[Mike51Merc]

The problem with overheating (to the point of boiling over) is the steam pockets that form from the loss of coolant. The pockets can allow superheating conditions to occur and the hottest spots in a flathead block are the Siamese exhaust passages. This is why a lot of flatheads have cracks in the middle cylinder exhaust ports up to the valve seats.

The moral of the story is to never allow your flathead to boil over. It's one thing to get hot, it's entirely another when it gets so hot that it blows out the coolant like a geyser.