Motor carnage!

[hawgsquatch]

If you look at you most fried piston you can see how the top has been pockmarked and eroded away. That is a sign of pre-detonation. Basically, it is caused by the gas oil air mix exploding rather than burning rapidly. It is usually caused by low octane fuel, but can be caused by advanced ignition timing. If that was the case the lower crank bearings are likely to be beat to death and new pistons will last right up until one of the cages spits some hardened bearing race into the combustion chamber. Check it out before you invest.