7 psi of steam pressure will blow a head gasket - but 800 psi of combustion pressure won't?
The steam is being relieved from the water jacket... back into the water jacket. It's a closed system, and all water passages are connected to each other.
The surface area touching water of a thick cylinder wall and a thin cylinder wall are exactly the same. What's different? The extra wall thickness allows heat to be conducted faster within the casting.
