As we all know, metric measurements aren't ever exactly equal to SAE measurements. I was just thinking: why is this? Where did metric even come from? Why do none of its measurements line up with the older system? It would make sense if millimeters were equal exactly to 1/16 of an inch, but they don't; they're an arbitrary length all their own, which makes conversion a nightmare and never exact. Also, in America anyone who works on cars or machines has to have two sets of wenches and sockets; one for SAE, one for metric, because you can't use a SAE wrench on a metric bolt and vice-versa; the sizes are always intermediate between each other for some reason. Metric and SAE scales are just completely out of sync. Why is this? What decided how long a millimeter is, and why is it nowhere close to a 1/16 of an inch? A yard isn't exactly equal to a meter either, and metric has nothing anywhere close to a foot. It has kilometers instead of miles, but that honestly makes sense given that its derived from meters. It doesn't change the fact though that the two systems don't have a single measurement that lines up exactly. How did this develop? I thought SAE was the older system, so in theory its measurements were still in use when metric was adopted, right? How could they be so different?
Why would you expect them to be the same?
Metric was designed to be a scientific system, whereby the meter (from which all other metric measurements are derived) was originally 1/10,000,000 of the distance between the pole and the equator, and was a system divisible in base 10.
Everything else are Customary units, which usually used arbitrary baselines in almost every country (Even today, US customary fluid measurements aren't the same as British Imperial ones) but which were mostly available to everyone without needing special equipment, and which would use many different bases between units. 1760 yards to the mile, 3 feet to the yard, 12 inches to the foot, and the inch divided upto 64 fractions - it shouldn't be surprising that nothing matches exactly with metric.
Even SAE don't quite match up with other inch sizes, it depends how the head is measured. SAE is I believe equivalent to AF in British sizes, but Britain has also had in the past BA and Whitworth sizes, which are again measured differently even though they're all based on the inch too. Thread pitch is defined too in each standard, and even today Whitworth threads (one of the first standards) are commonly still used even though the bolt sizes are pretty much totally obsolete now. So you can see it's a complicated business.