[olddog]

When you pull the pistons, look at the land area of the pistons above the top ring. If the pistons do not have enough clearance, when they heat up they can grow to a diameter larger than the bore, resulting in a break just like yours. A lean condition will heat a piston more and make it tighter. There is always some cylinder running leaner than the rest. If there is a lot of wear on the lands, that was very likely your problem.

I would measure the piston clearances in this engine. Then compare that to what these type pistons should have. Also if you have the build sheet, compare the clearance now to the clearance when it was assembled. It only takes a 0.001" change to make all the differance in the world.

Buy the way, although forged pistons can take the most punishment and are best for maximum performance, they also require the most clearance. If you have quiet exhaust, you can hear them slapping when cold. Warm up is critical.