[rotorwrench]

If the outer thread diameter measures near .704" then they are 18mm. My old Harley 45 uses the big 1-inch hex 18MM plugs NGK-6 if I remember correctly. My old Model A uses 7/8-18 straight thread Champion #3 plugs but they have larger hex than a 7/8" on the old plugs.

That link you posted has a good cross section of automotive sizes. Aircraft plugs use 5/8-24 & 3/4-20 thread and they are all 7/8" hex but they are all shielded types. I just assumed the 7/8 hex was an 18mm thread but I've been wrong on more than one occasion.