This has already been achieved as far as pipe is concerned. Pipe sizes were
never rational inch to start with thus switching them to rational
millimetres would serve no purpose. However a metric naming schemeis
already in existance. It is the "Diameter Nominal" scheme. A half-inch
pipe is called DN15 under this scheme. I guess because the inner diameter
is about 15 mm in diameter.
My memory is that a British standard iron half-inch pipe is known to French plumbers as *un treize* (a thirteen) and a British three-quarter inch pipe is *un dix-neuf* (a nineteen).
