I have a 350 that was rebuilt and only has 5 k miles on it since the rebuild. It was bored 30 over and all the rods are origionl and so is the crank. When I took the motor apart before the rebuild the crank bearings looked fine for 100 k miles on the motor. The otherday I had to take the motor back apart and the (new) bearingd looked very bad. Some were wore all the way to the copper. The crank measured to be within specs, but the my question is would the 30 over pistons cause the balancing to be off enough to do this. The pistong and bearings were the only thing that was replaces and the rest is all origional. Correct breaking procedures were taken. thanks

 

Probably the wrong oil... I'd guess too thick

 

I used 10w30. Same as when I did the breakin when the engine was brand new.

 

did you check your oil clearance when you assembled the engine? 30w dino oil is usually the recommended break in oil.