One wear point that's not visible is the pivot of the bellcrank in the starter nose cone that engages the drive gear into the flywheel. An oval-worn hole in the lever and/or a worn pivot pin will let the linkage push the switch button before the pinion engages the flywheel teeth. That pivot can't be inspected without dismantling the starter. The Y-shaped yoke that moves the starter drive can also wear and create just enough slack to cause a grind.
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!