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Help! Npt sizes not fitting!

  1. Sep 27, 2010 #1
    Ok, so here's a weird problem. I am trying to make a 1/4 npt fitting with two pieces of brass pipe.

    The tap says:

    1/4 NPT 18
    HSGG M21

    The die says:

    1/4 NPT
    Baystate -5

    So I threaded the pieces of pipe, expecting them to fit together nicely... and they didn't - far from it. I found that they were screwing in at an angle, and stopping after a short distance.

    Needless to say I'm mystified by this. I've tried threading several pieces, being careful to make nice straight threads, and still this problem remains. I suspect it must have something to do with that "-5" on the die, but I haven't been able to find any references explaining what it means.

    Please help!
  2. jcsd
  3. Sep 27, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    I would imagine because it isn't a 1/4 NPT 18 die. I don't know what "baystate -5" means and I don't think it has to do with the die size (manufacturer maybe?). Regardless, your die probably just doesn't have 18 threads per inch like the tap does.
  4. Sep 27, 2010 #3
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