Quote:
Originally Posted by Va68goat
So, I'm going to have to go through this whole thread to see who guessed the piston slap first so we can determine the winner. I will send the winner a Pontiac sign like I said I would. Hell, I'll even throw in some local Virginia beer. Hopefully we can all agree on the winner! Hahahaha!
I do have a question though. There seems to be some confusion if the bearing spun or not when it failed on the dyno. After reading the recent post, I do remember that it was discussed about the spun bearing. Has anyone heard of a piston skirt collapsing when a bearing is spun? Is that normal? What happens when a bearing is spun that could or would cause the piston to hit the head. Is that even possible? Considering that the knocking noise has been the same since when I first received the motor and stopped when that one piston was replaced, I would just like to get a better understanding of what happens when you have a spun bearing.
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If I don't win, I want a framed picture of the piston as a consolation prize. LOL
As far as collateral damage from a spun bearing, that's a theoretical debate that could keep this thread going forever. When a rod bearing spins inside the connecting rod, it rapidly self destructs due to friction and heat. During normal operation, the inside diameter of the rod's big end clamps the bearing halves in place (bearing crush). When bearing crush is lost for whatever reason, the inserts will spin inside the rod's big end. Loss of lubrication will also cause a spun bearing, when the friction wanting to turn the bearing insert overcomes the bearing crush keeping it stationary in the rod. Usually it makes a hell of a racket and the damage is limited to the rod and the crankshaft, unless the rod comes apart (thrown rod).
Since this engine didn't throw a rod, the rod bearing would have had to do one of two things to damage a piston:
1: Rod bearing is destroyed enough that the excess slack between rod's big end and crank journal allows piston to hit the head during TDC. That would have left witness marks on chamber and top of piston. Should result in damaged piston crown and pinched #1 ring land.
2: Rod bearing gets hot enough to friction weld itself between crank and big end of rod. Rod is then effectively welded to crankshaft. Rod tries to whip around with crank, restrained only by piston. The piston is damaged from the abnormal forces. However, this typically tears the rod cap off or breaks the rod, resulting in total carnage. Motor would have had to be shut down just before it chucks a rod across the room, to damage a piston like this but not chuck a rod.