I have seen this a few times: when screwing and tightening bolts in threaded "blind" holes (those not passing to the other side of the part) sometimes the air trapped inside gets compressed and the part walls, if thin enough or made of soft material, crack. You can hear a quick, short and loud pssst! you know the hole cracked. This commonly happens with tight thread steps, or precision bolts, like 1.0
Question is: why not drill a small hole through one side of the part wall reaching the threaded hole, perhaps as near to the hole bottom as possible? That way we could prevent hole or wall fracture, since the trapped air can get evacuated through the little hole, as we screw the bolts in.
I did some quick schematics for this. Thinking it better, maybe dirt/grease glop in the bolts acts like sealant, making the air in the hole more difficult to escape, uh? I also see that the cracks are more frequent in zones where the casting left those mold thin lines.