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British Association or BA screw threads are a largely obsolete set of small screw threads, the largest being 0BA at 6 mm diameter.
They are unusual in that they were originally defined in terms of the imperial units then in use in the United Kingdom, but were deliberately set to be exact multiples of the metric system units, rounded to the nearest thousanth of an inch. This anticipated metrication by about a century.
BA threads are still used in some precision instruments, such as optics and moving-coil meters.
BA thread sizes
The pitch of 0BA is 1mm and the pitch of each higher numbered thread is obtained by multiplying the pitch of the lower number by 0.9 so k BA has a pitch of . The angle of the thread is 47.5 degrees which is different from that used by Whitworth (55 degrees) and ISO Metric (60 degrees) so BA fasteners are never interchangeable with Whitworth/Metric ones even when the pitch and diameter seem similar.
| BA | Outer Diameter | Threads per inch |
|---|---|---|
| 0 | 0.2362" | 25.38 |
| 1 | 0.2087" | 28.25 |
| 2 | 0.1850" | 31.35 |
| 3 | 0.1614" | 34.84 |
| 4 | 0.1417" | 38.46 |
| 5 | 0.1260" | 43.10 |
| 6 | 0.1102" | 47.85 |
| 7 | 0.0984" | 52.91 |
| 8 | 0.0866" | 59.17 |
| 9 | 0.0748" | 64.94 |
| 10 | 0.0669" | 72.46 |
| 11 | 0.0591" | 81.97 |
| 12 | 0.0511" | 90.91 |
External links
- Bolt Science writeup.
- Early history of non-metric bolts including BA and Whitworth.
"British Association screw threads"