Since cast iron transfers heat very quickly, and combustion temperature can approach 2,000 degrees F. (confirmed by my measurements of exhaust port temps during dyno runs) cylinder walls thinned by big overbores tend to transfer a LOT of heat to the coolant during heavy throttle operation. Once it gets to the coolant, it MUST be dissipated by the radiator in a hurry. It's not uncommon to see some flash boiling around the cylinder walls unless there's plenty of coolant flow from a very efficient water pump and a high pressure radiator cap to raise the boiling point of the coolant somewhat. I grew up around flathead Ford round track engines bored .250" oversize and stroked 3/8", running 11:1 compression and alcohol/Hydrogen Peroxide fuel, and one of our biggest problems was keeping the things cool! Dad usually ran radiators from F-8 Ford dump trucks on the race car. When I ran dirt track 327's in the mid-1970's with 60 or 80 overbores, we always ran big radiators and 21-lb. radiator caps.
Jerry
