What are blown head gaskets?

Answer 1

There is a gasket between the head and the engine block. That gasket seals those two parts together allowing oil and coolant to flow freely between the two parts. If the gasket becomes cracked, warped, or otherwise damaged in any way the oil and coolant can mix together. The coolant can also enter the combustion chamber. In either case it will destroy the engine if it is not repaired immediately. In most cases a head gasket will blown due to the engine overheating.