First time I've seen this.  Very deep erosion around the spark area.  Next to the valves.  Between the valves, cutting under the valve seat.  Only three cylinders had it.  The heads had to be junked.  Deep, sharp pitting like, picking like erosion.  About 5mm deep.  

I don't have a sure idea what caused it.  Surely it is related to ignition or explosion or something of that kind.  

This engine has overheated badly.  One of the cylinders has even melted.  

I wonder, if anyone has seen any of this before.  And if any of you know what caused it.  

Best to all,
Zoltan