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I need to solder relatively close to a copper-steel adapter that has pipe dope.

I will put the adapter on a copper extension outside the assembly and then twist it on with dope, but the extension is only about 3-4" because it needs to make a turn due to a tight space.

I can't mount the elbow outside the assembly because then I won't have space to turn it.

My concern is that, if I heat with MAPP gas that close to a threaded joint with dope, I will cook the dope off the joint, and then might introduce a leak.

It's 1/2" pipe on both copper and steel side.

How close to the threaded, doped joint is it safe to heat the copper in order to apply a sweat fitting?

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    That's too close without a heat sink. Wrap a wet rag around the threaded fitting. When you're done, turn the rag to take out more heat.
    – isherwood
    Jul 13 '20 at 18:45
  • @isherwood I think that's an answer.+ Have done that splicing paper & lead utility cables.
    – JACK
    Jul 13 '20 at 18:50
  • Maybe, but the question asks "how close" rather than "is this too close". I'll leave it to one of our resident plumbers to make a more reliable estimate. Of course, it depends on technique as well.
    – isherwood
    Jul 13 '20 at 18:52
  • Wrapped pipe and adaptor with wet rag, only left about an inch to inch & half free otherwise you can get a cold joint.
    – Solar Mike
    Jul 13 '20 at 18:58
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    It is a two edged sword ; if you cool copper with a wet rag ,it will pull heat away form the joint so fast you probably will not melt the solder. Jul 13 '20 at 19:48
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When I sweat close to wood or another fitting I wet a rag(s) and wrap around the pipe. You want the rag dripping wet. I do this with both water pipes and on hvac with oxy acetylene using silver solder. The wet rag stops the heat but don’t take forever get it hot wet it with solder and get the heat off as soon as it wicks.

Do not move the rag let the joint cool if you move it prior to becoming solid you end up with a cold solder joint.

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  • Yeah, but you're cheating with oxy. To pull this off with mapp I use a dual headed AC torch head. With either, the whole operation should take like 15 seconds, and if you can't get it done in that amount of time, either your torch sucks or you're not good enough at it yet to do close quarters. I spent at least my first decade with both of those being the problem. After using oxy... man, wtf was I doing ;p
    – Mazura
    Dec 12 '20 at 3:42
  • I usually use mapp but provided this example because oxy is much hotter, true dual heads are better but with mapp I have been able to solder copper since 60/40 lead was outlawed , I tried with propane for quite a while and had leaks went to mapp and never had another problem with lead free solder. Unfortunately mapp is not hot enough for 15% silver or I would not even use oxy any more but I use that for refrigerant lines.
    – Ed Beal
    Dec 12 '20 at 14:48

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