Crankshaft Bearings
Crankshaft bearings are made of a softer material than the crankshaft journal or cap they sit in. This insures that the bearing material wears before the journal. The crankshafts journal doesn't actually ride on the bearing surface instead it rides on a thin film of oil between the journal and the 
bearings.
If the crankshaft journals are worn, scored, or out of round the gap between the two is affected. Whenever there is contact between the journal and the bearing there will be excessive wear.
Check the bearing for metal particles embedded into its surface. This would indicate two metal parts in contact with each other creating these metal shavings. It's important to find and repair this problem before proceeding. Whenever you find metal shavings in the bearings the oil pump must be inspected.
Look at the backside of the bearing insert for wear indicting that it has spun in its saddle. It's necessary to check the main bearing bore for misalignment.
There are several methods used to check the main bearing bore where the bearing inserts ride. Its easiest to use a straight edge and a feeler gauge. Use a blade half the thickness of the maximum clearance. The feeler gauge must not fit between the straightedge and the bores surface. Warpage can be corrected when it is not too far out of specifications. The block can be line bored and oversized bearings installed.
Bearings come standard, oversized, or undersized. They have the information stamped on the back on the insert. An oversized bearing has the same material thickness on the inside of the bearing facing the crankshaft journal with more material on the outside facing the bore. The oversized bearing is used after a block has been line bored. The opposite is true for an undersized bearing as it has more material on the inside facing the journal and the same amount facing the bore it fits into. These undersized bearings are for a smaller diameter or machined crankshaft journal.
