C'mon, haven't you guys ever worked on an old 300 inline-6 Ford pickup truck? Or a pre-'70 302? Almost all of them had 18mm plug threads. Those were the standard until they wanted to increase valve size and redesign combustion chambers for more efficiency.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch