I have some ings to add:
When I started on MV, there were GMC, Dodges, Jeep of WWII, all in inches. I had to learn that, no choise!
When I rebuilt my M715 the first time, I had to use the SAE System.
When I owned the M1009 the enigine is full metric except where SAE parts were connected. However, the US invented new wrench sizes for the bolts we were used and which are standardised in the metric system. You as the japanese use for M8 12 mm, we use 13 mm according DIN EN ISO standarts.
I own some repair manuals, where metric sizes, which realy need precission, are calculated into inches with 12 didgets behind the dot, just 90,00 mm.
And one more thing: Officially the US changed to the metric system in 1980, so 30 years time to learn.
However, we still used pound (German, which is 500 Gramm instead of the US pound of 453.6 gramm) and Zentner(Which is 100 German pounds or 50 kg) which officially became illegal in 1901.
The Britisch also use stone, cw, short ton, long ton.
The time is worldwide measuered in the Hexagonal system, so there is no need to change it.
When I started my University studis the pressure was measured in bar, then in Pascal, then in MPa, then in kPa. I had to get used to it.
So you should get used to the metric system!
Wolf