Spark plugs

Newbie question - please don't laugh!

Given that many of our cherished engines were built in the late 19th / early 20th century in UK and US, both bastions of imperial measurements, how come spark plugs have metric threads?

Good question!

My guess would be be (and it is just a guess) is that as the I/C engine originated in Europe they were first made with metric threads by Bosch ect and it just continued from there. English and US companies obviously had to make replacement plugs to fit existing engines.

Some of the american engines actually have a pipe thread for the spark plug, think it's 1/2 GAS thread.

During the early 1900s, France was the dominant manufacturer of spark plugs. Frenchman, Albert Champion was a bicycle and motorcycle racer who immigrated to the United States in 1889 to race. As a sideline, Champion manufactured and sold spark plugs to support himself. In 1904, Champion moved to Flint, Michigan where he started the Champion Ignition Company for the manufacturing of spark plugs. He later lost control of his company and in 1908 started the AC Spark Plug Company.

Thanks Villiers for a most informative answer - what this forum is all about.

Going somewhat off topic, I am reminded of the units of measure I leant as a nipper.
All were easily divisible by 12, 14 or 16.
We had three different 'ounces' depending on what we were measuring (as did the French, Spanish, Portuguese and Dutch)
Someone had sent a long time standardising our stones so that they were all 14 pounds (16 ounces)
I vaguely remember something to do with  perches, poles or rods and cricket pitches but wasn't paying attention.

Best of all were hundredweights, which weren't a hundred of anything. The Americans, sensibly, made theirs 100 pounds (but they got their fluid ounces slightly to big)

We still use them up in flat cap land
https://en.wikipedia.org/wiki/File:Hundredweight_cwt_weight_restriction_road_sign_Ilkley.jpg

Youngsters have it too easy these days.

I agree what with Gills,Quarts,Roods,Chains and the rest it was certainly interesting,although i must say i don't much about Metric, only a little about metres such as Gas and Electric.
Regards,
Chiefy.