I bought an Isenberg shower valve I need to stub out to 1/2" PEX and it only has 3/4" NPT connections. I will be using pipe dope on the 3/4" adapter but can I install it first and then sweat on the elbow after without screwing up the pipe dope connection on the adapter? Or should I install the adapter fully with pipe dope, tighten it down, mark the adapter and elbow, remove and solder, then thread the whole thing back in?

shower valve with copper and pex fittings

  • Do you have room to install a bit of straight pipe then elbow? If so could solder to the NPT then screw in, then solder the elbow
    – Chris
    Mar 21, 2022 at 3:45

1 Answer 1


Note that fittings to do this directly (in one piece) exist, and they are fixed. So you just thread them in until "tight enough" and "points the right direction" are both satisfied.

Image from supplyhouse.com, no affiliation nor endorsement

In other words, solder it and thread it in place without bothering to pre-fit, it's not needed, and the soldering heat will likely destroy your shower valve with threaded connections. Valves with solder-type connections should be built to withstand soldering heat, threaded ones are generally not.

Or return these parts and buy the single part that replaces all three of these parts, with less points of failure.

If that makes you nervous, use a straight MPT-PEX adapter and a PEX elbow, which will permit fiddling with the threaded part, since straight PEX connections act as a union for threaded pipe fitting purposes (you can rotate them.)

  • I have not seen a 1/2” PEX-A expansion x 3/4” male threaded elbow fitting. If you can point me to one, I’d appreciate it. I should have clarified I’m using expansion PEX (PEX-A). The pictured fitting is a PEX barb fitting.
    – Vincent
    Mar 20, 2022 at 17:33

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