A spun main bearing generally happens because of oil starvation. The bearing gets hot, melts, and "adheres" itself to the crankshaft and spins in it's jounel. That will spread metal particles throughout your engine and do damage to the journel.
Generally a spun bearing is cause for a rebuild, if for no other reason than to clean out the engine of metal pieces and line bore the bearing journels to repair and realign them. HOWEVER, I have personally known people to just drop the oil pan, unbolt the crank enough to replace the bad main bearing, button it up and on they go.
I would first, double check that it is in fact a spun bearing. IF it is a spun bearing and not something else that is easily repairable, then replace the engine with a remanufactured one. Don't mess with the old one. It's possible that it costs less to rebuild it, but downtime would be measured in weeks rather than days. I hope this helps.
Monday, March 5th, 2007 AT 11:21 AM