Timing and low octain gas(runs hot)
, it is called spark knock, it could be doing it and you couldn't hear it. Seen it eat little pin holes in the head and piston, the�valve seat�will come out to.

Subject: Erosion between valves

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