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Drill size for M1x0.25 tap

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  • Drill size for M1x0.25 tap

    What drill size should I use for the M1x0.25 tap? Should I use #69 or #68? The Machinery's handbook doesn't make any reference. The material is brass.


    [This message has been edited by Rotate (edited 12-09-2002).]

  • #2

    My Machinery's Handbook says that an M1 nominal thread should take a recommended 0.75 tap drill for an 81.5% thread, or alternately, a 0.78 tap drill for 71.7%. (25th Edition, page 1827)

    A #69 drill is 0.0292 inches or 0.7417 mm, which seems a bit small, while a #68 is 0.031 inches or 0.7874 mm, which is a bit large. I think I would go with the #68 to make tap breakage less likely.

    (And a hearty Thank You to JCHannum for catching my slipped decimal points!)

    Hope this helps.

    Rich Kuzmack

    Pi = 355/113 . . . to
    <85 parts per billion

    [This message has been edited by Indexer (edited 12-10-2002).]
    Rich Kuzmack

    Pi = 355/113 . . . to less
    than 85 parts per billion!


    • #3
      Drills normally oversize the hole, so would #69 drill a hole that's more like 0.75mm or am I being too hopeful?



      • #4
        To find tap size for metric taps, subtract pitch from diameter. ie 1.00-0.25=0.75mm drill size.
        You may find the .292 drill a tad on the large side. 0.0292 might be closer.
        Jim H.


        • #5
          Metric tap drills are always the size minus the pitch.

          1mm - 0.25mm = 0.75mm

          If you choose to go slightly larger or smaller is your choice.

          You can also check this website for tap drill sizes:

          That calculator lists the following tap drills for 1x.25mm tap:

          55% - 0.8213mm
          65% - 0.7889mm
          75% - 0.7564mm

          My favorite conversion program converts those numbers to:

          55% - 0.8213mm - .03233455"
          65% - 0.7889mm - .03105896"
          75% - 0.7564mm - .02977944"

          Based on the above, I would try the #68 drill.

          Mike L
          Amateur machinist, self-taught. I had a poor teacher, but I was a good student.
          Mike L
          Amateur machinist, self-taught. I had a poor teacher, but I was a good student.


          • #6
            As Mike points out subtract the pitch from the major diameter, this gives 79% approx. Works on English and Metric 60 degre threads.

            Good rule of thumb to remember, go through the numbers on a thread chart, you'll see that this is right. Also go for slightly larger, not smaller if drill bit for calculated hole is not available. Tapping force goes up really bad with an increase of thread percentage. Heard it said once that a 50% thread in steel, half the thread diameter in lenth would hold a common bolt to breaking. Like a bit more depth myself, but it's something to keep in mind.


            • #7
              Should have zoomed in closer on that Starrett drill chart on ebay


              • #8
                Mike L and Halfnut,

                Thanks for the info did not know that little trick.



                • #9
                  Tate: one of the village press magazines has a good write up on selecting drills for metric holes. Still on the news stands I think


                  • #10
                    Math formula for you. Take the major diameter and subtract out the pitch of the thread. For metric you convert to inches. In your example:

                    M1 = .0393
                    Subtract .25 mm pitch or .0098

                    .0393 - .0098 = .0295 or #69 .0292 drill.

                    Same goes for fractional:

                    1/4 - 20 tap

                    .250 - .050 (pitch) of thread) = .200 or a #7 .201 drill.

                    3/8-16 thread:
                    .375 - .0625 = .3125 or 5/16 drill.

                    Works every time for the 75% rule.

                    CCBW, MAH