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  • NPT tap use question

    Hi Group,

    I needed to tap a 1/4" NPT hole in a piece of 3/8" plate for a project. My question is after doing it I have a leak, the fitting has bottomed out in the threaded hole by a shoulder on it. Apparently I tapped it too deep using a hand tapper tap. What depth do you use or know when is enough?

    TX
    Mr fixit for the family
    Chris

  • #2
    I stop at around 5 full turns. I think that the NPT spec calls for 7 turns (full threads). If I check the fit and it isn't enough, I will give it one turn at a time until the fit is correct.

    Comment


    • #3
      There are several sources on line for charts giving the dimensions for pipe threads. Here is just one example:

      https://www.engineersedge.com/hardwa...pe-threads.htm

      Also, if you have a copy of Machinery's Handbook there is quite a bit of information there.

      Comment


      • #4
        4 threads from the top, back out, check fit, go one more if it's too tight. Or if you get lucky like I did, you score a full set of 1/16 thru 3" NPT thread gauges for cheap.
        I just need one more tool,just one!

        Comment


        • #5
          They make a thread plug gage and it has a flat ground on it.
          That is the depth you tap. With junky pipes, just creep up on it so you dont go too deep. Pipe tape ( Teflon ) can be your best friend when you go too deep. Not sure what you are doing, but can you counter bore it so you have a tighter thread?
          Attached Files
          Last edited by Fasturn; 01-30-2021, 09:25 PM.

          Comment


          • #6
            Originally posted by Fasturn View Post
            . . .With junky pipes, just creep up on it so you dont go too deep. . .
            Seems like the majority of chinese made male pipe threads are cut way too far and they bottom out before sealing.

            Southwest Utah

            Comment


            • #7
              Originally posted by chipmaker4130 View Post

              Seems like the majority of chinese made male pipe threads are cut way too far and they bottom out before sealing.
              I agree , ya just have to creep up on what you are tapping The pipe threads today, are all over the place dimentionally.

              Comment


              • #8
                Originally posted by Fasturn View Post
                ...The pipe threads today, are all over the place dimentionally...
                Pipe threads today are exactly what they've been since the NPT standard was established. What does
                vary to some extent is the length of pipe taps--some are longer than others and, yes, you can go too deep
                and make your (female) thread oversize. The only way to ensure that your thread is correct is too gage it,
                either with the fitting you're going to use or with a proper pipe gage. Counting turns will get you close but
                not always right. Of course, if you're doing multiple holes then, yes, you can gage the first one or two and
                then count turns to arrive at the same point every time...

                Keith
                __________________________
                Just one project too many--that's what finally got him...

                Comment


                • #9
                  I have always heard go X amount of threads from the end when using pipe taps.

                  Why don’t they just manufacture taps so the threads stop at the correct depth so you can just run the tap in until there are no threads showing and you are good.

                  Comment


                  • #10
                    Originally posted by Mr Fixit View Post
                    Hi Group,

                    I needed to tap a 1/4" NPT hole in a piece of 3/8" plate for a project. My question is after doing it I have a leak, the fitting has bottomed out in the threaded hole by a shoulder on it. Apparently I tapped it too deep using a hand tapper tap. What depth do you use or know when is enough?

                    TX
                    Mr fixit for the family
                    Chris
                    So the hole has a shoulder or inner ledge or the male thread has a shoulder? Which is it?
                    Either either way you're going to have to sneak up on it and or may have to trim the end of the male thread until you get the desired fit.

                    JL.....

                    Comment


                    • #11
                      I have some but have never used it but for oversized threads this is what I would try. Locktite 55 thread sealing cord. Don’t know how good it might work, but it might fill the gap.

                      Comment


                      • #12
                        Originally posted by Fasturn View Post

                        I agree , ya just have to creep up on what you are tapping The pipe threads today, are all over the place dimentionally.
                        ^^^^^^
                        It's amazing to me that china makes nearly all of the easily available "iron" fitting cheaper than the USA does. Given the weight of shipping and the fact that the garbage Chinese fitting don't fit that is. It's typical Chinese junk that's soft and the thread tolerance is all over the map.

                        Comment


                        • #13
                          The problem is not the pipe fittings.
                          The problem is the user does not know how to properly use a pipe tap.
                          No feel good excuses. Just lack of knowledge.
                          Not trying to be a dick, just saying what I interpret here.


                          -D
                          DZER

                          Comment


                          • #14
                            Gee Doozer, I thought that amateurs and others without experience came to this forum to learn, not be told they lack knowledge. But I guess not. They have to be perfect and experienced first.
                            Vitَria, Brazil

                            Comment


                            • #15
                              Is not lack of knowledge something you have before the learning process takes place?
                              What's your beef? Does stating the obvious offend you for some reason?
                              Why the butt ache ?
                              I will absolutely appologize if I was being a dick. But I was not trying to.
                              -D
                              Last edited by Doozer; 01-31-2021, 11:10 AM.
                              DZER

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