All the old “take apart” Champion plugs I have are 18mm. You are supposed to replace the copper washers on either side of the ceramic each time you took them apart to clean off the carbon. . impossible to find these washers today. – Some of the first Row Boat Motors were threaded to accept the ceramic insert held in place with a jam nut. These were developed as “Igniters” for the”coal oil” fueled steam engins of the day.
