I was explaining some of this to my daughter recently, who was very polite but was probably thinking, “another long dad story about the olden days”.
Prior to World War 1 threads, by and large, were not standardised, even up to the advent of the Second World War there were lots of proprietary sizes. Fasteners were often made by individual manufacturers or groups of manufacturers within certain geographic regions. They would make nuts and bolts for their own unique usage in their own plants.
It was only when manufacturers needed to work together to make a complete assembly that this became an issue and standardised sizing became more prevalent. When Rolls Royce sent the plans for the Merlin engine to the US for them to make; the US engine manufacturers tolerances were more than ten times greater than the standards used in the UK, they couldn’t build it. There was a lot of catching up that needed to be done for US manufacturers to be able to achieve the necessary tolerances.
Being part of the broader British Empire, Australia was part of a more standardised grouping than existed within other jurisdictions. The British Standards were widely used and reinforced across the British sphere of influence. This was more economically efficient and had huge strategic value as, warships, for example, could be more easily resupplied and repaired where components would actually fit off the shelf. A steam engine made in Scotland and exported to Victoria could be repaired using Australian made components and someone wouldn’t need to make a specifically indexed thread for a bolt or wait for spares to arrive from Scotland. The engineers all knew what the dimensions meant and what tolerances applied within those dimensions.
While 1\2” BSF may be uncommon now it was much more common previously. Usage of British Standard threads has largely been supplanted by metic threads with the US persisting with the standards they somewhat reluctantly adopted 80 years ago and now refuse to modernise.
