Ah Doc! That's where just about all of the remaining 413's are then...
I've got FOUR of these undiscovered torque-monsters waiting for a good set of cheap slugs to hit the market. I contacted KB several years ago, but at the time they didn't have any plans of making pistons for the 413.
BTW, here's something to ponder about... The smaller diameter pistons of a 413 would also mean less ring contact-area in the cylinders, so less ring-drag. Freeing up torqy horses. If my stone-age calculations are somewhat correct, there's a combined total of roughly 13.8" (!) less ringcontact on a 413-piston compared to a 440-piston.
Smaller piston-tops also mean flame-travel is shorter across the top, meaning faster completion of burn? Which might also give less chance for detonation (Less area where pre-ignition can occur?).
