Interesting. I have had the grinding issue as well but just replaced the starter with a new one last week (reduction type). I noticed the shaft the starter pinion moves on also moves about a quarter inch which could be the culprit. Seems to be quite sloppy in there so I am thinking a spacer or washer has come apart and is causing the play in the shaft. My original starter lasted 900 hours, replacement about 150 before the grinding started, so some day I will open it up to see if I can rescue it for a spare.