British Standard Fine (BSF) Size Chart

The BSF (British Standard Fine) thread is a series of standards that belong to the British Standard 84:1956 - Parallel Screw Threads of Whitworth Form. This is an alternative thread form to the BSW which has a coarse pitch as opposed to the BSF thread which has a fine pitch. The BSF thread has the same profile as of a BSW thread, except that it has a finer thread and is used in applications where a finer pitch is required for a specific given diameter. However, all the BSW and BSF measurements are derived from the pitch of the thread. British Standard Fine Threads have been used around in the United Kingdom along with BSW threads since the year of 1908 when it was first introduced by the British Engineering Standards Association. First developed by Rookes Crompton and his assistant George Field in the year 1908, these threads were used in applications where there was a requirement for finer pitches. The angle of the thread of BSF thread measures as 55 degrees. BSF threads were used in cars, machinery and other appliances which needed a finer thread before industries began replacing BSF threads with UTS threads and Metric threads.

BSF threads can be considered as the British equivalents of the American thread standard UNF (Unified National Fine Thread). However, while the American UNF threads have a thread angle of 60 degrees, British BSF threads have a thread angle of 55 degrees. Also, the thread profiles of BSF threads differ from the UNF threads as well. BSF threads have a thread profile that is wavelike. Meanwhile, UNF threads possess a thread profile that has a theoretically flattened crest and root in it. These flattened crests are slightly rounded within a maximum outline in order to ease the manufacturing process.

Although, BSF threads and UNF threads may look similar to each other, but functionally have their own huge differences. As BSF and UNF threads have an entirely different TPI (Threads per inch) value, they do not fit with each other. Currently, BSF threads are used as stage lighting suspension bolts with the standard ⅜ and ½ sizes. Although these threads got replaced by metric threads, they began using BSF again as there were constant complaints that fine metric threads were difficult to set up and were time-consuming. Also, garden gate fittings use BSF threads to date, which is a standard use in the UK.