BA Thread Sizes
The pitch of 0BA is 1 mm 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 major diameter is given by and the hex head size (across the flats) is 1.75 times the major diameter.
BA sizes are specified by the following British Standards:
- BS 93:1951 — Specification for British Association (B.A.) screw threads with tolerances for sizes 0 B.A. to 16 B.A.
- BS 93:2008 — British Association (B.A.) screw threads. Requirements
The shortening at the crest and root is given by 0.26817p with a radius of 0.18083p at both crest and root. 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 (e.g. 0BA which appears similar to M6x1mm). Standard BA sizes are defined between 0BA and 22BA. The very small sizes are not used very often and other standards tend to be used (e.g., NHS).
| BA | Outer Diameter |
Threads per inch |
Threads per mm |
Tap drill |
|---|---|---|---|---|
| 0 | 0.2362 in / 6.0 mm | 25.38 | 1 | 5.1 mm / 7 gauge |
| 1 | 0.2087 in / 5.3 mm | 28.25 | 1.112 | 4.5 mm / 16 gauge |
| 2 | 0.1850 in / 4.7 mm | 31.35 | 1.234 | 4.0 mm / 21 gauge |
| 3 | 0.1614 in / 4.1 mm | 34.84 | 1.372 | 3.4 mm |
| 4 | 0.1417 in / 3.6 mm | 38.46 | 1.514 | 3.0 mm |
| 5 | 0.1260 in / 3.2 mm | 43.10 | 1.697 | 2.65 mm / 37 gauge |
| 6 | 0.1102 in / 2.8 mm | 47.85 | 1.884 | 2.3 mm |
| 7 | 0.0984 in / 2.5 mm | 52.91 | 2.083 | 2.05 mm / 46 gauge |
| 8 | 0.0866 in / 2.2 mm | 59.17 | 2.330 | 1.8 mm |
| 9 | 0.0748 in / 1.9 mm | 64.94 | 2.557 | 1.55 mm |
| 10 | 0.0669 in / 1.7 mm | 72.46 | 2.853 | 1.4 mm / 54 gauge |
| 11 | 0.0591 in / 1.5 mm | 81.97 | 3.227 | 1.2 mm |
| 12 | 0.0512 in / 1.3 mm | 90.91 | 3.579 | 1.05 mm |
Read more about this topic: British Association Screw Threads
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