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If it were a bolt, there should be a thread or two still visible. It looks to be too big of a hunk of crap to be a nut or washer. If it bounced around inside the combustion chamber it may have damaged the piston ring groove which may score the cylinder wall. The pistons are aluminum and are easily beat up with a piece of harder steel.
FOD potentially can bounce from one cylinder to another by getting spit back into the manifold and down another intake runner.
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