just clearing a post that was double entered and left here for a long time.
Answer could be:
a lot of miles on the engine. oil change interval factory clearances at max allowable from the factory but allowable so reduced life normal wear, nothing lasts forever engine pinging. detonation causes abnormal stresses on bearings. Parts not perfectly made from the factory but passed inspection: (passed allowable tolerances but over time tolerances grew) a. defective bearing b. defective crank c. defective rod d. defective wrist pin e. defective piston Did you loose oil pressure? Is it a rod bearing? Crank bearing? Cracked flex plate? Loose harmonic balancer? Piston slap? Bad lifter?