The rivets that hold the main bearing ball cage together will cause this type of damage when they let go .

Rick

 

Take a look at the pins in the piston that keeps the rings from spinning make sure they are still there if the come out they can do alot of damage

 

The damage you are seeing is from the piston slapping the head. There is no little piece on metal (ufo) that needs to found. You need to split the cases and rebuild the bottom end with all new bearings (case and rod) and seals. I would also suggest that you replace the rod at this time as well.

Be sure to use a reputable machine to do your bore (if you need one). At a minimum, I would hone the cylinder.

 

Im gonna havce to disagree with you on this emmerth, there was deffinately somethind in there. Thest marks are deep and consistent in size... plus theres a couple of marks in the dome where the aluminum has beem "torn" back by something, no way the piston can contact the head in this area.The bottom was completely fresh with the exception of the lower rod bearing, which seems to be in bad shape. Only reason this isnt new is because the crank has to be pressed apart, and
back together using a special jig.

 

Yep! I looked in there last nite and found a small chunk of copper colored metal. It also matches the couple of pices that I picked out of the piston