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I messed up the soldering of this 1" copper union (solder went into the threads) and the threaded joint (not the soldering) is having a very small leak, yet noticeable.

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I will replace the union but I feel it could be stopped by teflon tape. I understand unions are designed to not require tape/dope but I was wondering if there is any harm in using it if it stops a leak.

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  • Does that not have an O ring inside to seal the union? If the solder is stopping you from tightening the nut all the way you may be able to just file it out, but the o-ring makes the seal.
    – JPhi1618
    Oct 21, 2021 at 15:52
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    Not all unions have an o-ring seal, many are just straight compression of mating surfaces.
    – JRaef
    Oct 21, 2021 at 16:46
  • no O-ring @JPhi1618
    – amphibient
    Oct 21, 2021 at 18:49

2 Answers 2

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A leaky union is probably caused by one of the following:

  • misalignment of the two parts -- not sufficiently close to a straight line
  • debris between the mating surfaces (maybe there's solder here too..?)
  • not enough compression squeezing the surfaces together
  • damage to the mating surfaces of the two parts

Examine the work carefully and figure out which of these could be at play.

It's possible solder on the threads could be preventing the nut being tightened enough. It might not be necessary to remove the solder.. merely making it flow around into a thinner layer might be good enough. Spread flux all around and re-heat it so that the solder flows over a wider area.

Solder can also be removed with some effort by sanding, as with the emery cloth you might have used to polish up the copper pipe before soldering.

Tape or dope on the threads definitely won't help the leak -- it'll just start leaking out the other side of the nut instead.

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    Re point 2: I quote the first line of the OP's question, "I messed up the soldering of this 1" copper union (solder went into the threads)". You seem to acknowledge that after the listing of leak reasons, but this was the first thing that hit me - "OP knows why the joint is leaking!". I guess your point is that may not actually be why it's leaking...
    – FreeMan
    Oct 21, 2021 at 16:05
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I have wrapped the male mating surface in ptfe tape when the surface is slightly damaged and it can then seal.

But it is not the best way.

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