Before I purchased a SB Chevy Melling Select oil pump, I E-mailed Melling and discussed this with George Richmond, their PR
man. He said the thin castings were breaking just below where they attach to the main cap, mainly in engines that had solid motor mounts and some 4WD off road vehicles.
He went on to say that GM requested that the oil pump be made of thinner castings to save a few pennies. That sounds ridiculous but i believe him because that is the way the bean counters at GM does things.