One quick way to check for starter position is to spray-paint the teeth on the starter gear, then start the engine a few times. The paint wears away and shows exactly where the teeth are meshing. They SHOULD NOT bottom out- - - -there needs to be a small area at the bottom of the tooth with paint remaining on the pinion gear. Adding shims moves the starter housing down, increasing the clearance at the tooth root. Ideally, there should be about 1/16" of clearance at the bottom of the teeth.
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!