Re: Metric threading

Paul Alciatore

Why 25.4mm instead of 25.0? Well, both the inch and the meter were already established when today's standard was adopted in the 1950s and the 1960s. Both had gone through several refinements before that point and more problematic was the fact that different countries had slightly different standards, at least for the inch. When the 25.4mm factor was adopted as the exact conversion, that distance for the inch was 1.7 millionths longer than the existing imperial inch and 2 millionths shorter than the existing US inch. I believe other countries also had differences. In short, it was a compromise and, being only a three digit number, it was a very convenient one. Two parts in a million was almost a negligible difference for almost all practical purposes.

If the inch had been redefined as 25mm at that time, then that would have been a difference of almost 1.7% and that would have been noticeable on almost every ruler and other measuring device that used inches. Virtually every one of them in the world would have been instantly inaccurate and would need to be replaced. Just for one example, a one foot ruler would be off by about 3/16". Even a first grade school child would notice. The confusion in the world of engineering would be enormous.

But, as I said above, the 25.4mm figure was only off by millionths of an inch. So, a 100 foot tape measure would only be off by 0.0002". For all practical purposes every existing measuring device could be used with no worry about it's accuracy. Only the most demanding measurements in physics and astronomy would need to ever consider the differences between the old standards and the new one.

So the 25.4mm figure was a magnificently practical compromise. I was getting my degree in physics (the science of measurement) in the early 60s and there was absolutely no reason to correct for the change in any of the experiments or calculations that I did in that program. The blueprints for bridges and buildings and automobiles and everything else were just as good after the change as before. And the same measurement devices were still used with no problems.

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