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Soldering MPT to FPT

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  • Soldering MPT to FPT

    Repaired a Central shower mixing valve that had a union that was cracked on the FPT side. While buying a new one a salesman (retired master plumber w/34 years in the trade) told me that's a common problem. He said the way to do it right is to clean and flux the FPT and MPT, hand tighten them and solder the joint together. Worked beautifully. Can't tell you the number of times I tightened threaded joints and soldered the adapter just to have it leak at the threaded joint. Thought I might pass this on.
    Buy cheap, buy twice.

  • #2
    Re: Soldering MPT to FPT

    I've seen that problem occasionally and a few soldered MPT/FPT connections, few and far between though.

    Usually I don't like to directly sweat a male adapter thats screwed into a FPT. If I really need to due to extenuating circumstances, I usually teflon tape first, then add Hercules Mega-Loc instead of Pro Dope. I also wrap a strip of wet rag around the threaded area for a heat sink.

    If I have the room, I will cut further back on the pipe. Pre sweat the male with a short nipple, then connect it to the FPT, wrap it with a wet rag (depending on distance), then solder the coupling last.

    Thanks for the tip!



    • #3
      Re: Soldering MPT to FPT

      I usually solder in the mip or fip before I thread it onto the fitting. I may not be doing something different.

      Ben and I were discussing this today on the phone, I am curious what the others are going to say about this, good job posting it Ben

      On tight spots, anyone use the Heat putty, you can hold the copper while soldering it, with the putty in the middle, between your flame and your hand.

      I priced it at the supply house, $35.00, I will continue to do what I have done for years