All Small Six Spark plug thread size for small and big sixes?

[62Ranchero200]

Just (I hope) a simple question: Is it true that small six heads (including the CI/VI aluminum head) use spark plugs with 14mm threads, while big sixes use spark plugs with 18mm threads? Since I'm running the AL head and don't have an iron head at the moment, I wanted to be sure that stock iron heads ran plugs with 14mm threads.

If so, does anyone have any insight as to why (as far as I know) only this one part on the entire engine/car (mine is a 250 in a '62 Ranchero) is metric? I apologize if this has been asked before, I did search and couldn't find it.

Pic for attention, a bit out of date as I have done a few upgrades since then.

TIA

 

[bubba22349]

Hi 62Ranchero200, your 250 is looking great! The Iron Small Six Heads also have the same Spark Plugs with a 18 mm thread as the Big Six's. Yes the Spark Plugs have always been Metric Threads I think the reason they were is because they were mostly made over in Europe. I imagen that was because the first to Patten them and start to make those First Spark Plug's in the mid 1800's must of also wanted it to be that way. Here is a brief History of those who are suppose to have been most responsible for the idea and making of the Spark Plugs. https://www.thoughtco.com/inventors-of-the-spark-plug-4074529

 

[Lazy JW]

I think it was Bosch that had the patent so everyone had to use the metric thread in the beginning, after the patents expired it was just too expensive to change threads since the machinery was all set up that way