Jake,

    There is 14 MM and older Fo*ds used 18 MM size threads. You have 14 MM.
Any good "Flaps" should have a thread chaser, use that instead of a tap. Go
very slow, by hand until you get it started.
    Good luck.

Howard

I somehow managed to burr the spark plug threads on one cylinder last night.
All the tin on these air cooled motors makes changing the plugs much harder
than it should be.