[NeighborsComplaint]

Originally Posted by GTOEARL

Read every entry in this blog. Very interesting information and very unfortunate for you. I hope it’s an easy fix. Obviously you’ve heard from a lot of people and a lot of different scenarios of what might’ve happened. Hopefully it’s a spun bearing that allowed the crank to move just a bit to kiss those spark plugs.. The very best of luck for you in solving this issue..

How does the crank move with a spun rod bearing? It's upward travel is limited to the normal stroke plus the main bearing clearance which is restrained by the main cap. Even with a single main bearing failure, the unsupported part of the crank between adjacent bearings does not deflect enough to allow a piston to contact the combustion chamber.

As regards theory that a flat top piston can hit the spark plugs, try this at home: Screw a spark plug into a cylinder head. Turn the cylinder head upside down and and place a flat top piston on the deck. What does it do? It sits nice and flat on the deck without getting anywhere neat the sparkplug.

My current 455 motor was drag raced and had 670 heads with Venolia pop-up piston domes. The motor had repeatedly spun a rod bearings and the owner gave up and had the motor disassembled. Every piston had a ding in the corner of the dome from contacting the edge of the combustion chamber due to interference as a result of the block being decked too severely. Even with spun bearings and an interference fit with the chambers, the piston domes were not hitting the spark plugs.

The pistons (3) hitting the sparkplugs and no piston to valve contact is a good story, but it doesn't add up with "no other damage other than a spun bearing and sheared cam gear keyway.