MM, AF, BSW, BSF Spanner Conversion Chart

If your classic was built - or at least designed - before 1960, there is a strong chance it will contain a combination of Imperial AF and Whitworth bolts and fixings. If it's Continental, metric fittings will be the standard.

The following chart allows you to convert easily between all of the major standards - MM, AF, BSW and BSF - making it a lot easier to find a spanner or socket to fit.

Metric bolts, which are measured in MM across flats (edge-to-edge) of the bolt head, are the most common fittings for continental vehicles. In an attempt to standardize certain fittings, some metric sizes which have an Imperial equivalent have been used for major fittings (such as wheel nuts) for many years. After around 1970, wheel nut sizes have virtually been globally standardized at 19mm, an almost exact match for a 3/4in AF socket in Britain. Likewise, earlier vehicles have 18mm wheel nuts, corresponding directly to 3/8in Whitworth (BSW).

The most common Imperial standard in classics built after 1960 is the 'increments of one inch across flats' or 'AF' system. A good quality AF socket set will fit the vast majority of fittings on a British classic car.

The 'Whitworth standards', of which the most common are BSW (British Standard Whitworth) and BSF (British Standard Fine), are a collection of standards based on the Whitworth thread. Devised by Victorian Industrialist Joesph Whitworth in 1841, it became the World's first standard for fittings, hugely benefitting Britain's Industrial Revolution. Whitworth set out the thread angle and pitch of a bolt, which increases in a ratio the larger the bolt becomes. The size of the bolt head also increases proportionally. The 'sizes' of Whitworth spanners refers to the diameter of the threaded stud, and not the bolt head itself: it is not an AF system. For this reason, BSF spanners are one size larger than the equivalent BSW, despite the fact that the bolt head is exactly the same size. The difference in measurement refers to the fact that the threaded section is thicker.
If you own a classic built or designed before 1960, and you are regularly struggling to find a spanner or socket to fit nuts and bolts (particularly on bodywork), it is a wise idea to invest in a set of Whitworth tools.