I suspect the beginnings of the piston damage happened at the time he smoked the plugs. Generally, here's the sequence of things- - - - -a lean condition develops, either from wrong carburetor jetting, a clogged jet, or possibly a vacuum leak, and the cylinders affected by the lean condition start burning hot enough to melt things. Usually, the plug conductors, both insulated and ground, melt and/or vaporize. There's enough hot oxygen in the cylinder after all the fuel is gone that it starts eating aweay at whatever's handy, plugs, valves, and piston crowns. Once anything inside the cylinder gets hot enough to ignite the incoming fuel (pre-ignition) during the compression stroke, the hammering action of the burning fuel opposing the upstroke of the piston (detonation) starts melting and/or breaking away small pieces of the piston crown. Most of these are small enough to find their way past the exhaust valve, or sometimes they will weld themselves to the cylinder head somewhere. I suspect the engine in question here began the piston damage during the initial vacuum leak, and even though the leak was stopped, the damage progressed from there. In the case of severe detonation, like in an alcohol-fueled blower engine, the cylinder head can actually be blown off the block.
Hot oxygen is like a sailor on shore leave- - - -it's definitely going to combine with something- - - -RIGHT NOW! The piston is the closest thing, and it also has the lowest melting point. If you're obsessed with either maximum performance, or maximum fuel mileage, remember that gasoline is cheaper than pistons! Don't go too lean!
Jerry
