Threads and their origins – a classic car owner’s guide

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Did you ever start to do some mechanical work on your classic car only to discover that none of your spanners fit correctly? We recently had a customer complaining that he couldn’t find a spanner to fit particular bolts on his car, only for us to point out that this was Whitworth rather than imperial or metric sizes. These tools with their odd and unusual markings were once very common indeed my Father gifted me a number when he decided to stop repairing his, admittedly fairly modern, Ford in the 1970’s. Now they are getting harder to find as people’s memories of what they were all about fade into the mists of time… however, not only can Leyton Classics source Whitworth sizes for you, our tool supplier; King Dick can supplier the correct spanners and wrenches, British made and brand new.


Before the mid-nineteenth century, nuts and bolts were individually crafted, specifically matched and were generally not interchangeable. Serious efforts to standardise screw threads began in the 1840’s, when Joseph Whitworth proposed a standard screw thread form based on a constant thread angle of 55 degrees. This became known as the Whitworth thread, and gained acceptance across British industry. Outside diameters of bolts began at 1/8″ and increased by increments, with a whole number of threads per inch specified for each diameter…indeed the Whitworth bolts and spanner became synonymous with earlier classic cars.


Roughly the same time, an American (Sellers) developed a screw thread system based on a 60 degree thread angle, which was originally called the Sellers thread, then the USS thread, and finally in 1948 the Unified National Series, including UNC (coarse), UNF (fine), and UNEF (extra-fine). The UNF series is sometimes called SAE (Society of Automotive Engineers) or ANF (American National Fine). These threads became popular on many post war classics.


The Germans, Swiss and French each developed their own metric screw thread forms. The metric world eventually agreed in 1898 on the (SI) metric thread series, with a 60 degree thread angle.


British Association (BA) screw threads have a thread angle of 47-1/2 degrees and are based on the Swiss thread. Screw threads in these series are found, particularly, on all older British vehicles. BSF is commonly used on engines and drive train components and body fasteners, BA is used to attach small clips and electrical parts, BSPP can be found on the banjo bolts of water pumps and SU carbs and fuel pumps.


In 1965 the British Standards Institution urged British industry to regard BSW, BSF, and BA as obsolescent, to be gradually replaced by International Standards Organization (ISO) metric thread. Hence the gradual move during the 1970’s from imperial to metric sizes across the British car industry.


It is very important to realize that it is really not safe to repair damaged Whitworth threads with UNF taps and dies, or to mix and fit BSF and UNF fasteners together. The pitch or number of threads per inch is the most obvious difference, but more important (and potentially disastrous) is the reduction in surface contact area between the threads, which can lead to binding when torqueing, loosening in vibration or complete failure under load. I once saw a friend’s wheel hub shear off the car whilst in motion, due to this very reason. BSW and UNC sizes in most cases have the same number of threads per inch, so it is very possible to get these mixed as well. If you have a ruined fastener it is worth the trouble to get the right one rather than take a chance.


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