anecdote about bolt sizes pre 1960

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7 Jul 2023
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chertsey uk
I served my apprenticeship from 1959 at Acton Bolt Company at Park Royal London and very good training it was. Making bolts in main factory We were very conversant with bolt thread sizes. There was British Standard Fine BSF or British Standard Whitworth, not British Standard Coarse!. Changes appeared and we had to adapt to unify threads. They first Changed to American threads ANC and ANF. To amalgamate They were changed to Unified FIne UNF and Unified Coarse UNC, To make it easier?. that stayed a while then we had to change to Metric. say 10mm fine or 10 mm coarse. O/k getting confusing as some of our machines were still lease lend from America from WW2? We adapted, but having been imperial for so long so long we could go to , say !/2" to 12mm, ah but the length threw us!! 1 3/4 length bolt was about 44mm which we could not visualize, so for years we would ask for a 12mm bolt, 1 3/4 long. I still 'think' in imperial!
I trained as an aircraft mechanic in 1974/5. Aviation bolt lengths are specified by the diameter and the grip length, the unthreaded portion, the threads being the thickness of the corresponding nut plus three threads. Easy to work with. Just measure the thickness of the material to be held and get/order that length. Commercial world uses overall bolt length with a great deal of the threads buried in the material or sticking out past the nut a bunch. To this day I hate getting commercial bolts so I cut off the extra threads because to me it is wrong. 🤷‍♂️