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Capillary gaps required for silver soldering.

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Andrew Tinsley17/03/2020 17:42:05
1170 forum posts

I have made half a dozen loco boilers from 3.5" gauge to 7.25" gauge. Also a series of smaller boilers for stationary engines.

I have had virtually no problems with poor penetration of joints. I was idly thumbing through Tubal Cain's book on soldering and brazing, when I saw a chapter about setting up the job with the correct capillary gaps. Suddenly I had a guilty feeling, because I have never knowingly bothered about this aspect of silver soldering!

I must have been very lucky. So to correct my bad ways, how does everyone else produce the ideal gap?


Tim Stevens17/03/2020 17:58:46
1259 forum posts

A simple experiment might help. Set up two small flat plates of your material upright on your soldering base so that one edge of each plate is touching the other, and there is a very small angle (1 degree) between the plates. Then apply your normal flux, heat the assemblage, and run your hard solder into the joint as you normally would. This will tell you how far into a narrow gap the solder will penetrate.

My money is on 'all the way' but others seem to have alternative ideas.

Regards, Tim

Roger Best25/07/2020 17:51:40
82 forum posts
15 photos

I have that book and I remember the comments.

Part of the warning was about gaps being too small on precision parts that were tight together. The other part was about using the solder as a filler for bad work. He is a demanding man. wink

Its not so much about the ideal gap as the ideal solder for the quality of geometry. Will you need something runny to penetrate, or pasty to fill? The book tabulates loads of options but the message I took away was to ask a reputable supplier when I need some and to expect to have to buy a variety for any given model.

IanT25/07/2020 18:32:15
1581 forum posts
145 photos

I'm certainly not an expert Andrew - but it is reasonable to assume that the solder requires some 'gap' to flow into - and also that too large a gap will not be bridged by the solder either. If there is no 'gap' then you may well get a fillet of solder at the 'top' of the joint but may not get good penetration actually into it. With boiler parts, I try for a sliding 'Goldilocks' fit - not too tight and not too loose but just right.

For fabrications, I use Kozo Hiraoka's method of lightly centre-punching a few spots around the area to be brazed - effectively making miniature stand-offs. This seems to work very well for my needs and is a useful way of thinking of the size of gap required generally - e.g. some but not much.

I also try to apply the heat in such a way that the solder is drawn into the joint where possible (it will flow towards the hottest area).

So "some" gap and thoughtful application of the heat is the best I can advise others.



pgk pgk25/07/2020 19:53:27
1887 forum posts
288 photos

I'd guess that the main effect causing flow into a gap is surface tension (as in a meniscus and capillary tube?) I that's the case then I find it interesting that silver solder flows towards the hot end since with water the capillary pressure decreases with temperature...
Presumably the flux applied into a joint before assembly also acts as a spacer?


Andrew Tinsley25/07/2020 20:39:14
1170 forum posts

Sorry for neglecting answers to my post. I have another two boilers to make so I shall be extra careful of my "gaps". It is odd that in all the boilers I have made, I have had only two very small leaks. I was quietly thinking that I was becoming an expert boiler maker. However Mr Walshaw took me down a peg or three!

I must have been taking note of clearances without really taking it on board, Otherwise I would not have got away with so few leaks. All the books on silver soldering / boilermaking make great play of the correct gaps and even quote figures. None of them give any tips on how one is to achieve this! Ian's mention of someone using ccowellsentre punch marks to define the gap is the first example I have heard of that is practical.

I put my success (luck!!!) down to using very large propane burners helped out with the odd oxy -acetylene torch. Like most people, my early mistake was not having the joint hot enough.



Andrew Tinsley25/07/2020 20:40:28
1170 forum posts

Well I never, how did the Cowell's advert get in on the act???????


Bob Stevenson25/07/2020 23:21:36
425 forum posts
7 photos

It's curious how this keeps coming back on this site.....I think it must be factored by what you are soldering and what you desire as the ideal outcome........ In makingn clock parts I try to get invisible joints that finish well with total penetration. as such i always try to get as tight a fit as possible and have never had any problem with silver solder faioing to 'run' when using enough heat and good flux. The danger with the 'gap method' is that molten silver solder is extremely 'runny' and percolates the tiniest tightest gaps by capillary action but is very bad at filling any larger gaps......

For clock parts I will stick with the methods that have served me well and always test that the solder has run thru to the other side of the a fine line which is easy to polish out to invisibility.

pgk pgk26/07/2020 07:38:01
1887 forum posts
288 photos

When using a centre puch to create a solder gap are you punching from outisde the joint surface to raise a dimplel through the material or punching within the (future) joint surfaces and depending on a raised crater edge?

