How deep to tap NPT internal threads

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  • Bill Hopcraft
    Junior Member
    • Mar 2007
    • 15

    How deep to tap NPT internal threads

    For internal NPT threads, I've found lots of references that talk about running a tap in for twelve threads to get the proper fit. What's the math behind this?
  • japcas
    Senior Member
    • Nov 2004
    • 1149

    #2
    At work we use npt gages so there is some consistency from one person to another. At home I usually just buy a pipe plug the same size that I'm tapping and use it for a gage to determine how deep I need to tap the hole. Just use the "looks about right method" with the plug. It's cheap, simple, and works. Just the way I like it.
    Jonathan P.

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    • PixMan
      Senior Member
      • Sep 2009
      • 2077

      #3
      My old instructor's command is the "3 to 3-1/2 turns" thing. He told me to shoot for 3-1/2 turns, meaning: Tap the hole conservatively. Using a pipe plug, screw it in to finger tight, then count how many full turns it takes to pop out of engagement and be loose in your hand. When you have 3-1/2 turns, it's done.

      I don't have any of the official gauges at home now, so this method is all I have. Thankfully, it seems to work.

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      • winchman
        Senior Member
        • Aug 2003
        • 4030

        #4
        This should help:

        Any products mentioned in my posts have been endorsed by their manufacturer.

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        • FabRight
          Junior Member
          • Oct 2009
          • 9

          #5
          I usually run the tap in until there are 7 threads left showing above the part. Not very scientific, but it works for me.
          Mike

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          • winchman
            Senior Member
            • Aug 2003
            • 4030

            #6
            A little more looking yielded this:

            "To achieve the basic thread depth, a machinist typically drives the tap into the workpiece 12 turns."

            "The application, with proper gaging, will determine the appropriate depth for the proper mating of the parts."

            From: http://www.ctemag.com/dynamic.articles.php?id=59
            Any products mentioned in my posts have been endorsed by their manufacturer.

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            • j king
              Senior Member
              • Apr 2003
              • 909

              #7
              I leave 5-7 threads showing on tap. Never had problems.

              Comment

              • macona
                Senior Member
                • Aug 2006
                • 9425

                #8
                Originally posted by winchman
                A little more looking yielded this:

                "To achieve the basic thread depth, a machinist typically drives the tap into the workpiece 12 turns."

                "The application, with proper gaging, will determine the appropriate depth for the proper mating of the parts."

                From: http://www.ctemag.com/dynamic.articles.php?id=59
                I have used the 12 turn method and it works fine.

                Comment

                • lane
                  Senior Member
                  • Aug 2005
                  • 2691

                  #9
                  Was taught to leave 5-7 threads out . Found out by gaging many many pipe taped threads that 6 thread will be just about perfect. so hardley even use gages any more.
                  Every Mans Work Is A Portrait of Him Self
                  http://sites.google.com/site/machinistsite/TWO-BUDDIES
                  http://s178.photobucket.com/user/lan...?sort=3&page=1

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                  • Oldbrock
                    Senior Member
                    • Dec 2007
                    • 1157

                    #10
                    yup

                    tap until there are 7 threads showing. Seems to work for lots of us. Peter
                    The difficult done right away. the impossible takes a little time.

                    Comment

                    • JoeLee
                      Senior Member
                      • Aug 2009
                      • 10873

                      #11
                      I've ran into this problem several times in the past. I also had a thread going a while back. I guess I'm not the only person that has problems with threadding pipe, or pipe threads. I have finally dawn the conclusion that plumbing is the art of cobbing, so just thread to fit.

                      JL.............................

                      Comment

                      • thebusdweller
                        Member
                        • Nov 2005
                        • 32

                        #12
                        The depth depends on the pitch dia of the tap. The 1/4-18 npt taps I use at work range from .690 to .550 deep. A guage would be best to be 100% sure.

                        Comment

                        • BobWarfield
                          Senior Member
                          • Nov 2005
                          • 1644

                          #13
                          It's in the thread spec:



                          See, 5 turns there.

                          Since it's a tapered thread, it matters how many. Note the "hand tight" spec as well.

                          Cheers,

                          BW
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                          • ulav8r
                            Senior Member
                            • Oct 2002
                            • 1274

                            #14
                            BW,

                            I see 7 1/2-8 turns??????????
                            North Central Arkansas

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                            • BobWarfield
                              Senior Member
                              • Nov 2005
                              • 1644

                              #15
                              5 still sticking out.

                              BW
                              ---------------------------------------------------

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