Historical Overview of Fasteners

British Standard Thread forms:

In 'British' thread form fasteners, the spanner size correspond to the bolt; a 1/2" Whitworth bolt is used in conjunction with a 1/2" Whitworth spanner. Whitworth sizes were laid down in 1841 by Sir Joseph Whitworth (1803-87), they featured an oversized head to accommodate the crude tolerances of the production methods of the day. With the introduction of BSF (British Standard Fine) and BSW (British Standard Whitworth) in 1908 the head sizes of Whitworth were reduced by one size, to avoid new tooling being needed for production and assembly. The old Whitworth 1/4" bolt was used for the new BSW/BSF 5/16" bolt which is more in line with the modern day fasteners we are used to. The new BSF/BSW bolts were specified a head across flats of 1.5 x the diameter of the shank, BA were geared at 1.75 x the diameter of the shank.

The majority of 'Whitworth' nuts/bolts we now see and use are likely to be the 1908 version of BSF/BSW head size. However some spanners are still marked '1/4 W 5/16 BS' which indicates they sized for the old 1/4" Whitworth bolt, and will be useable for the 1908 5/16" for BSF/BSW. The 1908 BSW bolts/nuts use the same coarse thread form as the original Whitworth. BSF bolts have the same angle of thread profile but have more teeth per inch, thus providing a stronger fastener and better vibration resistance for high tensile materials.

BSF/BSW Fasteners are commonly found in British equipment designed before metrification became the acceptable standard. However some machinery made in countries that were part of the Commonwealth still continued to use BSF/BSW long after the UK went Metric.

BA sizes were formulated in 1884 and put into a standard 1903, 5 years before BSF/BSW were standardized. BA were recommended for usage below 1/4 in mainly instrumentation and electrical apparatus, thus attempting to discard BSF/BSW sizes below 1/4". The common sizes used in BA threads were evenly numbered but some manufacturers used oddly numbered sizes, in relation to motorcycle AMAL used 1BA. Although odd numbered BA sizes have been made and are listed, they are quite rare.


Metric fasteners took a hold in the 1970s due to several reasons (one being on a scale of economies of manufacture of fasteners) the majority of sizes listed are the common ISO dimensions circa the 1980s. There have been international recommendations to reduce head sizes on certain items and sizes of metric fasteners and some of these now float the fastener market, further confusing standardization issues as direct replacement parts must be the same as the ones they are replacing. It is possible to find M10 hexagon heads with 17mm, 16mm, 14mm AF sizes; M12 with 19mm, 18mm, 17mm AF sizes. These non standard head sizes have been mainly utilized by the automotive industry and now we can find M7 fasteners becoming more commonly used.


UNF and UNC (Unified National Coarse/ Unified National Fine) are mainly found on machinery either manufactured in USA or by companies that are recognized as being American manufacturers. The USA stopped using BS standard fasteners after WW2 but they can still be found on old machinery manufactured before then. For smaller fasteners numbers are used as were used in the BA system, with the even numbers being the most common. Currently the USA has made significant moves towards metrification with certain fastener distributors specializing in them, again it could be argued it is purely down to cost of manufacture for the Far Eastern Manufacturers.

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