VTSteam,
You should remember that, originally, it was going to be 10,000 km from the equator to a pole. They screwed up that calculation and ended up setting 100 km as the distance between Paris and London -- and had to move the markers several times in the 19th & 20th centuries to make that work. In 1967 (just after I had purchased a rather expensive set of metric micrometers and dial indicators) the length of the mm was revised causing that entire "purchase" to become scrap.
Sometime between 1971 and 1975 the temperature scale changed from Centigrade to Celsius. This is because it was found out that true zero°C had been miscalculated. Celsius' scale actually set "zero" as water's boiling point and 100° as water's triple point. Fahrenheit's scale was based on the temperature at which Baltic Sea water would freeze to (hemp) ropes making it too dangerous for sailors to work (0°F). The other end of his scale (100°F) was the body temperature at which a sailor should be held in sick bay. Which system is more "scientific?"
The Unified National system of threads was created by the Joint Industrialization Committee of the American-British-Canadian Alliance Council established in the run-up to WWI. They designed the Coarse thread pitch set to function with low-shear materials such as zinc and (pre-Duralim) aluminums. The Fine thread pitch was designed to provide 15% greater pull-out and equivalent pin strength than a Coarse thread pitch. The Extra-Fine thread pitch was designed to provide 15% greater pull-out and equivalent pin strength than a Fine thread pitch. The minutes of those meetings (which I had to read for a contract back in the early-1970's) are truly amazing in the manner in which they show how a well constituted bureaucracy can work!
The (post-1972) ISO metric threads do not have a pitch that works well with low-shear materials. Moving from the (post-1999) standard to fine pitch in ISO threads gains you an average of 3% on your screw properties (and about 4% on nut properties). The attempt to standarize metric screwthreads under ISO auspices was begun in 1947. It took them until 1999 to reduce things to standard and fine pitches based on major diameter. As of 2010 (the last time I spent the time and money to dig through the relevant ISO documents) there were still five separate and incompatible "standards" (American-ISO, French-ISO, British Standard, DIN, and JIS) for tolerances and allowances based class of fit!
The thing is that units of measure apply to humans. The Romans discovered that, if you want underfed peasants to pick up and accurately place stones when building a roadway, then the stones should weigh no more than (roughly) 14 lbs. This is where the old Imperial measure of stone came from. It is a very good data point to have when figuring out production lines. The Roman unit of distance, the league, was the distance a full-loaded legionnaire was expected to walk in an hour. A furlong is 1/4th of a league -- good things to know when figuring out how long it will take a worker to move around a plant. In older measures (and redefined to make things confusion), the staff or pike was roughly 16 feet long and represents the longest item that a single individual can accurately move and line up (coming from the pike that represented the end of the plate-armored knights). Another older measure that has been redefined several times and is inconsistently identified across time is the shear or the chain which represents the longest piece of pole that can be raised using manpower and shear lines.
I am not suggesting that we use leagues, stones, or even fortnights (though calculating acceleration in furlongs/fortnight˛ was one of my favorite physics examination questions back when I was in high school) as units of measure. But knowing where they came from has value...
