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1/16"- 27 NPT Drill Size ??

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  • 1/16"- 27 NPT Drill Size ??

    I know I asked this once before but I can't find the post.

    Drill size for 1/16" -27 NPT........ I'm finding two different drills listed for this size.

    C which is .242 and D which is .246 which one should I use ??


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

  • #2
    I would imagine, like all threads, it depends on the percentage of thread desired. With the 2 sizes you listed, they are only 4 thou apart so the difference in thread percentage will be minimal. Personally, I would try the smaller size first and see how hard it taps, if its uncomfortably tight then open up the hole with the larger bit. A few percentage difference in thread depth only makes a very minimal difference in strength but can make tapping much easier.

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    • #3
      Without a tapered pipe reamer I'd go with the D size size drill, especially if the material you are working with is something harder than brass or aluminum. Correct me if I'm wrong but I believe the C size is for when using the reamer.
      With stainless for example I always give myself as much leeway as possible. Works for me.
      Home, down in the valley behind the Red Angus
      Bad Decisions Make Good Stories​

      Location: British Columbia

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      • #4
        Originally posted by JoeLee View Post
        I know I asked this once before but I can't find the post.

        Drill size for 1/16" -27 NPT........ I'm finding two different drills listed for this size.

        C which is .242 and D which is .246 which one should I use ??

        JL...............
        This table lists two sizes depending on whether or not you ream the hole prior to tapping.

        http://www.carbidedepot.com/formulas-tap-pipe.htm

        Comment


        • #5
          1/16 -27 data is hard to find. It's not on most charts. Hence the question, I suppose. I found a calculator in a link:

          http://www.amesweb.info/Screws/Drill...e_Threads.aspx

          It recommends letter drill "C" or .242 dia. The pipe OD is .0.312

          Who actually has a 1/16 - 27 NPT tap? The only common use I'm aware of is little zerc and some Bijur fittings. You can't buy them over the counter in most fastener stores. You have to order them. The only reason I have one is because it came in a big box of loot given to me by the family of a passed-away friend.
          Last edited by Forrest Addy; 05-22-2017, 03:47 PM.

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          • #6
            Originally posted by Willy View Post
            Without a tapered pipe reamer I'd go with the D size size drill, especially if the material you are working with is something harder than brass or aluminum. Correct me if I'm wrong but I believe the C size is for when using the reamer.
            With stainless for example I always give myself as much leeway as possible. Works for me.
            In one of the charts I found it did mention something about a reamer. I'm tapping bronze so I guess I could go with the smaller size drill.

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

            Comment


            • #7
              Originally posted by Forrest Addy View Post
              1/16 -27 data is hard to find. It's not on most charts. Hence the question, I suppose. I found a calculator in a link:

              http://www.amesweb.info/Screws/Drill...e_Threads.aspx

              It recommends letter drill "C" or .242 dia. The pipe OD is .0.312

              Who actually has a 1/16 - 27 NPT tap? The only common use I'm aware of is little zerc and some Bijur fittings. You can't buy them over the counter in most fastener stores. You have to order them. The only reason I have one is because it came in a big box of loot given to me by the family of a passed-away friend.
              McMaster has them, that's where I got them from. And yes, you are correct Bijur uses that size also.

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

              Comment


              • #8
                Originally posted by reggie_obe View Post
                This table lists two sizes depending on whether or not you ream the hole prior to tapping.

                http://www.carbidedepot.com/formulas-tap-pipe.htm
                I did see that chart in my searches and that was the one that prompted me to ask which is correct, C or D. That chart says 15/64" drill with reamer and D size without.

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

                Comment


                • #9
                  'D' drill, according to both my Dormer wall chart, and on my old antique Cleveland Twist Drill wall chart.

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                  • #10
                    Originally posted by JoeLee View Post
                    McMaster has them, that's where I got them from. And yes, you are correct Bijur uses that size also.

                    JL................
                    It appears to be the size for nitrous. Looks like all the auto places have it: Advance, Summit, etc. Zoro has it. Looks real easy to find. Just google it. Many hits.

                    Sent from my SM-G900V using Tapatalk

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                    • #11
                      I have been trying to develop a tap drill calculator that accurately calculates for any size Vee thread. I can see from putting in some of the numbers from this problem that it will need a lot of more work with tapered pipe threads. Using the outside diameter of the small end of the external threads from Machinery Handbook, I came up with a #3 or #4 drill. I suspect that will be far too small and the indicated that a larger OD must be used. A curious thing that I noticed is that the truncation at the root and crest of these pipe threads is fairly small. While standard screw threads generally use between 10 and 12.5 percent truncation, the pipe threads seem to use a smaller percentage. The 1/16 - 27 NPT in particular uses between 0.0012" (minimum) and 0.0036" (maximum) truncation at both areas. That translates to percentages between 3.74% and 11.2%. I have found that using a truncation of about 12.5% for standard screws seems to give results that are in close agreement with published tables. I would be curious to find out just how much truncation is used on commercial pipe taps.

                      When I get some time I am going to scale this out in my CAD program to see just what the thread will look like with the recommended drills, C and D. (#3 = 0.213" and D = 0.246") I suspect there will be a lot of material that is removed at the small end of the taper. What I wonder is just how many fully formed threads will be left.
                      Paul A.
                      SE Texas

                      And if you look REAL close at an analog signal,
                      You will find that it has discrete steps.

                      Comment


                      • #12
                        Originally posted by Paul Alciatore View Post
                        A curious thing that I noticed is that the truncation at the root and crest of these pipe threads is fairly small. While standard screw threads generally use between 10 and 12.5 percent truncation, the pipe threads seem to use a smaller percentage. The 1/16 - 27 NPT in particular uses between 0.0012" (minimum) and 0.0036" (maximum) truncation at both areas. That translates to percentages between 3.74% and 11.2%. I have found that using a truncation of about 12.5% for standard screws seems to give results that are in close agreement with published tables. I would be curious to find out just how much truncation is used on commercial pipe taps.
                        .
                        I'd guess it has something to do with the sealing. NPTF probably has even less truncation.
                        Location: Helsinki, Finland, Europe

                        Comment


                        • #13
                          After reading the OP's other post on balancing some rings with weights that use the taper of a 1/16 NPT plug to expand and lock them, it becomes obvious that the exact size of the tap drill is not very important. There is no need to worry about the threads sealing or even about any pull-out strength. I would use a tap drill that is large enough to make the tapping process as easy as possible.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

                          Comment


                          • #14
                            Originally posted by MattiJ View Post
                            I'd guess it has something to do with the sealing. NPTF probably has even less truncation.
                            American National Standard "Dryseal" Pipe Threads, referred to as NPTF, have a larger truncation of the thread form at the root than at the crests, so that the sharp crest makes contact with the wider root before the flanks of the thread form come into contact.

                            Comment


                            • #15
                              Originally posted by Forrest Addy View Post
                              1/16 -27 data is hard to find. It's not on most charts. Hence the question, I suppose. I found a calculator in a link:

                              http://www.amesweb.info/Screws/Drill...e_Threads.aspx

                              It recommends letter drill "C" or .242 dia. The pipe OD is .0.312

                              Who actually has a 1/16 - 27 NPT tap? The only common use I'm aware of is little zerc and some Bijur fittings. You can't buy them over the counter in most fastener stores. You have to order them. The only reason I have one is because it came in a big box of loot given to me by the family of a passed-away friend.
                              I still use them on rare occasion. I used them a lot in my apprenticeship, (mid 60's) the prints we worked off of specified them.

                              Sarge

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