Standard Tap pre-drill sizes

Standard		Metric
Tap Sizes	Pre-tap	Tap sizes	Pre-tap
2-56	.070	M2 X .4	.063
4-40	.089	M2.5 X .45	.081
6-32	.109	M3 X .5	.098
8-32	.136	M4 X .7	.128
10-32	.161	M5 X .8	.166
10-24	.147	M6 X 1	.196
1/4-20	.203	M8 X 1.25	.265
1/4-28	.214	M10 X1.5	.332
5/16-18	.257	M12 X 1.75	.404
5/16-24	.272	M14 X 2	.469
3/8-16	.312	M16 X 2	.547
3/8-24	.332

Roll Form Tap pre-drill sizes

Standard		Metric
Tap Sizes	Pre-tap	Tap sizes	Pre-tap
2-56	.078	M2 X .4	.072
4-40	.101	M2.5 X .45	.090
6-32	.125	M3 X .5	.110
8-32	.150	M4 X .7	.144
10-32	.175	M5 X .8	.183
10-24	.172
1/4-20	.218