- Thread starter awemawson
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Braze or silver solder, using a borax-based flux. Ordinary tin-based solders will not produce a decent joint. Heat would be best delivered using an oxy-fuel torch. Gas-air mixes won't be hot enough. Find a few pieces to practice on before going for the proper joint. 0.15mm is quite a large gap for some silver solders!
Kayos
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I'd use a big compression or press fit adapter on that, sod soldering 54mm
https://www.bes.co.uk/compression-straight-coupling-54mm-x-54mm-17594
https://www.bes.co.uk/compression-straight-coupling-54mm-x-54mm-17594
Seadog
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Agroshield
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Maybe give the people at CuP Alloys a ring. They are very model-engineer friendly and are always willing to give advice.
Stueeee
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I would soft solder that joint if you don't need a high degree of resistance to vibration. I've used this flux on stainless steel joints: https://soldersandfluxes.co.uk/P584/A8-Stainless-Steel-Flux.html the solder flows on stainless just like it would on brass or copper. The fumes are hideous so make sure you have good ventilation.
If you need high mechanical strength or good vibration resistance silver solder would work, but a joint of that size would need a lot of heat, even Tenacity flux (AKA HT5 flux) might well get cooked during the heat up. Making a silver soldered joint would certainly be easier if you can get hold of some easiflo No2 rather than its 55% Ag Cadmium free replacement which really doesn't flow around a joint especially well in my experience.
If you need high mechanical strength or good vibration resistance silver solder would work, but a joint of that size would need a lot of heat, even Tenacity flux (AKA HT5 flux) might well get cooked during the heat up. Making a silver soldered joint would certainly be easier if you can get hold of some easiflo No2 rather than its 55% Ag Cadmium free replacement which really doesn't flow around a joint especially well in my experience.
Seadog
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As an aside, in domestic boilers you often see heat exchangers with no direct coupling. The means of connection is with an internal or external fitting sealed by O-ring. The joint being maintained by the mounting of the exchanger.
That flux is zinc chloride / HCl - based. I can't comment on the specific product in the link, but "simple" ZnCl/HCl flux was never good on stainless.
I received a little bottle of Zinc Chloride based flux this morning from CUP Alloys called Harris STAY-CLEAN. A quick test on a (much smaller) random stainless fitting shows that it wets nicely with tin / lead solder so I must pluck up courage to attack the actual 54 mm fittings
brightspark
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its not a problem doing it as long as the stainless is rubbed up clean and the proper flux used I used to have to re solder spouts on stainless steel russel hobbs kettles when they bust off
Seadog
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I assume you've discounted my comment at #11? It may be easier to turn a couple of pieces and groove them for O-rings than to attempt soldering a large lump like that. But then again, you do like a challenge 
I worked out the cost of either stainless or brass slugs to machine a pair of O ring fitting retained by a V clamp. Metal was in excess of £40 (needs minimum of 75 mm to match the existing flange) and the V clamps that I could find weren't really suitable as the exiting flange lip is only 3.75 mm so somehow I would have had to make the V clamp
Seadog
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I might have guessed you'd looked at it.
Why do you need V-clamps. Since you seem to be mounting it against a flanged part, would that in itself not be sufficient to hold the assembly together? The tubes just need to be a nice slide fit to remain pressure tight.
Of course, obviously I don't have a picture in my head to understand how this all goes together, but it doesn't stop me suggesting things you'll be sorry to hear
Why do you need V-clamps. Since you seem to be mounting it against a flanged part, would that in itself not be sufficient to hold the assembly together? The tubes just need to be a nice slide fit to remain pressure tight.
Of course, obviously I don't have a picture in my head to understand how this all goes together, but it doesn't stop me suggesting things you'll be sorry to hear
Impressed. What grade of stainless was it, out of interest, and did the solder definitely flow down the gap between parts? Looking at the images, the heat exchanger ports are brazed, or silver soldered on, so trying to braze could have ended in tears.
