Try making a "positive stop" for finding top dead center on a stovebolt engine. It's necessary to put an angle on the threaded rod that's in the spark plug shell or it never touches the top of the piston. Extended tip plugs on a stovebolt have exactly zero risk of touching a piston crown. Just make sure no threads on the plug shell stick into the chamber, even if they have to be machined off. Hands-on experience trumps even an "educated guess" every time.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!