The last paragraph nails it. There isn't a cylinder head on any engine ever that doesn't lift and flex. There has never been one.
If there was, you could machine the head and block to flat and smooth and not use a gasket.
That's why you need a gasket. The head lifts and flexes and moves all over hell. Without a gasket you'd never get it sealed.
As one more proof I'm not nuts...if you use dead soft copper gaskets you know what a beeeeeeeeotch to get sealed. Not compression sealing, but coolant and oil sealing. That's because that dead soft copper gasket can not follow the head as it moves away from the deck and flexes with every single firing cycle.
My buddy had a blown iron 392 hemi we built and I begged him to let me Oring the the block and heads but some gasket maker promised him his dead soft copper gaskets would seal 10 pounds of roots boost without Orings. So I did it without the Orings.
It was in a roadster. When we fired it up and got it warm I walked around the engine and stuck my hand down by the head/block interface at all 4 corners. He said WTH are you doing...and I say I'm feeling the combustion leaking out past the gaskets!!!
So he sticks his hand down there and he feels it. He was pissed. That very day the engine came out, I took it back to work, pulled it all the way down and did the Orings.
You couldn't feel the combustion leaking at idle any more, but I promised him at full boost and full song it was leaking!
It's just the way that it is.
