[olddog]

I do not know exactly where the limits are on the factory blocks. Not sure if it is RPM and balance or torque that kills them. Perhaps it is how hard you drop the clutch.

There are aftermarket blocks, both cast iron and aluminum. They are advertised to take more power. I do not know exactly what they can take. If I was spending the money on one of these blocks, I would get the 4.125 bore block.