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Based upon the limited view of the teeth on the starter gear, it looks like it may not have been properly aligned when installed and was not kicking back far enough. Are the teeth showing signs of wear further back? It looks like the ends were chewed off, which would suggest that it was only partially engaging and that is why they were torn off. It's hard to tell, because there is not a good view of the teeth further back to see if they are shiny.
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