British Association Threads
A British system of threads with 47.5° included angle with rounded roots and crest. Even numbers are the preferred sizes.
| B.A. | Tapping Drill | Clearance Drill | ||
| inches | mm | inches | mm | |
| 0 | 0.189 | 4.8 | 0.242 | 6.147 |
| 1 | 0.166 | 4.216 | 0.213 | 5.410 |
| 2 | 0.147 | 3.734 | 0.191 | 4.851 |
| 3 | 0.1285 | 3.264 | 0.166 | 4.216 |
| 4 | 0.111 | 2.819 | 0.147 | 3.734 |
| 5 | 0.098 | 2.489 | 0.136 | 3.454 |
| 6 | 0.086 | 2.184 | 0.116 | 2.946 |
| 7 | 0.076 | 1.930 | 0.104 | 2.462 |
| 8 | 0.067 | 1.702 | 0.0935 | 2.375 |
| 9 | 0.0595 | 1.511 | 0.081 | 2.057 |
| 10 | 0.0465 | 1.181 | 0.073 | 1.854 |
| 11 | 0.042 | 1.067 | 0.067 | 1.702 |
| 12 | 0.037 | 0.940 | 0.055 | 1.397 |
| 13 | 0.035 | 0.889 | 0.055 | 1.397 |
| 14 | 0.028 | 0.711 | 0.0469 | 1.191 |
| 15 | 0.025 | 0.635 | 0.040 | 1.016 |
| 16 | 0.0225 | 0.572 | ||
| 17 | 0.020 | 0.508 | ||
| 18 | 0.018 | 0.457 | ||
| 19 | 0.0145 | 0.368 | ||
Reference
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
See also: Acme Thread, British Standard Whitworth, Metric Threads, Thread, UNC, UNF.
Subjects: Mechanical Engineering