Thanks.
Do you know why SAE/Unified won out?
Why do they call it Unified?
I (barely) understand that British Standard and BSW have different ways of measuring fastener sizes, eg SAE 5/8" is different from BSW. Can you give insights as to how British Standard and BSW determine fastener sizes?
For many years, Britain and the U.S. used their own systems.
The British Whitworth system was the oldest, using a thread angle of 55 degrees. The U.S. system used a thread angle of 60 degrees, but otherwise the systems were similar, and both used bolts with dimensions in fractions of an inch. A 5/8 fastener is a 5/8 in both systems, though the thread pitch, as well as the angle, may differ.
The major area of difference was the bolt heads. British bolt heads, in the original system, were simply twice the bolt diameter, measured across the points. The U.S. system used bolt heads of specified sizes, measured ‘across flats’.
Both systems offered coarse and fine threads, BSW being comparable to American ‘National Coarse’, and BSF to ‘National Fine’.
BSW is British Standard Whitworth and BSF is British Standard Fine. Both might be referred to as ‘British Standard’, as would the wrenches, which fit both!
After the war, with the possibility, of both countries having to fight another major war, (Canada too) it was thought preferable that we all standardised on the same system. That was ‘Unified’. It was decided that a 60 degree thread would be standardised, as that angle is easier to construct geometrically, and with the benefit of a century of engineering knowledge, probably better.
In practice, this wasn’t a big change for the U.S. as most thread pitches stayed the same. A few vanished though, such as the larger sizes in the ‘numbered’ series, and some thread pitches changed. On many products produced in the U.K. in this era (late 50’s) you will see the ‘Unified’ symbol of 3 interlocking rings to denote it’s made with Unified threads.
As for aircraft, in the 50’s and 60’s the only major aircraft producers in the western world were the U.K. the U.S. and Canada. They had all just decided to use the Unified system, so that was that!
