British Association (BA) tapping drill sizes.


Number TPI Tapping drill Dia mm
No.0 25.4 5.10
No.1 28.2 4.50
No.2 31.4 4.00
No.3 34.8 3.40
No.4 38.5 3.00
No.5 43 2.65
No.6 47.9 2.30
No.7 52.9 2.05
No.8 59.1 1.80
No.9 65.1 1.55
No.10 72.6 1.40
No.11 81.9 1.20
No.12 90.9 1.05
No.13 102 0.98
No.14 109.9 0.80
No.15 120.5 0.70
No.16 133.3 0.60

British Association thread (BA)

The BA threads were first developed in 1884 by the British Association for the Advancement of Science and were standardised in 1903.

The thread diameter has a relationship with the thread pitch in mm, this being proportional to a power of the logarithm to the base 10.

Similar to the Whitworth thread, it has a rounded root and crest, however the included angle is 47.5°.

The British Standards Institute (BSI) declared the BA thread obsolete in 1966, replacing it with the ISO threads.