British Standard Fine (BSF) is a screw thread form, as a fine-pitch alternative to British Standard Whitworth (BSW) thread. It was used for steel bolts and nuts on much British machinery, including cars, prior to adoption of Unified, and later Metric, standards. For highly stressed conditions, especially in motorcycles, a finer thread, British Standard Cycle (BSC), was used as well.
BSF was developed by R. E. B. Crompton, and his assistant George Field. BSF threads use the 55 degree Whitworth thread form. It was introduced by the British Engineering Standards Association in 1908.
The table shows suitable tapping drill sizes.
TPI stands for Threads Per Inch, Means the total number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch. In general, smaller fasteners have finer threads, so the thread count is higher.
Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters. In general, smaller fasteners have finer thread so they have lower thread pitch.
|Nominal Inch||TPI||Pitch||Thread Depth||Major Diameter||Effective Diameter||Minor Diameter||Tap Drill Size (mm)|
|1 1/8||9||0.11111||0.0711||1.125||1.0539||0.9828||25.50 mm|
|1 1/4||9||0.11111||0.0711||1.25||1.1789||1.1078||28.50 mm|
|1 3/8||8||0.125||0.08||1.375||1.295||1.215||31.50 mm|
|1 1/2||8||0.125||0.08||1.5||1.42||1.34||34.50 mm|
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