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  • Screw hole size

    There are plenty of places on these aircraft that use machine screws. I'm embarassed to ask such an elementary question, but I couldn't find a definitive answer through Google.

    What drill bit size is considered "best practice" when drilling for these screws (#4's, #6's, and #8's)? I can find the actual diameter of the screw on Google (and using a caliper) but I imagine the hole should be slightly larger and I'm sure there is a table out there somewhere that specifies this. Is this in 43.13B? Thanks,

    -Nic
    #1217

  • #2
    The information about clearance hole sizes can be found in the Machinery's Handbook under clearance hole sizes for machine screws.
    Last edited by S Lathrop; 12-04-2017, 01:43 PM.

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    • #3
      Originally posted by S Lathrop View Post
      The # sizes correspond to number drill sizes, Those are the through hole sizes for that size screw. So for a #10 screw, you use a #10 drill for the clearance hole.
      Do you use a #8 drill to drill for a 8-32 screw?

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      • #4
        Thanks for the response. My Google skills are challenged, but the drill charts I can find show that #6 and #4 drill bits are larger than 0.2" while the screw sizes are much smaller, like 1/8" or less. Larger #d drill sizes represent smaller sizes, and vise versa for screws. What am I missing?

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        • Bdflies
          Bdflies commented
          Editing a comment
          Nothing. I think you got it just about right...

          Bill

      • #5
        Google "clearance hole for machine screws". There are a number of charts out there to show the "tight fit" and "free fit" hole size for each screw size.

        John Ciolino
        Patrol

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        • #6
          The number size is the major diameter of the screw. That is the diameter if the sides of the threads were extended to a point. Because the threaded proton of a screws will not equal to the major diameter of the of the screw, the major diameter will give plenty of clearance for a screw.

          And you can go tighter but you need to look that diameter up or calculate it based on the pitch diameter of the screw thread you are working with.
          Last edited by S Lathrop; 12-03-2017, 09:01 PM.

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          • #7
            Nic, I found this one: http://eberhardt.bz/GME_Wood_Land/GM...rill_table.pdf. I know it says "wood" in the link but the title on the chart says machine screws.
            Last edited by Flygirl1; 12-03-2017, 09:20 PM.

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            • #8
              #6 screw, use a #28 drill bit.
              #8 screw, use a #19 drill bit.
              #10 screw, use a #12 drill bit
              I can't remember what the clearance hole size is for a #4 screw, but if you need it, I can post that later

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              • #9
                #4 uses a #32 drill for close fit and a #30 for free fitt.

                I apologize for posting wrong information. I got the above numbers information from a copy of the Machinery's Handbook. And I should have looked in the correct place the first time.
                Last edited by S Lathrop; 12-04-2017, 01:44 PM.

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