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All About Threads

Screw Threads - a potted history 

What's it all about?

Whitworth, BSF, BA, BSP, BSC, UNC, UNF...all terms often accused of striking terror and loathing into the hearts of anyone who's ever been involved in the repair or restoration of any old British machine, whether it be an aeroplane, motorcycle, car or washing machine. OK, maybe forget the washing machine. But hang on, instead of gnashing our teeth and pulling our hair out in bloody clumps we should fling up our hands in honour of these mysterious standards as, without them, things would be even more complicated. Why? Well here's a bit of history...

Back in the mid-19th Century, when the industrial revolution was at full bore, almost every factory and manufacturer had it's own design of screw thread and, consequently, fasteners were often made specifically for the job in hand. This meant that compatibility and interchangeability between companies was just about non-existant. This is technically known as "A Bad Thing" or, less technically, as "A Proper Pain in the Arse". Something had to be done.

Enter Sir Joseph Whitworth, a brilliant engineer, who decided that a standard was required and, in an attempt to retain the small amount of hair that he hadn't already torn out by the roots, set about developing one. In order to do this he got hold of as many existing threaded fasteners as possible, measured them, averaged them out and in 1841 announced the "Whitworth" thread form to a grateful nation. Whitworth's new-fangled thread had a pitch and depth based on the diameter of the bolt - the larger the bolt, the coarser the thread. Love and praise were heaped upon him, the Whitworth became the mother-of-all threads and all was well in Great Britain and the Empire. Later, the British Standards Institute adopted the Whitworth as their standard threadform and announced that,

"...henceforth, and from this day forward the thread formerly known as 'Whitworth' shall be known as 'British Standard Whitworth' or 'BSW'. God bless her and all who screw her.".

Actually, I made that last bit up, but anyhow the much improved situation now became technically known as "A Good Thing". Marvellous!

There was a problem however.  Whitworth is a relatively course thread and while that's great for holding a steam engine or the Forth Bridge together, it's less well suited for smaller stuff. So, in 1908, a finer thread was developed based on the Whitworth form and given the snappy name of British Standard Fine (BSF).  The BSF retained the 55 degree internal angle of the Whitworth, but with a finer pitch making it far more suitable for smaller machinery with less likelihood of it vibrating loose.  In addition, the British Association (BA) thread, with a 47.5 degree angle, was introduced for screws smaller than 1/4" in diameter and often used in electrical and instrumentation applications.

All this excitement meant that the writing was on the wall for pesky non-standard threads worldwide and, in 1918, the United States Society of Automotive Engineers proposed and adopted their own standard threads, with a 60 degree angle, named National Course (NC) and National Fine (NF).

This is great stuff, but add British Standard Pipe (BSP) for self-sealing pipework and British Standard Cycle (BSC) for bikes and motorcycles plus the metric threads that had become standard in continental Europe to our existing list of BSW, BSF, NF and NC, and things are starting to get a bit hair-raising again.  Some wag once said "the trouble with standards is that there are too many of them" (think about's irony) and these wise words were proven during World War II when thousands of lend-lease tanks, jeeps and dodgy chocolate bars arrived in dear old Blighty with incompatible American nuts and bolts.   As a result, and once the distraction of the war was dealt with, Britain, the US and Canada got their heads together and decided to standardise with a unified thread form.   This was based on the National Fine and National Course standards which were imaginatively renamed Unified National Fine (UNF) and Unified National Course (UNC).

The changeover was quite gradual though, lasting well into the 1960's, so it's not uncommon to have a mixture of the older British threads and these new, jumped-up unified jobs on the same aeroplane.  Instruments are a good example, although they are usually marked with a OOO symbol indicating when they are fitted with UNF/UNC threads.

