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6-48, 8-40 Tap Drill Size ??

Discussion in 'Gunsmithing and Repairs' started by brassbullets12, Mar 4, 2008.

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  1. brassbullets12

    brassbullets12 Member

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    I Have Misplaced My Info On Proper Tap Drill Size For 6-48 And 8-40, Coundn't Even Find It In My Machinist Handbook Thanks Darrell
     
  2. highorder

    highorder Member

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    edit

    6-48 NS #31 drill (.120")

    8-40 NS #28 drill (.1405")

    2A class fits.
     
  3. Zeke/PA

    Zeke/PA Member

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    Here's an easily remembered formula.

    Tap Drill Size = Major Diameter minus Thread Pitch.
    Example : 5/16 x 18 UNC
    1/18= .055" 5/16 = .312"
    .312"-.055"= .257" ( "Letter "F" Drill)
    Works on smaller sizes and Metrics too!
    Zeke
     
  4. 35 Whelen

    35 Whelen Member

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    Drill sizes

    Just Google "tap drill sizes". You'll get all the info you need!
    35W
     
  5. PTK

    PTK Member

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    Funny how the two most common scope/sight mounting screw sizes aren't used in the Machinist's Handbook or on Starrett's tap guide, isn't it?
     
  6. BWB

    BWB Member

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    They are both "specials", used only for the purposes indicated. Specials are usually not listed in the fine or coarse thread standard tables.
     
  7. wolf_from_wv

    wolf_from_wv Member

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  8. Clemson

    Clemson Member

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    I use a #31 for 6-48.

    Clemson
     
  9. highorder

    highorder Member

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    its all about thread engagement. at small sizes like these, a few thousandths make a big difference in fit. But, considering the permanence of a scope mount, a little loc-tite makes up for alot of tolerance.
     
  10. rsilvers

    rsilvers Member

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    They are not listed because they are not standard screws. Basically, whoever selected them in the first place made a very poor decision by not picking something that was used for other things.
     
  11. BBBBill

    BBBBill Member

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    The Brownell's catalog has a chart of tap and clearance drill sizes. Surely you must have one. :)
     
  12. jimmyraythomason

    jimmyraythomason Member

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    I have used both and they work equally well.
     
  13. rsilvers

    rsilvers Member

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