by Tim Harris

In the early days of the Industrial revolution, screw threads were all hand-made to designs and sizes determined by the individual engineer. Not a good recipe for interchangeability. Around 1841, the brilliant engineer Joseph Whitworth realised that this was not a good idea, and decided to try to establish some kind of standard. He did this by measuring every thread he could get his hands on, and coming up with something that represented an average. As an indication of his abilities, itís interesting to note that as early as 1856 he exhibited a measuring instrument controlled by a screw which detected differences in length as small as one millionth of an inch!.

What he came up with was the Whitworth thread - this had an internal angle of 55 degrees, and a depth and pitch of thread that varied with the diameter of the thread - ie, the bigger the bolt, the coarser the thread. Very sensible.The size of the spanner required is determined by the shank size of the bolt.

Here's a picture illustrating:

After a while, it was realised that the Whitworth thread form was a touch coarse for all applications, and the British Standards Institue recognised the British Standard Whitworth (BSW) thread and introduced the British Standard Fine (BSF) thread in 1908.

There was also introduced around this time the British Association (BA) thread, commonly used for smaller (eg electrical) fittings (which is a bit curious because it uses a metric thread but measured in imperial units),  and British motorcycle manufacturers had introduced the Cycle Engineers Institute (CEI) thread in 1902 to cope with, among other things, the need for a large diameter but fine pitch thread. The CEI thread then morphed into the British Standard Cycle (BSC) thread - same thread form, but fewer sizes.

To further enrich the panoply of threads available, we had British Standard Pipe (BSP) - used to this day even on 'metric' cars like BMWs, British Standard Taper Pipe (Rc Series), and let's not forget ME (Model Engineer), British Brass, and Acme threads (not that you'll ever find these even on the most quirky of Morgans).

Meanwhile, our impudent rebel colony in the America ;-) introduced their own standards in 1918 with the National Fine (NF) and National Coarse (NC) threads approved by the Society of Automotive Engineers (SAE). These were still imperial sizes, and had pitch sizes measured in teeth per inch (TPI), but the thread form had an included angle of 60 degrees, and the spanner size was determined by the 'across flats' measurement of the hex bar from which the bolt was formed. Be careful not to confuse the letters AF when used to mean 'across flats', with AF used (erroneously) to indicate 'American Fine' thread. This is actually 'National Fine' and should be called NF or UNF.

After the war, we recognised that this rich choice of threads and sizes, in which there was a perfect size for every occasion, was maybe possibly a little bit confusing for some people, and so Britain, Canada, and the USA agreed that it would be best to standardise. So it was that NF and NC became UNF (Unified National Fine) and UNC. Most large car and motorcycle manufacturers switched to these, but of course the smaller operations tended to still use whatever they wanted.

The Metric thread system started to be introduced in Britain after we joined what is now the EU. This uses a 60 degree angle, like the unified series, but of course the measurements are all in millimetres, and the thread pitch is given as the millimeter distance between thread crests. Thus a 6 mm  thread (generally described as M6) will have a pitch of 1mm (or 25.4 TPI). There is a defined relationship between the core diameter, the thread diameter, and the pitch for metric threads. This relationship is different for the Metric Fine, Metric Coarse, and Metric Special pitches available.

If you are going to work effectively on an old(ish) British car, you will therefore need 3 sets of spanners, sockets, nut spinners, etc.

Metric sizes are measured in mm between the jaws (ie across flats)
UNF/UNC sizes are measured in fractions of an inch between the jaws (across flats)
Other Imperial sizes are stamped with sizes that bear no obvious relationship to the distance between the jaws, these are used on whatever will fit ;-).

Depressingly for the home mechanic, there is no obvious easy way to correlate the 'non-standard' (ie not Metric or Unified) onto standard sizes. They don't even correlate together

For example, a 3/16 BSW spanner, a 1/4 BSF spanner, and a 7/16 AF spanner are EXACTLY the same size. But although a 1/4 BSW spanner is the same as a 5/16 BSF, it is a bit bigger than 1/2 AF. A 1/8BSW spanner is the same as a 3/16BSF, and is very nearly the same as 2BA - close enough not to be a problem if the nut's in a reasonable condition.

In effect, I believe that the only BSF/BSW sizes that you really need are:

1/8 BSW (3/16 BSF)
1/4 BSW (5/16 BSF)

Other sizes can be managed with AF or Metric spanners. I have been furtling in my spare time with British bikes and cars for well over 25 years and have never needed anything else. Mind you I've got a 16" Bahco adjustable which I'm not afraid to whip out should the occasion merit it.

One practice which I learned many years ago is to maintain a small toolbox separate from your main collection. Every time you use a tool for a routine job on your car, bike or whatever, you put it back in the small toolbox, not in the main collection, and after a while you will end up with the small box containing a set of all the spanners, screwdrivers, etc that you really need to do 99 percent of the jobs on your vehicle. Then you can go out and buy just these sizes from a really good tool supplier like Snap-On, Facom, Beta, or whoever. This becomes your main and travelling toolkit, whilst all the stuff you never or rarely need can sit in the rollercab in your workshop for that fateful day when you realise that the offside trunnion pin retaining nut on your newly acquired 1933 Grindlay-Peerless is actually a 19/64 Taper Pipe thread, and you're sure you got a spanner for that from the autojumble you attended in Nempnett Thrubwell in 1967...

Finally, if anyone has a problem converting easily between millimetres and inches, and easy way to do it with not too great a sacrifice in accuracy is to consider an inch to be equal not to 25.4mm, but to 25.6mm Then you can simply get to most of the significant imperial sizes quickly and easily:
Corrected by Webmaster & by Peter Bready)
1  inch  = 25.4mm
½ inch = 12.7mm
Webmaster note:
Here is a comparison chart, kindly donated by Fred Sisson, comparing Metric, Whitworth and American spanner dimensions. Note that some Whitworth heads are between the  available American and Metric wrench (spanner) sizes. Buy the right wrenches or you can file one to fit the head...Whitworth Comparison File