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Message: <blockquote data-quote="BigTang" data-source="post: 489714" data-attributes="member: 10950">spun bearings are typically caused by 2 things. RPM and lack of proper oiling. Talk to your engine builder about the tolerances that your engine needs to have and make sure you use at the very least a new oil pump, if not an upgraded one.a spun bearing is when the rotating part, in this case the crankshaft, contacts the bearing(remember it rides on a slight cushion of oil) and grabs a hold of the bearing and twists it in its seat trashing the crank and causing you to need a line bore on the mains in the block. The cams actually don't use any bearings on the 4.6's and rely on the bore in the head itself as its bearing surface, if you screw up that surface you have to get it line bored and have bearings installed.</blockquote>