British Association BS 93

British Association Standard Thread. Used for instruments and clocks. Replaced by the metric ISO thread and the ISO fine thread.
The flank angle is 47.5°.

BA Thread No. Diameter
mm
Tapping Drill Size
mm
Pitch
mm
BA No. 0 6.00 4.80 1.00
BA No. 1 5.30 4.22 0.90
BA No. 2 4.70 3.73 0.81
BA No. 3 4.10 3.22 0.73
BA No. 4 3.60 2.81 0.66
BA No. 5 3.20 2.49 0.59
BA No. 6 2.80 2.16 0.53
BA No. 7 2.50 1.92 0.48
BA No. 8 2.20 1.68 0.43
BA No. 9 1.90 1.43 0.39
BA No. 10 1.70 1.28 0.35
BA No. 11 1.50 1.13 0.31
BA No. 12 1.30 0.96 0.28
BA No. 13 1.20 0.90 0.25
BA No. 14 1.00 0.72 0.23
BA No. 15 0.90 0.65 0.21
BA No. 16 0.79 0.65 0.19
BA No. 17 0.70 0.50 0.17
BA No. 18 0.62 0.44 0.15
BA No. 19 0.54 0.37 0.14
BA No. 20 0.48 0.34 0.12
BA No. 21 0.42 0.29 0.11
BA No. 22 0.37 0.25 0.098
BA No. 23 0.33 0.22 0.090
BA No. 24 0.29 0.19 0.080
BA No. 25 0.25 0.16 0.072