Even this supposedly 'unified' situation became problematical in Britain, with our nearest geographical neighbours all using metric threads, so in 1965 the British Standards Institute decreed that the ISO metric system should also become standard in the UK.  Interestingly, BA threads were retained for instrumentation and electrical applications with "preference given to the even sizes" i.e. 2BA, 4BA etc.. BSP was also incorporated into the metric standard as the thread of choice for pipes and pipe fittings.

OK, so which #@$% spanner do I use?

Ah, now we're edging back into "A Pain in the Arse" territory as, for the inexperienced, spanner sizes for the old BSW, BSF, BA etc. fasteners seem to have been picked out of the ether with no apparent relationship to the size of the nut sitting in front of you.

Metric and Unified sizes are easy - just measure the distance across the flats of the nut (in millimetres for metric, fractions of an inch for Unified) and that's your spanner size, i.e. the distance between the jaws. "Unified" spanners are generally marked as AF which simply means "Across Flats".

BSW and BSF spanner sizes are based on the diameter of the threaded section of the bolt and not the dimension across the flats of the head.   Because of this method of sizing, there are instances where different sizes of bolt, from different thread types, have the same size head and therefore fit the same spanner.  For example a 3/16 BSW bolt has the same head size as a 1/4 BSF.  In this case the distance across the flats is (more or less) 7/16" and so a 7/16 AF spanner will fit all three!  By the way, how BA sizes were arrived at is a mystery on a par with the Bermuda Triangle or the Marie Celeste.  It's completely beyond the wit of man to explain so you'll just have to accept it in the same way that you accept your fridge light really does go off when you shut the door.

Unfortunately, instances of the same spanner fitting a number of different heads are few and far between (this whole business is merciless I tell you, merciless) and in reality there is no easy way to cross-reference one to another.  Luckily, most older British cockpits use a fairly small number of different sizes, e.g. 2BA for fixing small panels, junction boxes, brackets etc., 4BA for holding instruments in their panels and the smaller Whitworth sizes for larger fittings.

So, now for some numbers.  The following table shows the outside diameter, in millimetres, of some of the more commonly found sizes, i.e. the diameter of the hole (add on a bit for clearance) that the bolt would pass through;

Whitworth and BSF
3/16" 4.763 mm
1/4" 6.35 mm
5/16" 7.938 mm
3/8" 9.525 mm
6BA 2.8 mm
4BA 3.6 mm
2BA 4.7 mm
0BA 6 mm

This table shows the measurements across the flats, again in millimetres, for a variety of bolt hex heads, i.e. the distance between the jaws of the spanner that fits it;

Whitworth BSF
1/8W 8.7 3/16BSF 8.7
3/16W 11.4 1/4BSF 11.4
1/4W 13.5 5/16BSF 13.5
5/16W 15.3 3/8BSF 15.3
3/8W 18.1 7/16BSF 18.1
7/16W 21.0 1/2BSF 21.0
6BA 4.9 1/4 5.5
4BA 6.3 5/16 8.0
2BA 8.3 3/8 9.6
0BA 10.5 7/16 11.2
M5 8    
M6 10    
M8 13    
M10 17    
M12 19    

That lot may well have put you to sleep and there is no shame in that, it IS extremely boring and I can only apologise for burdening you with it.  However, it does tell us a couple of things;

1. There are just about no common spanner sizes between different standards.  The exception being Whitworth and BSF, but they damn well ought to work together because, if you've been paying attention, you'll recall that BSF was developed from Whitworth as a finer pitched alternative.

2. Some spanner sizes from different standards are just close enough that they can, if the nut/bolt head is in good enough condition, be used if the correct size is not available.  Don't get too excited though for goodness sake, there are precious few that fall into that category with 2BA-1/8W-3/16BSF and 3/16W-1/4BSF-7/16AF being the only immediately obvious ones.

So that's it then?

Well, there you have it.  Threads and spanners are a nightmare.  It's official.  You could spend a fortune, buy a set of spanners for every standard then invite your friends around to admire them.  Alternatively you could do what I suspect many people do...stuff the theory, just use whatever seems to fit!