How did spark plugs end up having metric threads ?

[1937 LaSalle coupe]

Hello and Merry Christmas and happy holidays to all. From what I have seen, spark plugs started out having pipe threads. You could also take them apart to clean off the deposits. Worked fine for a while. Maybe then spark plug manufactures found out a cheaper made smaller plug still produces a hot spark. My question is: why did the spark plug manufactures decide to produce them with Metric threads ?  I have seen 10mm, 14mm, and 18mm. I don't about aviation or marine use. Seems like Corvair did use a SAE thread briefly. Thanks for any answers.... John C. Lehman... CLC # 26365....

[TJ Hopland]

I too will be curious to hear if anyone knows why this was.  Wonder if it perhaps relates to Bosch somehow?  But even if it did why did seem to stick?

I have seen other odd metric things like that before in other industries.   In the carbon arc lighting business, not early days of lighting but specialty lighting most of the sizes of the carbon rods were identified in mm for diameter and inches in length.  This seemed to have been the case back in the 30's well before there was a lot of 'global' trading going on.   Metric did seem to be a cleaner way but that's the case with just about everything metric and for the most part we have not changed.