I've seen that - generally, there are two potential causes:
- Overtightening of the screw. More common with iron parts than with aluminum (usually, the internal threads in the aluminum part just pull right out, you're left with a fairly smooth hole.)
- Void or other fault in the casting. I'd be interested in spreading that crack to see if there was an "air pocket" in there that would have led to the failure. Else, you've got a minor (relatively) internal stress riser that was caused by uneven cooling of the cast part, which could have also caused a casting failure (this will depend highly on whether or not that boss is "cast solid" or they have a core in place - since there will be a hole there anyhow. Depends on how the foundry does their work, and how well they recover their chips for reuse - better recovery means no-core bosses, which is better.)
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