I confess, I am the one who said "HELICOIL, HELICOIL, HELICOIL!!!" to Miguel, however, that was for a cylinder/head stud so maybe I can weasel out.
I recently helped my neighbor with his outboard and it's problem was that the end coil of a helicoil insert in the spark plug hole extended into the cylinder, heated up and caused pre-ignition. The other cylinder had been repaired with a one piece insert, much cleaner job and equally easy to do.
Contact auto parts, speed shops and motorcycle shops for someone with a kit.