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Soldering stainless steel pipe

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Tim Stevens05/02/2021 11:17:12
1502 forum posts

I need to join an old brass adaptor to a stainless steel pipe about 32mm (1.25" diameter. The brass is nicely tinned from the old pipe - with lead solder.
The original pipe has been lost in the post ... and the pipe benders say they are only able to supply and bend stainless of the correct diameter.

Because of the large size and weight of the brass fitting, I do not think a soldering iron will serve, so i will use a propane flame, and this rules out using flux with inflammable solvents.

Is there a flux which will serve?

The pipe feeds water from the radiator into the engine of a pre-war MG.


Cheers, Tim

J Hancock05/02/2021 11:36:01
781 forum posts

Is there any possible way you can make this a 'rubber pipe' connection, or compression fitting , anything to avoid

a stress fatigue vibration failure at the brass/stainless joint if using low temp alloys.

noel shelley05/02/2021 11:38:10
873 forum posts
19 photos

For stainless you will need a special flux. contact Cup. Noel.

Nick Clarke 305/02/2021 11:47:09
1293 forum posts
52 photos

As there is lead solder on there already you basically have a choice of leaded solder, lead free soft solder or an adhesive.

On the web you will find people who say you must not mix leaded and unleaded solders and others who say that there is no problem. It seems to be the case that lead free solders can be more brittle than those with lead in.

My suggestion is to use an acid cored soft solder (I have used the Activ8 brand) which is intended for stainless and aluminium (But less successfully than with stainless in my personal experience)

But remember to clean off thoroughly afterwards!

Edited By Nick Clarke 3 on 05/02/2021 11:47:44

Tim Stevens05/02/2021 12:08:26
1502 forum posts

Further details:

It would be fairly easy for me to reduce the amount of lead solder - with a scraper - so that any new tin-based solder was not contaminated much. Even better, perhaps, I could run a blob or two of tin solder around in the heated joint, and tip it out again, so removing more of the lead. Using Tin solder seems a good idea anyway as the soft solders for aluminium seem to be tin based rather than lead.

There is no room for a different type of fitting - the engine block, and complex manifolding, would be in the way - and the engine-end flange is a two-bolt (carburettor-style) fixing. But the free end of the pipe can be held in a clamp where it joins a rubber pipe, giving good support against fatigue effects.

And PS - silly emoji whenever I put something in brackets is becomming a severe irritation. Smiley they are NOT.

And I have contacted CuP - but they sent an Ap for a mobile phone which I do not use. 

Cheers, Tim

Edited By Tim Stevens on 05/02/2021 12:23:13

An Other05/02/2021 12:29:37
220 forum posts
1 photos


About 8 years ago I had to make up a 'custom' water connection for a heating system in our house (not possible to buy the right thing), and eventually I found a short length of stainless steel pipe (which I think came from a shower unit - definitely not chromed steel, and not magnetic). I eventually silver soldered this to a brass nipple at each end to make the connections, using easy-flow flux. (I checked it for magnetism because like you I was anxious about its provenance before soldering, eventually I had no choice but to get on with it)

It is still in use, and has water at a continuous pressure of around 3 to 4 bar in it. It is part of a heating system, and the water is cycled from hot to cold on an almost daily basis.

I did not have any old solder on the parts - the pipe was 'salvaged', but clean, and the nipples were new brass.

I would have photographed it to show you, but now it it is not possible to remove it without draining a lot of water, and I can't get a shot in place. I hope this is some reassurance.

JasonB05/02/2021 12:30:30
21652 forum posts
2495 photos
1 articles

Can you not clean off all traces of the old solder and then silver solder it, HT5 or Tenacity No5 flux will do.

Tim Stevens05/02/2021 12:39:41
1502 forum posts

I am not prepared to risk the hard soldering method. First, the old solder will be difficult to remove completely, and second, the brass fitting is not available as a spare.


Stueeee05/02/2021 12:46:57
105 forum posts

I've used this flux for soldering stainless steel. I was using a MAPP torch rather than a soldering iron. Weblink

I found it really effective; I used a tin/lead solder as recommended by the supplier. Usual disclaimer applies; just a customer etc.

John Rutzen05/02/2021 12:55:55
332 forum posts
19 photos

You need Harris Stay Clean flux for stainless steel. It makes it as easy to solder as brass.

Tim Stevens06/02/2021 12:10:20
1502 forum posts

Thanks, all of you - I now have a pot of flux on the way, and a big roll of no-lead solder, and encouragement all round. The pipe itself has not arrived, but I am prepared for when it does.

Cheers, Tim

Ron Laden06/02/2021 14:39:52
2253 forum posts
446 photos

Well I have learnt something, I never knew that solder would take to Stainless, you live and learn.


noel shelley06/02/2021 14:48:07
873 forum posts
19 photos

Ron, You've never been to a chip shop and noticed that the frying range cabinets are soldered together ? Noel.

Ron Laden06/02/2021 15:35:59
2253 forum posts
446 photos

No I have never looked to be honest I,m too eager to get my hands on what I ordered..smiley

If I had to guess though I would have thought they would be welded but obviously not..

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