Strange Main Bearing Failures?
 

Has anyone heard of misaligned main bearings in the 90s Vortec 5.7s that cause the crank to knock against the block? I just bought a 99 Suburban with a knock in its 5.7 and had the pan off expecting a rod failure. To my surprise this was not the case. However, there was a ton of copper in the pan as well as magnetic metal on the plug. I can't remember where I read it, but I remember something about these vortec blocks having issues with main bearing caps basically causing pre mature failure due to a poor hone alignment. GM supposedly would fix them by installing .001 bearings in specific places (upper or lower) in the #3 and #5 bores to align the crank properly. Is this true? I haven't had the engine out yet to verify the cause of the knock being the mains, but it seems possible in this case. It has 60 psi on cold start up and around 25 psi hot idle. So maybe just the thrust washer on the #5? I've no idea.

The knock is loud on start up, and softens once the engine is hot. Once hot at idle its barely noticeable, but under load gets very loud. So I don't think its piston slap. It hasn't affected power as I can tell. There is a small misfire randomly under load, but it may be a bad spark plug wire.