Starter pinion gear does not engage

- The one way engage gear (the Bendix gear) may broken internally or externally
- Throwout fork for the Bendix gear may be bent up or partly broken
- Teeth all worn out on the flywheel. Do/did you ever have a grinding of the Bendix against the flywheel?

Bottom, line: pull the starter and look at the flywheel and check the Bendix/throwout, etc.