It seems Herr Robert Bosch and Monsieur Albert Champion sort of grandfathered metric into spark plugs. Champion lost his company and started A.C. (Albert Champion) Spark plug Co. Model T, A and other engines used pipe threads and SAE threads but were phased out and the Metric won.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn