Tapping and Clearance Drill Sizes for Unified Threads
Nominal Tapping Drill size Clearance Drill size
Size mm inch mm inch
1/4 x 20 5.20 13/64 6.50 17/64
1/4 x 28 5.50 7/32 6.50 17/64
5/16 x 18 6.60 17/64 8.00 21/64
5/16 x 24 6.90 Letter 8.00 21.64
Drill I
3/8 x 16 8.00 5/16 9.80 25/64
3/8 x 24 8.50 Letter 9.80 25/64
Drill R
7/16 x 14 9.40 3/8 11.30 29/64
7/16 x 20 9.90 25/64 11.30 29/64
1/2 x 13 10.80 27/64 13.00 33/64
1/2 x 20 11.50 29/64 13.00 33/64
9/16 x 12 12.20 31/64 14.75 37/64
9/16 x 18 12.90 33/64 14.75 37/64
5/8 x 11 13.50 17/32 16.25 41/64
5/8 x 18 14.50 37/64 16.25 41/64
3/4 x 10 16.50 21/32 19.50 47/64
3/4 x 16 17.50 11/16 19.50 49/64
7/8 x 9 19.25 49/64 20.25 51/64
7/8 x 14 20.50 13/16 22.75 57/64
1 x8 22.25 7/8 22.75 1 1/64
1 x 12 23.25 59/64 25.80 1 1/64