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Withworth Threads - Taps and Drill Sizes

Taps and drill sizes used for Whitworth - ISO 7 and ISO 228 threads.

British Standard Whitworth (BSW) is an imperial-unit-based screw thread used for ISO 7-1 and ISO 228 threads (formerly BS-2779).

Withworth Threads - Taps and Drill Sizes
Size Taps
(inches)
TPI75% 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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