Lots and lots of possible reasons listed for why/how that cap is broken. Just for fun, I will add one more. Look at the bolts protruding through the cap. It;s possible the bolts bottomed out and there was little clamp load on the cap. Pontiac blocks are counterbored in the block so the threads don't pull on the surface. But those bolts look to have 2-3 full threads exposed below the cap? Hard to say from one pic. Look at the broken area and see if it's grainy through the entire break. If it is, likely an impact break from dropping it at some point. If any rubbed areas or smooth areas in the break, the two pieces were working or rubbing together while running.
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