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Thread: Pipe thread taper angle

  1. #1
    Join Date
    Jan 2009
    Location
    Salmon Arm, BC, Canada
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    191

    Default Pipe thread taper angle

    Tried a search, but no joy.
    Anybody know the taper angle of 1/8" NPT ?
    Trying to make my own weld bung from a piece of 303 stainless. Tap says "Drill 21/64" on it but that's no easy deal, screwing that tap into a hole that size to any depth. Got it down to where a fitting screws in about 4 threads and "I think I better stop before I break something". I want my fitting to screw in deeper, I'm thinking I should bore a tapered hole, but don't know the angle.
    Anybody ?
    Thanks !

  2. #2
    Join Date
    May 2007
    Location
    Live Oak, TEXAS
    Posts
    1,540

    Default

    All NPT threads is 1⁄16 per inch (3⁄4 inch per foot) measured by the change of diameter (of the pipe thread) over distance. The angle between the taper and the center axis of the pipe is 1 47′ 24″ (1.7899).
    NPT Reamers work well too.

  3. #3
    Join Date
    Aug 2006
    Location
    Taylorsville Ky
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    5,882

    Default

    The correct depth of the pipe tap allows the fitting to go in the hole 3 to 4 turns by hand. If your using a taper pipe tap in stainless steel you need a tapered pipe reamer.
    It's only ink and paper

  4. #4
    Join Date
    Jul 2005
    Location
    Burnet, TX
    Posts
    2,128

    Default

    I have been a longtime advocate for dark cutting oil but for tapping stainless MolyD works better. Use a tapered pipe reamer and go full depth. The OSG elektraLUBE taps are the best I have found. The Interrupted thread taps also help in difficult situations.
    Byron Boucher
    Burnet, TX

  5. #5
    Join Date
    Jan 2010
    Location
    Kansas City area
    Posts
    3,240

    Default

    For doing tapered pipe threads I have found that going in 3/4 of the tap works out about right when the fitting is tightened up. If you take the tap to full depth the fitting (usually) runs out of threads before it's tight. The tapered pipe reamer does help a lot, as do the interrupted thread taps.

  6. #6
    Join Date
    Dec 2007
    Location
    Castlegar BC Canada
    Posts
    1,153

    Default

    Leave 7 threads showing on the tap usually works for correct depth
    The difficult done right away. the impossible takes a little time.

  7. #7
    Join Date
    Jan 2006
    Location
    Preston, Connecticut
    Posts
    524

    Default

    While the info posted is correct (1 degree 47 minutes 24 seconds) may I add HOW to find that info by calculation.

    The formula to find the angle of a taper with the center line (half the included angle) is Tan 1/2A = TPF/24

    So, if the TPF (taper per foot) of a pipe thread is .750 then the Tan1/2A = .0313 and the angle is 1 degree 47 minutes 24 seconds.



    Errol Groff

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