Since 1908 this thread, in conjunction with BSW (British Standard Whitworth)
was the mainstay of British Engineering, and was used when finer pitches were
required.
Do note, this thread is similar, but not same pitch as UNF (sometimes labled NF, or even ANF). Using the wrong tap or die can easily result in a damaged thread with regular gaps. BSF is very common in pre-war cars, and can be found into the 1970's. As can UNF - you've been warned!
Angle of thread = 55o degrees
| Diameter Inches |
T P I | Pitch inches |
Core Diameter |
Tapping Drill Size |
| 3/16 | 32 | 0.0313 | 0.1475 | 5/32 inch (3.97 mm) |
| 7/32 | 28 | 0.0357 | 0.173 | Number drill 15 (4.6 mm) |
| 1/4 | 26 | 0.0385 | 0.2008 | Number drill 6 (5.3 mm) |
| 5/16 | 22 | 0.0455 | 0.2543 | 17/64 inch (6.75 mm) |
| 3/8 | 20 | 0.05 | 0.311 | Letter drill P (8.2 mm) |
| 7/16 | 18 | 0.0556 | 0.3663 | 9.7 mm (0.3819 inch) |
| 1/2 | 16 | 0.0625 | 0.42 | 7/16 inch (11.11 mm) |
| 9/16 | 16 | 0.0625 | 0.4825 | 1/2 inch (12.7 mm) |
| 5/8 | 14 | 0.0714 | 0.5336 | 14mm (0.5512 inch) |
| 11/16 | 14 | 0.0714 | 0.5961 | 15.5 mm (0.6102 inch) |
| 3/4 | 12 | 0.0833 | 0.6432 | 16.75 mm (0.6594 inch) |
| 7/8 | 11 | 0.0909 | 0.7586 | 25/32 inch (19.84 mm) |
| 1 inch | 10 | 0.1 | 0.872 | 22.75 mm (0.8957 inch) |
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