Withworth Threads - Taps and Drill Sizes
British Standard Whitworth (BSW) is an imperial-unit-based screw thread used for ISO 7-1 and ISO 228 threads (formerly BS-2779).
| Size Taps (inches) | TPI | 75% Drill Size for Pipe Taps (inches (mm)) |
|---|---|---|
| 1/8 | 40 | #39 (2.6) |
| 1/4 | 20 | #9 (5.1) |
| 3/8 | 16 | 5/16 (8) |
| 1/2 | 12 | 27/64 (10.7) |
| 5/8 | 11 | 17/32 (13.7) |
| 3/4 | 10 | 21/32 (16.6) |
| 7/8 | 9 | 49/64 (19.5) |
| 1 | 8 | 7/8 (22.3) |
| 1 1/4 | 7 | 1 7/64 (28.3) |
| 1 1/2 | 6 | 1 5/16 (34.0) |
| 1 3/4 | 5 | 1 9/16 (39.6) |
| 2 | 4 1/2 | 1 3/4 (45.4) |
| 2 1/4 | 4 | 2 (51.1) |
| 2 1/2 | 4 | 2 1/4 (57.4) |
| 2 3/4 | 3 1/2 | 2 1/2 (62.9) |
| 3 | 3 1/2 | 2 3/4 (69.2) |
| 3 1/4 | 3 1/4 | 3 (75.0) |
| 3 1/2 | 3 1/4 | 3 1/4 (81.4) |
| 3 3/4 | 3 | 3 3/8 (87.1) |
| 4 | 3 | 3 5/8 (93.5) |
- TPI = threads per inch
- 1 inch = 25.4 mm
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