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NPT tap use question

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  • #16
    Originally posted by LKeithR View Post

    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...
    That's not true, there is a lot of pipe threaded by home depot and hardware stores with a toy of a threader. I watched one guy miss the finger stop by 1/8 of an inch. So this is why the taps are longer. Never seen a plumber use a ring gage ?? The NPT standards are there and you are correct, that has not changed. You will see pipe with O/S root minors where their dies or tool was damaged. So we tap for the correct engagement. Most dont own pipe rings or plug gages, so I it's old school to get things right. There is a lot of questionable pipes floating around out there ?

    Now back to folding table napkins : • 》
    Last edited by Fasturn; 01-31-2021, 01:01 PM.


    • #17

      No offence taken, but I was asking to learn, as I learned the hard way this time. The tap does not come with instructions on how to use it so maybe a understanding is in order. The internet was not always available, not as an excuse because it is now, but still just learning is always helpful. maybe you could teach me something.
      Next time I will know more and that will help me be successful. I did not run the tap all the way through or even .75 but when checking the 5-7 turns I did go over that. I was able to fix the problem as the fitting is a brass 90* and has the machined shoulder on it so I turned it down and with teflon tape and pipe dope it sealed and all is good.

      Mr fixit for the family


      • #18
        I was told by a pipe fitter
        3 threads to start
        3 threads to seal
        3 threads to show

        and pipe dope is your friend....


        • #19
          The pipe depth gauges are the way to go, but way too expensive for hobbyists.

          Second best and cheapest is to get an American made fittings to use as gauges.

          Be careful not to use Chinese threads as a gauge. I was fixing an under sink leak in old galvanized piping at one of my rentals. Went to Home Depot and bought all new pieces for the repair as I've done many times in the past. I could not get the joints to stop leaking no matter what, Teflon tape, joint compounds, every trick in the book I could think of. Somebody suggested going to a professional plumbing supply. Good old American made fittings did the job.


          • #20

            Once you know the proper depth (possibly by sneaking up on it),
            then grind one flute of the tap away, just above your surface.
            Thereby making your own stop gauge out of the tap that you use.
            I have several pipe taps that I have ground away one flute above
            the gauge line. You never need those threads anyways, and if you
            had to push it deeper, the other 3 flutes would still cut.



            • #21
              Turn the tap in until you have 6 tap threads still showing. That should get you close to threads that gauge correctly. Works for me. Of course if there are 6 threads showing at one flute there will be a fraction more or less at others.


              • #22
                Well, besides tapping next size and bushing, you can machine away a bit of the fitting to allow the thread to enter deeper.
                use lampwick and pipe joint compound together.
                use a nipple instead of a fitting as the pipe has no shoulder and can enter deeper, then use coupling or other fitting to connect to whatever you need.


                • #23
                  Old-timer at work (he was 85...) used to mix horse hair with nickel anti-seize.... damn it worked great, every time... sealed up against hundreds of lbs pressure and yet you could almost unscrew them by hand.


                  • #24
                    Just yesterday I silver soldered some mounts to the bottom of a motorcycle gas tank. While I had the torch out I decided to solder around the welded petcock bungs too. When I was done, I ran a 1/4 NPT tap into two of the holes to clean up the threads and it went ALL the way in. I checked them with a brass fitting and it’s really loose too. Now I’m going to have to come up with a solution before I put sealer in the tank and paint it. I’ve had the tank for five + years and don’t really even remember who I bought it from. Se la vie
                    Illigitimi non Carborundum 😎
                    9X49 Birmingham Mill, Reid Model 2C Grinder, 13x40 ENCO GH Lathe, 6X18 Craftsman lathe, Sherline CNC mill, Eastwood TIG200 AC/DC and lots of stuff from 30+ years in the trade and 15.5 in refinery unit operations. Now retired. El Paso, TX