I HAVE TO THINK ITS CLEARANCE PROBLEM. TWO MUCH CLEARANCE BETWEEN CRANK AND RODS OR CRANK AND MAINS WILL DO IT . IM AN AUTO MACHINEST AND HAVE SEEN THIS HAPPEN A LOT . YOUR MACHINEST COULD HAVS ASSUMED THE CRANK WAS STD WHEN IT WAS REALY UNDERSIZE . IT HAPPENS WHEN YOU GET IN .A HURRY. MACHIEST ARE HUMMAN TOO.