I have a copper water pipe that had a plastic "reducing adapter" on it, which I believe is 3/8 inch x 1/4 nptf. I unscrewed it, and now I can't screw it back on. It feels like the copper pipe is just a little too big for the adapter.

I went the hardware store and was able to screw the reducing adapter on their size tester. I bought a new reducing adapter which also fits the size tester, but the new one won't screw onto the pipe either. Does anyone know how this could happen and what I can do about it?

This suggests that there is something wrong with the copper pipe, but I run my finger around the threads and find no issues.

The pipe carries cold water.

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  • You might reconsider attaching male metal threads to female plastic threads. For tapered threads, anyway.
    – popham
    Commented Oct 25, 2023 at 2:35
  • It's very likely that the metal threads mangled the plastic ones and that's why it won't go back on.
    – FreeMan
    Commented Oct 25, 2023 at 12:57

3 Answers 3


It looks as if you have in your hand a copper water pipe, a brass male-to-male adapter, and the white plastic fitting.

Have a closer look at the male threads. From here it looks as if that brass adapter might have a compression thread on top, not a pipe thread. The bevel in the mouth of the fitting is one feature that brought this to mind, but also the threads on the top half of the fitting look a little more fine-pitch than the threads on its bottom half.

If I'm correct about that brass part having a compression thread then you have the wrong kind of adapter. In fact, there do not exist adapters that would only screw to the outside of a compression fitting. This is because in a compression fitting the threads do not make the seal -- that bevel in the mouth of the fitting does.

  • Is the theory that someone in the past had just forced it on? Commented Oct 25, 2023 at 6:52
  • @GlennWillen OP might just have got a new adapter with the wrong type of threads. Unless an expert on pipe threads, usually better to bring both halves when matching.
    – crip659
    Commented Oct 25, 2023 at 12:12
  • THIS. I agree that the brass adapter on the end of the pipe is a pipe thread to compression thread adapter. You will need to remove it and get the proper size pipe thread to pipe thread adapter. Remove it and take all those fittings to a plumbing shop instead of the ubiquitous big orange box store. Commented Oct 25, 2023 at 15:24
  • 1
    @GlennWillen Yes, that's what I think may have happened. The top-half threads are covered in white PTFE tape residue. If it is a compression fitting those threads should be completely clear of sealant since compression threads don't seal. A person who puts PTFE tape on compression threads might also force entirely the wrong fitting onto those threads, unaware of their mistake.
    – Greg Hill
    Commented Oct 25, 2023 at 16:46

Typically this means you don't actually have the two items properly aligned, and they are "cross threading" rather than going together as they should due to the misalignment.

You can attempt to use the "turn it backwards until you feel it drop oneo the thread" trick, but that only gets the starting thread in the right spot, it does not compensate for an angle that shouldn't be there.

Given that pipe threads are tapered, the end of the male fitting is normally quite a bit smaller than the end of the female fitting, but that very thing also makes it easy to misalign them.


It's bad plumbing etiquette to install a metal male fitting into a female plastic fitting (the female fitting will fail--if not from overtorquing, then from expansion of the metal part under corrosion), so you should throw away that plastic fitting. Take the male-to-male adapter off of the water line, noting which end was connected to the existing water line. This process takes two wrenches: One to work the fitting and one to constrain the water line from stressing too much as you work the fitting. Take the adapter to the store with you and check the threads of the noted end, not the end that refused to mate with your plastic fitting. These are probably NPT/MIP threads. Find an adapter, plastic or metal, transitioning your water line's female threads to your sought joint type.

  • Carp! Do I have to go pick up the Miss Manners of Plumbing Etiquette's book now? makes a note...
    – FreeMan
    Commented Oct 25, 2023 at 18:08
  • 1
    @FreeMan, I looked and looked, but couldn't find anything in my copy of Emily Post. codes.iccsafe.org/content/IPC2021P1/…: "Joints shall be installed in accordance with the manufacturer’s instructions." codes.iccsafe.org/content/IPC2021P1/…: "Mechanical joints shall be installed in accordance with the manufacturer’s instructions."
    – popham
    Commented Oct 25, 2023 at 18:46

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