Originally Posted by cragdweller
Detonation causes cracked pistons. Overheating can contribute to this, but I would look at fueling (both mixture and grade) and timing first. That piston died from detonation.
Actually that was my first thought too, hence my Q's.
I believe the octane rating in the US isn't too high, so was surprised to see 11:1 mentioned. Here we've got 95, and my CRF230 with a 11:1 piston in pings low down when hot and loaded. Being aircooled this may be expected, accepted even as long as you pay attention, but on the big one I'd stay away from it. This is why I bought a 10.5:1, after all I'm not racing it.
FlyGuy may want to look, with a timing light, at the ignition. Maybe it's out a little, a worn key maybe? Check anyway, if it pings the best piston will give way..