The design of the BA thread form was first introduced in 1884, following a study of Swiss engineering practice, sometimes referred to as the Swiss Small Screw Thread System, it was standardised in 1903.

Although widely regarded as obsolete, the BA thread form is still used in miniature instruments and in model engineering.

It is also commonly found in electrical fittings and accessories.

Due to the thread angle of 47½° this thread form is ideal for use in thin sectioned parts, because of this it is very popular with clockmakers.

BA Threads Information Chart. British Association.


TPI = threads per inch
" = inch
Ø = diameter

Number TPI Pitch" Tapping drill
Ø mm
No 0 25.4 0.0394 5.10
No 1 28.2 0.0354 4.50
No 2 31.4 0.0319 4.00
No 3 34.8 0.0287 3.40
No 4 38.5 0.0260 3.00
No 5 43 0.0232 2.65
No 6 47.9 0.0209 2.30
No 7 52.9 0.0189 2.05
No 8 59.1 0.0169 1.80
No 9 65.1 0.0154 1.55
No 10 72.6 0.0138 1.40
No 11 81.9 0.0122 1.20
No 12 90.9 0.011 1.05
No 13 102 0.0098 0.98
No 14 109.9 0.0091 0.80
No 15 120.5 0.0083 0.70
No 16 133.3 0.0075 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.