Drill sizes for tapping threads

Helpful for race car, hot rod, and custom car builders.

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Size of Thread Threads per inch Std drill for approx 75% threads
10 24 No. 25
10 32 No. 21
12 24 No. 16
12 28 No. 14
1/4 20 No. 7
1/4 28 No. 3
5/16 18 F
5/16 24 I
3/8 16 5/16
3/8 24 Q
7/16 14 U
7/16 20 25/64
1/2 13 27/64
1/2 20 29/64
9/16 12 31/64
9/16 18 33/64
5/8 11 17/32
5/8 18 37/64
3/4 10 21/32
3/4 16 11/16
7/8 9 49/64
7/8 14 13/16
1 8 7/8
1 14 15/16

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