BSF: the British Standard Fine thread
BSF is the fine sibling of Whitworth: same profile, same 55° flank angle, same rounded crests, only with more threads per inch. It is found wherever a British design needed a finer pitch, for example in engines and gearboxes. VÖLKEL makes 117 hand taps for it to DIN 2181 in HSS-G, from BSF 3/16 x 32 to BSF 2.1/2 x 6.
The difference from BSW in numbers
At 1/4 inch BSW has 20 threads, BSF 26. At 5/16 inch it is 18 against 22. Diameter and profile are identical, the pitch is not, so the two series do not fit together even though they look the same. With a British thread you have to count the threads, not just measure the diameter.
Two-piece instead of three-piece
Because the thread depth is smaller, two tools are enough for BSF: taper tap and bottoming tap. There is no second tap. The same applies to the other fine series such as UNF, while the coarse series like BSW and UNC stay three-piece.
Identifying it
A thread gauge with 55° blades and the thread count over one inch are enough to tell BSF reliably from BSW and from the American series. The flank angle rules out UNC and UNF, the thread count separates BSF from BSW.
Complete imperial sets are under BSW and BSF, dies for the matching external threads among the round dies, tap drill sizes in the article calculate tap drill size.