Does anyone place scraps of silver solder foil near ,say, rivet points to prevent those being overtightened?


Brian H26/07/2020 07:47:50
1741 forum posts
112 photos

I believe that the normal method of providing a gap is to use a punch to make raised edges to the edges.

If using silver solder foil there is no need (or point) in using a centre punch to provide a gap, the foil has already done that.


Andrew Tinsley26/07/2020 10:45:20
1170 forum posts

Thinking back to making boilers. I have always used what I consider to be a sliding fit for the components. I now am becoming more convinced that it is the flux itself that creates the gap on assembly

Bob you make an interesting point about tight fits. I am not too sure that some of the higher melting point solders are that runny, those with a high solid to liquidus range especially. However as long as you are well above the liquidus range, I suppose it doesn't matter.

The use of foils on assembly sounds a good idea and one that I have considered. However I have a nagging doubt about this method. Somewhere I remember reading that there is a snag . Maybe it was in Keith Hales book, wherever it was, I can no longer find this reference, perhaps someone on the forum can remember seeing it? Maybe it was the price of the foils, but i think it was a more fundamental problem.


Neil Wyatt26/07/2020 11:38:50
18128 forum posts
713 photos
77 articles

I think the problem with foil is that if the two halves of the joint are not able to 'settle' it may be partially starved of solder.

I generally go with the centre punched dots.

I think the gap can be pretty small, as long as it's there.


Graham Meek26/07/2020 11:43:59
253 forum posts
184 photos

I use a spring loaded centre punch to get consistent punch dot sizes and thus a consistent gap around the joint.



Norman Billingham26/07/2020 14:03:37
39 forum posts

The capillary gap can be very small indeed. There are published data for tensile strength of silver brazed copper joints with different gaps. The optimum gap is about 1.5 thou which gives a tensile strength of 0.9 GPa (135,000 psi). Smaller gaps give weaker joints by a very small amount, so it falls to about 0.7 GPa (100,000 psi) for a nominally gap-free fit. It’s thought that may be because it’s harder for the molten alloy to push flux out of a very small gap. However, the tensile strength is still 0.34 GPa (50,000 psi) at a gap of 20 thou. The ultimate tensile strength of annealed copper is about 0.2GPa (32,000 psi) so even with a 20 thou gap the joint will still be stronger than the metal. Bearing in mind that the diameter of a silver atom is about 350 pm (0.000014 thou), a gap of 1 thou will be enough space for more than 70,000 atoms of silver.

Andrew Tinsley26/07/2020 15:11:38
1170 forum posts

Thanks Neil. Your comment on the use of foils rings a bell. I am sure that is the objection i have come across. I wonder where it was?

ron vale 103/08/2020 23:31:02
39 forum posts
5 photos

Cup alloys IIRC recommend a gap of 0.1mm

Hopper04/08/2020 06:25:51
4768 forum posts
104 photos

Luckily my tinsmithing is rough enough that the small gaps are inherent.

Yet with any other piece of metal we are told we have to surface grind and scrape and lap and hone, rub with engineer's blue on a surface plate supported on three equilateral points set perfectly level to the nearest body of water and invoke magical incantations to get things to match each other closer than a thou or two.

Edited By Hopper on 04/08/2020 06:28:59

CuP Alloys 104/08/2020 07:47:34
249 forum posts

The joint gap is fundamental to the success of silver soldering.

The laws of thermodynamics explain why silver solder flows to where it is hottest.

The flux does not create the gap.

This is explained in my book along with techniques on creating and maintaining gaps.

There is 53 years experience of silver soldering between the book's covers.

For more information, or to obtain a copy, send me a message.



Usual disclaimer does not apply!

Michael Gilligan04/08/2020 07:57:02
16190 forum posts
706 photos
Posted by CuP Alloys 1 on 04/08/2020 07:47:34:


Usual disclaimer does not apply!


yes Nicely put


Andrew Tinsley04/08/2020 10:06:36
1170 forum posts

,Hello Keith,

I do indeed have the book! There has been lots of replies, but only a couple answered the question. To remind people, it was "How do you set up the correct gap for the alloy that you are using" I may well be wrong, but I don't think that even your book addresses the question.

There are lots of references saying what the gap should be, but virtually nothing on how to achieve said gaps. Apart from some information in Alec Farmers book re brazing tubes in the inner firebox plate.

For example I have seen people, including myself, riveting a structure to hold things together during brazing. The gap around the rivet must be close to zero, yet it still seems to solder well.

I must have been very lucky , because I never gave gaps a thought in the past, unless it was subconsciously.and I have never had a major problem.



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