Overview of BSW and BSF Thread: Dimensions, Angle, Differences & Size Chart

In the British Threads Standards, BSW and BSF are important standards for screw threads. Understanding BSW and BSF threads, we are going to introduce their meaning, angle, uses, differences, and dimensions along with a BSW/BSF size chart.

What Is BSW Thread (Full Form)?

BSW (British Standard Whitworth) is a type of screw thread standard that was devised by Joseph Whitworth in 1841 and later adopted as a British standard. BSW thread dimensions are expressed in inches. BSW refers to a coarse thread, the thread pitch increases in defined steps as the diameter increases, and peaks are rounded with a 125-degree arc at the top and rounded roots as well. BSW threads were widely used in British industries in the 19th century due to their standardization and are still used today for applications such as British Standard Pipe threads.

What is the angle of the BSW thread? It uses a thread angle of 55 between the thread flanks, this is shallower than many other standards like metric (60 degrees) or UNC (60 degrees).

What is a BSP thread? The British Standard Pipe (BSP) is derived from British Standard Whitworth (BSW), and includes two different types, Parallel (BSPP) and Tapered (BSPT).

Can BSW and UNC threads be interchanged? Yes, Unified Coarse (UNC) and British Standard Whitworth (BSW) threads can technically be interchanged, but BSW has a 55° thread angle, while UNC is 60°, this means the threads won’t fit perfectly together and the joint won’t be as strong. In addition, BSW has rounded roots and crests, while UNC has flattened tops. This further reduces the fit and sealing ability.

What Is BSF Thread (Full Form)?

BSF stands for British Standard Fine thread, a type of imperial screw thread standard that was established in the United Kingdom. BSF threads have the same 55-degree thread profile as British Standard Whitworth (BSW) threads, with rounded roots and crests. Whereas BSW is a coarse thread, BSF has a finer thread pitch, meaning the threads are spaced closer together for a given diameter size. BSF threads are intended for smaller diameters or applications requiring a finer pitch than standard BSW can provide.

BSW vs BSF Thread, What Are the Differences?

Pitch: BSF has a finer thread pitch than that of BSW, meaning the threads are spaced closer together.
Dimensions: BSF and BSW have different dimensional specifications for things like thread depth, minor/major diameters according to thread size.
Application: BSW is designed for coarser threads and applications requiring strength over precision. BSF is used when a finer pitch is required for a given diameter.
Standardization: BSW was the initial British standard established in 1841. BSF was created later when an even finer thread pitch was needed for some applications.