This file to fit myth is becomming an universal legend...
At first, this file to fit myth was here and elsewere (See Jay Leno...) applied to the British Merlin production (which was already wrong), and now T.A. Gardner tries to apply it to German production, which is as wrong as it could be!
Germany started a large scale standardization of its industrial production not directly after the outbreak of the war but three years later (note, I’m talking about WW1). For this purpose the DIN norm system was established which is still the basis of the contemporary ISO System. The DIN institute standardized dimensions, calculation methods, proving methods and of course tolerances. It was the first time, that a general tolerance system was developed and it worked so well, that it is still in worldwide use (Except maybe in the USA, where individual tolerances for fittings are still popular). By unifications of the tolerances for bore and shaft (like H7/k6) it universal quality gauges could be used to check if every part is within the specs. This system is allows easily to use Company A to produce a 30 H7 bore which can be perfectly combined with a 30 k6 shaft from company B. It took longer than WW2 before the Anglo saxes started using this simple universal fitting system….
