I recently spoke to an OEM customer about a project they had for a new customer. One of the jobs they were quoting included a demand for the US equivalent to an old BSW (British Standard Whitworth) Thread that was to be used in the re-building of British Classic Cars here in the US.
As I was born in the UK and emigrated to the USA from England in 2008, I was very aware of the older British Standards for threads. But I have to admit, it had been some time since I was asked if I recognized the thread type BSW.
– Alan Bate (National Sales Manager)
In the early 18th century, there were no standard measures for threaded products. Parts were individually engineered, and nuts and bolts were made to fit like a pair and were not interchangeable. Henry Maudslay was one of the first to recognize the importance of standardization and interchangeability of machine parts.
Joseph Whitworth chose Maudslay’s works as the starting place for his soon-to-be-successful career. Whitworth began his task of devising a standard for threads. His work started with a collection of bolts from all over England. He noted which sizes had shown to be most useful and the results of various thread forms.
Whitworth is credited with being the first person to offer a serious attempt at standardizing threads when he presented his paper, “The Uniform System of Screw Threads,” to the Great British Institute of Civil Engineers in 1841. He proposed as a standard a thread form with an included angle of 55°, and the tops and bottoms of the threads rounded with a radius equal to 0.1373 times the pitch. Whitworth’s thread system became the British Standard for Fastener Threads and is still known to this day as BSW.
Demand for this thread type has been steadily decreasing post World War II when the USA adopted the Unified Thread Standard (UTS) in 1949. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millimeter value. This steadily became “the” recognized thread standard until 1960 when Europe, the UK, and the rest of the industrial world adopted the Metric thread system.
To this day, the Unified Thread system (UNC & UNF) is still preeminent in the USA, and the ISO Metric thread rules in Europe, the UK, and the rest of the world.
Please find below a comparison chart between the three thread types.
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