trouble any boiler inspectors with that little detail. It would probably " take up" with a few steaming anyway.

John Rutzen20/06/2019 08:34:15
348 forum posts
19 photos


I think we could do with a model engineer article or two on fixing minor leaks. As you say the received wisdom nowadays seems to be that any new boiler with a leak is scrap. This thinking is stopping many model engineers from even attempting to make boilers. I have a friend with a boiler with a crack round the silver solder on one of the flue tubes in the firebox and who doesn't want to try re- silver soldering it. I had a leak round a rivet on the foundation ring. That one responded to a few taps with a hammer and has not leaked since. Fortunately my boiler inspectors accepted that fix after a few discussions.

fizzy20/06/2019 09:00:39
1843 forum posts
120 photos

Of the several hundred boilers I have made it has only happened once and was entirely my fault for not testing the flue prior to assembly. It went in the scrap bin. If you start with a defective joint it is never going to get better. The sealants out there are for car radiators and central heating systems, running about 15psi and 90 deg C - this is a long way from a boiler running 60psi up to 140 deg C. On a very small boiler they might be worth a try.

Brian Oldford20/06/2019 09:12:34
686 forum posts
18 photos

How many full size locomotive boilers are completely leak free?

Nick Clarke 320/06/2019 10:00:14
1395 forum posts
61 photos

I agree that many small leaks can apparently be stopped in a boiler - potato peelings, mustard, china clay and porridge have all been suggested, and according to what I have read all work to a greater or lesser extent.

If an issue with insurance were to arise for operating in public, at a exhibition for example, then the appropriate boiler code would need to be followed which will specify the steps it is acceptable to take.

if it is only going to be run in private then you make your own decision and take responsibility as seems appropriate.

duncan webster20/06/2019 10:11:44
3928 forum posts
61 photos

Full size boilers are rather different. Firstly they are steel, which rusts and tends to seal any very small leaks, second they have rivetted lap joints, so you have a long leak path. Even a rivet is going through 3/4" or so of metal.

The accepted practice for putting stays into model boilers used to be screw thread sealed with comsol. Whilst I wouldn't advocate going back to this, sealing a small leak in a silver soldered boiler with comsol would seem to be OK provided it was just sealing the leak and not providing any strength. Tinning the whole of the inside sounds heroic, and a bit of a sledgehammer approach

Werner Schleidt20/06/2019 10:27:22
146 forum posts
179 photos

Hello Mick,

years ago I had as pinhole leak in a boiler with a max working pressure of 3 bar. As I testet it with 6 bar with water it was fine as a needle.. I took the water out and then Ifilled it again with mineral water from the bottle . It takes about 2 min under pressure and then it was perfect tight. And there was never again an issue.

The same is possible with a mixture with water Na2SiO3


martin ranson 220/06/2019 10:54:59
135 forum posts
2 photos

To Mick please ... I hope you can fix the leak with some of the methods above ... I have had the same problem with SF 55 ... I do not think it flows as well as the old EF 2 ... I know this does not solve your problem, but when I produce a boiler with cross-tubes I use SF 24 as a first fix .... clean it all up and then go round everything again with SF 55 ... use LOTS and LOTS of FLUX for the SF 55 ... I now get through much more flux than with EF 2 ... yes the soldering looks heavy and messy but nobody sees it inside the boiler ... I agree with one of the replies above, I would scrap it and put it down to experience ... I have had to do this with one of my boilers ... nearly in tears, but I did not think it would seal itself ... do not forget that the cross tubes up at the boiler top may be partially out of water if the level gets a bit low ... this could ruin any sealant you use.

As a P.S ... when you silver solder the main tube into the boiler, try to stop the heat travelling down the tube ... I have several very scruffy "heat stoppers" made from copper sheet ... 1 mm or 1/16 thick ... imagine a 1 penny coin with a handle like a bent spoon ... different diameters for different tubes ... bend it over the side of the main tube end ... the more mucky and grubby the better ... less chance of gluing it in place with silver solder.


mick H20/06/2019 18:29:09
766 forum posts
28 photos

Thanks for that bunch of very interesting posts gents. I shall re-read them all several times and tuck the advice away for future reference. As for my boiler, I seem to have cracked the problem with a bit of comsol and she is holding pressure well. We shall see what occurs when I get it into steam.

Touching again on "chemical sealants" as a solution. Radweld is an obvious lo-tech example. I note though that the same maker markets a product called Wonderweld which purports to seal cracks in motor car cylinder heads and engine blocks. Pretty extreme conditions and pressures there I would have thought.


martin ranson 220/06/2019 18:44:36
135 forum posts
2 photos

To Mick please ... as a PPS to the above ... if you do end up cutting it all apart you should be able to carefully re-use all the bushes so they are not scrap ... if you gently cut through the leaking tube and examine it under a magnifier you should be able to see where the silver solder has penetrated to ... is it only on the outside of the main tube ? does it penetrate right through to the inside ? is there a decent fillet on both sides of the joint ? could I ask how you made the holes for the cross tubes ? are they reamed ? drilled ? there are always a lot of ideas how to make the best joints ... my way has been in use for many years and came about following my early failures ...I drill the holes slightly undersize and gently file them out until the tube is a tight slide into place ... using a small triangle needle file make 3 tiny notches round the circumference ... by tiny I mean only a few thou. ... if it is properly fluxed you ought to get a double fillet of solder ... at least if you have to scrap the boiler it will be useful knowledge for the next one.


Former Member20/06/2019 19:24:23
1329 forum posts

[This posting has been removed]

duncan webster20/06/2019 22:58:13
3928 forum posts
61 photos

I'm sure the CuP guy will be along soon, but in case he misses this post, silver solder works by capillary action through the joint. there has to be some clearance, typically 0.001" to 0.005" all round to leave the silver solder somewhere to go. To stop the tubes falling through the holes, just bell out the ends a bit. I anyone doesn't believe me see **LINK** or **LINK**

And another thing, we are often told that metal must be squeaky clean before we start. I went to one of CuP's talks where he said this was all wrong, and the flux did the work. Next day I dug up a piece of copper tube which had been buried in the garden for a couple of years (launching rockets on fireworks night), cut 2 pieces out of it, fluxed and silver soldered. perfect. Having said that I'd still clean it up, just don't go mad.

John Rutzen21/06/2019 07:41:16
348 forum posts
19 photos

I have Alec Farmer's book on boiler making and he says the tubes should not be a tight fit. There must be a gap of 2 to 5 thou. He also said don't try to re-silver solder a leak, you'll probably end up with several leaks. Either use a punch to close the leak or use a tiny piece of comsol with the appropriate flux.

Ian S C21/06/2019 12:10:12
7468 forum posts
230 photos

Soft soldering needs more cleaning to get is right.

Ian S C

Brian Oldford24/06/2019 12:49:40
686 forum posts
18 photos
Posted by duncan webster on 20/06/2019 10:11:44:

Full size boilers are rather different. Firstly they are steel, which rusts and tends to seal any very small leaks, second they have rivetted lap joints, so you have a long leak path. . . . . . .

Although they do usually have riveted lap joints the majority of fireboxes are copper. There are one or two "new builds" that have gone down the welded steel route with "variable" results.

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