Wore out teeth.... does it every time. Again make sure your starter bendex gear teeth are good as a bad ring gear WILL EAT UP a new starter which inturns WILL EAT UP the new ring gear! Been down that road with a cheby... only to find a spun bearing after...

 

well without having anything to measure them with and no reference point they all looked good. . . compared to each other.

 

I tell you something else abotu your type of problem.I'll bet that every starter you have had has a 9 tooth bendix.in some of the broncos ford used a 10 tooth.I own a starter/alternator rebuild shop and have been down this road before.sad thing about this is that know body even has a listing of has or any idea about this difference.I had a customer bring a starter in for rebuild I replaced the bendix with out checking his old one.
when he installed the starter and tried to start his truck he has serious problems.when he returned unit I could find nothing wrong.luckily I had not thrown his old one away .when i started checking things closer I discovered my error.when i put the ten tooth in his problem was solved.

 

all my starters are 9 tooth and i just replaced the flywheel and all is right again

 

make sure you put the block plate in. not using it may kill another starter eventually or worst case knock out the crank thrust in the engine