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.


  1. you dont know how much trouble you have just saved me with this conversion chart. We have been trying to adjust a couple of components on a really old heating system and need to find out what spanners to use.

  2. Great info resource for mini engine mechanics. Thanks

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  4. Whitworth is the most intelligent system as you need fewer spanners; however, the SAE system has long been the US standard, and metric the continental system since Napoleon was a boy. The interchangeability chart looks useful, but you really do need a set of each standard because tools vary (very slightly) in the clearance allowed by different makers (cheap tools have more clearance). Also, cheap tools wear or become deformed with hard use, increasing clearances. Buy yourself a good set of each standard, and it will save you headaches big time. German-made DIN standard tools are excellent - you don't necessarily have to go to Snap-on or Stahlwille which are aircraft standard, but they are nice to use. Early Sidchrome and early Dowidat are also good. I have a mixed bag that are mainly Sidchrome and Minimax (a reasonable Japanese brand no longer around), with a few others that I have come by over the years.

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  6. Instead of investing in Whitworth tools - try looking at a set of Metrinch sockets and spanners. These grip the flats on the nut and not the corners, so one spanner can fit metric, SAE, AF, Whitworth, BSW, BSF and BA. They are also useful for removing rounded off nuts and bolts. I've been using them for over 20 years and they are really good quality and work extremely well. You can get standard sets in 1/4", 3/8" and 1/2" drive and you can also get impact sockets as well. Check out

  7. Great chart. I looked for ages to find a decent conversion chart between Metric, Imperial, & Whitworth. You left out 3/4" WW (BSW) though. That's used commonly here in .AU even today for the Toe Ball nut on vehicles. .-.-.

  8. Replace all your spaners from 10mm-32mm, 3/8"-1 1/4" i bought 2 Knipex Multiple Slip Joint Spaners 87 41 250 for counter tightening situations £49.97 From= Wont work for under kitchen counter bolt locations check out Raptor pliers-You tube or Knipex Pliers Wrench for smaller nuts both seem like better options than adjustable spaners the raptors heads quite slim even if it is side mounted they grip like a mother i couldnt twist a bolt out of the jaws sideways really impresed rusty nuts ude probably sheer, bring out the WD 40!

  9. Just started fettling a late sixties Triumph motorcycle. The company was in the throws of changing thread forms from CEI to BSF, properly entitled ASF 'cos it was American standards they were adopting.
    But Triumph didn't change them all at the same time - you can only imagine the mix I've got on one bike!!
    Thanks for this chart, at least some sanity can be preserved.