When checking/tightening all of your nuts and bolts during periodic maintenance, be sure to check the bolts on the starter. If the top bolt starts to loosen, it will torque when the starter is engaged. The starter teeth will then be out of alignment with the flywheel, and will shear off. The next time you go to start it, it wont engage as there are no teeth left on the starter gear. Ask me how I know...
