Photo below of spark plug out of M103 engine. Something has hit the end of the plug don't know what. Must have been a valve or piston, but how?

I just put head back on after doing heat gasket. Apparently I screwed something up. Before removing head engine was set at TDC for Cylinder one and cam grear sprocket was put back on exactly as taken off. Chain was put on at same place on cam gear as it was removed

What could have caused my problem and what can I do to fix it?


Edited: The spark plugs are all less than one year old but could it be that that one plug just goes to far in the hole that it threads into? I don't think I can shim it out since the plugs seat into hole, ie no washer.

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