The “metric system” of measurement is just about the only living remnant of the French Revolution. The idea was to make a logical system of weights and measures based on something very basic: the size of the Earth.
They took the best number available for the section of the Earth’s circumference from the Equator to the Pole through Paris, and called 1/10,000,000 of this value “1 meter” (or “metre”). So measurements up and down from that are in increments of 10s. And then a cube 1/10m on a side is a “liter” or “litre”, (about 10% larger than a quart), units again going up and down in increments of 10s. And 1 milliliter (1/1000 liter) of pure water at its maximum density is 1 gram.
This involves auto stuff in two major ways: engine displacements measured in liters; specifications quoted in millimeters, and hardware made in metric dimensions. A lot of people hate metric specs, but it isn’t hard to convert with a couple of basic equivalents.
On engine sizes, 100 cubic inches is 1638 cc (cubic centimeters, equivalent to milliliters) or 1.638 liters. So a 1600cc engine is very close to 100cid; and a 1.0L engine is about 61cid. (My own tickler for remembering this is that back in the late 1960s, the smog requirements exempted anything under 50cid, which is 819cc. So Fiat underbored the 1968–70 850s to 817cc to get under the wire.)
The easiest way to deal with length measurements is to remember that 1 inch is 25.4mm (millimeter), and 25:1 is close enough for most things. So 1/4″ is just a little bigger than 6mm; 1/2″ is almost 13mm; 3/4″ is almost exactly 19mm. In common bolt sizes, 5/16″ is just a few thousanths smaller than 8mm. So 50mm is just about 2″, etc. In bearing and piston sizes, 1.0mm is very close to .040″, so that a .010″ undersize or oversize is .25mm, etc. And of course the easiest thing about it is that if the 6mm tool is too small, look for a 7mm or 8mm without figuring fractions.
This is all neat stuff to know if you’re working on most European or Japanese cars, but there are exceptions. The Brits didn’t “go metric” until 1975, and they did it the same way as the Americans: new tooling is metric, but old tooling still in use was not retooled. So most common English cars are not metric, except for things like the air cleaner bolts on the late MGBs and brake hoses on 1975 & up Jaguars.
The other thing you run into on the earlier English cars, like MG-TCs and such, are British Standard Fine and British Standard Whitworth fasteners. These are just weird; some will kind of interchange with some inch or metric tools, but don’t count on it. Some Whitworth bolts may the same pitch (number of threads per inch) as a the common American bolts, but the angle on the thread is different so they don’t actually interchange correctly.*
Some Volvos were assembled in England during the 1950s and 60s, used a lot of English and American stuff in them, and were inch spec until ’75. One bit of hardware trivia: the early Nissan/Datsuns were copies of British Austins, and so you will find a lot of inch-sized hardware on the Datsuns through the 1960s, and even later where early parts are used on later cars.
The oil pressure switches on most Japanese cars are 1/8″ BSP (British Standard Pipe), which is similar to US 1/8NPT except for a sharper taper and about 1 thread per inch finer; and both are slightly smaller and finer than the metric M10 (10×1.0mm). So when trying to put an accessory oil gauge onto a Japanese car, you have to find a BSP adapter or T-fitting. (We carry a T-fitting that allows you to retain the light switch.)
And the Brits went to American thread pipe sizes in the ’50s and ’60s!
The US is one of the last three countries in the world (and the only industrialized nation) that has NOT adopted the metric system, even though it was originally planned for the Seventies.
* Despite being much maligned, Joseph Whitworth brought a standard to thread sizing. Prior to Whitworth’s work, most manufacturers used their own threads. Whithworth changed all that and his original “odd” nut sizes are really quite logical. They were 1.732 (root 3) x the bolt diameter — all based on the dimension of a hexagon. The size of the flat equals the bolt diameter. However, the bolt head and nut sizes changed (smaller) during World War Two in order to save steel.