NPT thread length to tighten

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Lakee911, Oct 12, 2009.

  1. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    I need to get some 3/4 pipe cut and threaded. On average, how much threads in to the coupling?
     
  2. nhmaster

    nhmaster Master Plumber

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    There should be 7 perfect threads and 3 to 4 imperfect threads. tighten 3 turns by hand and 3 by wrench.
     
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  4. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    Sorry .... I should have been more specific ...

    From fitting to fitting let's say I need 18" ... how long should the pipe be? In other words, how much (in length) threads in?
     
  5. nhmaster

    nhmaster Master Plumber

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    Ah, 3/8 + 3/8 or 3/4"
     
  6. Winslow

    Winslow Plumber

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    measure from center to center then deduct 3/4 inch per fitting (11/2 inches total).
     
  7. jimbo

    jimbo Plumber

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    3/4 per fitting is probably closer to accurate. But you have to remember that this is never an exact science. Threads vary, depending on who cut it , how the dies were set on the machine, etc. Factory threads on a piece of pipe will often be different that field cut threads. Adding up a fitting on each end of a pipe, there could easily be 2 to 4 turns total difference in "tight". The experience gained over time ( we call it wisdom, others call it learning from your mistakes!) helps to make these things work out.
     
  8. hj

    hj Moderator & Master Plumber Staff Member

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    engagement

    The setting of the die, the way the fitting was tapped, the size of the pipe wrench, and YOUR build will all conspire to determine the answer to your question. It is NOT an exact science so you have to use some discretion in your measurements.
     
  9. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    Thanks. :)
     
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