On my 94 530iT BMW. One of the head studs stress fractured and the nut/threads got lodged between the cam and the head and killed the head. Ironically, the car still ran albeit like a dying diesel. It happened at low speed and I drove the car a few blocks trying to figure out what the noise was before fear of the enevitable finally made me stop/park. It was cheaper to replaced the engine with a used 53K mi motor($3K+$2K labor) than rebuild($10K+$3K labor). At 106K mi, I rolled the dice and 20K mi. later, it still runs like a top. Now I have no way of knowing if that thing was lying arpound under there for a day, a week or a year. But somehow it made its way over to the cam and caused the pistons to have a stroke(pun intended)
