Tap Drill Sizes

National course & fine threads
THREAD
 		DRILL SIZE
inch		DRILL SIZE
mm
0-803/641,25
1-6453 (0.0595")1,50
1-7253 (0.0595")1,55
2-5651 (0.067")1,80
2-6450 (0.07")1,90
3-485/642,10
3-5646 (0.081")2,15
4-4043 (0.089")2,35
4-4842 (0.0935")2,40
5-4039 (0.0995")2,65
5-4437 (0.104")2,70
6-3236 (0.1065")2,85
6-4033 (0.113")2,95
8-3229 (0.136")3,50
8-3629 (0.136")3,50
8-4031 (0.12")3,50
10-2425 (0.1495")4,00
10-3221 (0.1590")4,10
12-2417 (0.173")4,65
12-2815 (0.18")4,70
1/4-208 (0.199")5,35
1/4-283 (0.213")5,50
5/16-18F (0.257")6,80
5/16-24I (0.272")6,90
3/8-165/168,25
3/8-24Q (0.332")8,50
7/16-14U (0.368")9,65
7/16-20W (0.386")9,90
1/2-1327/6411,15
1/2-2029/6411,50
9/16-1231/6412,60
9/16-1833/6412,90
5/8-1117/3214,05
5/8-1837/6414,50
3/4-1021/3217,00
3/4-1611/1617,50

Estimating drill size for taps

To determine the ideal drill size for appx. 75% thread on inch-sized (imperial) special pitch taps, use the following formula:

Drill Size = (Major Diameter of the tap) - (.975 divided by the number of Threads Per Inch)

For example, to find the ideal drill size needed for a 8-40 tap:

Drill Size = (0.164") - (.975 / 40 tpi)
Drill Size = .164" - (.975 / 40)
Drill Size = .164" - .0244"
Drill Size = .1396"

I might use a 9/64 drill.
0.1396" x 64 = 8.936/64ths", or approximately 9/64".

If I had metric drills, I might use a 3,55 mm drill.
0.1396" x 25.4 mm/inch = 3,546 mm.

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