What Are The British Standard Fine (BSF) Of Fastener?

The British Standard Fine (BSF) thread is a fine pitch thread series that was widely used in the United Kingdom and the British Commonwealth for fasteners. It is part of the British Standard Whitworth (BSW) thread family, which was introduced by Sir Joseph Whitworth in 1841. The BSF standard is defined in the British Standard 84:1956. While the BSW thread is characterized by its coarse pitch, the BSF offers a finer pitch alternative, which allows for finer adjustment and stronger connections in some applications.

BSF threads have a 55-degree thread angle and a rounded root and crest. The fine pitch of the BSF series makes it suitable for applications where a finer thread is required for greater precision or strength. It was commonly used in British cars, motorcycles, and machinery before the widespread adoption of metric standards.

The sizes of BSF threads are usually denoted by the major diameter (in inches) followed by the number of threads per inch (TPI). For example, a 1/4″ BSF bolt might have a thread pitch of 26 TPI.

Here are some common sizes for BSF threads, though this is not an exhaustive list:

3/16″ x 32 TPI

1/4″ x 26 TPI

5/16″ x 22 TPI

3/8″ x 20 TPI

7/16″ x 18 TPI

1/2″ x 16 TPI

5/8″ x 14 TPI

3/4″ x 12 TPI

7/8″ x 11 TPI

1″ x 10 TPI

It’s important to note that as the metric system has become more prevalent globally, the use of BSF (and BSW) threaded fasteners has decreased. However, they are still used in certain applications, particularly in the restoration and maintenance of vintage and classic British vehicles and machinery.

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