this should be explanation, page 36
http://www.cabletiemotorsport.com/Pistons.pdf
I must also note that damage at all pistons was only at exhaust side. The true is that I probably too much advanced ignition after WI installations. As I never hear detonations, with WI a feeled safe. But not all dets can be audible.....
It wasnt forgeds, just standart pistons.