Electrodes breaking off dont melt pistons mate but they can damage them as we see........it takes a hell of a lot to melt a piston though.
If the car was running lean because the pump wasnt sucking up fuel,as it was, and the plug tips HAD remained in tact.........the pistons would have still been knackered cos of the heat in the cylinder in turn melting a piston or two maybe more in time, no ? So if the plugs HAD remained in tact..........the damage would possibly have been substantially more with melted pistons and a fooked block / bores ?
So the plugs arent really in the equation. Because the AFR problem was the main factor in the chain of damage. The plug damaged the top of the pistons.....but it could have been worse if they had kept in one piece and the car had kept running lean.
The damage caused to the piston in this instance is physical.......as in chunks out of it................not heat related from a lean AFR which was apparent in this car,this in turn caused the plugs to overheat by the looks of it and tips to break off. The problem isnt plug related, it was AFR related due to a fuelling prob.
Michael