You're really flying; phone poles flash by in a blur as you nail the pedal to the floorboard, and then it happens-oil pressure drops to zero and the thunder turns to death rattle as the oil film between the spinning crank and main bearings vanishes. Bereft of lubrication, it only takes an instant for heat to form a sticky bond between the crankshaft and the bearings before they start spinning in the block like some berserk lathe from hell. Congratulations, you're a pedestrian.
Spun main bearings are a disaster, but if you kill the ignition before things get critical and the parts hastily leave the crankcase, you can usually save the block. Here's a primer on how a quality machine shop repairs the damage.
Even after steel abrading...
Even after steel abrading to clean the block for surgery, the Nos. 2, 3, and 4 main bearing saddles still show score marks left by the spinning bearing shells.
In severe cases, the spinning...
In severe cases, the spinning bearing shell can overheat the block and cause cracks in the saddles. A magnetic inspection hunts them down using a ferrous (iron-based) powder that is magnetically attracted to cracks. Our demonstration block, however, was crack-free.
This block wasn't so lucky....
This block wasn't so lucky. The pointer indicates a deep crack that travels into the saddle and intersects the threaded main cap bolt hole-a structural meltdown waiting to happen. While some cracks can be repaired by brazing or machining the damaged saddles to accept a pinned saddle insert, the costs quickly begin to outweigh the benefits. Most blocks with cracked saddles go to scrap.
Spinning main bearing shells...
Spinning main bearing shells also damages the caps. The pointer shows the telltale score marks. Like the block, the caps must be magnetically inspected for cracks before repairs begin.
Sal Alcarez showed us this...
Sal Alcarez showed us this broken cap and said that the motor still ran when it came into JMS. Its owner, a road racer, complained of a vibration. Amazingly, the crankshaft and block were not damaged.