No experienced engine builder would intentionally bolt a cylinder head to a block without installing a gasket between the block and cylinder head.
Why? Because the gasket seals the block deck surface to the cylinder head surface. As we all know, a piece of metal that looks smooth is actually rougher and wavier than it appears. That microscopic surface roughness and waviness creates leak paths, especially under high cylinder pressures.
From the pages of RPM Magazine August 2021 – Read the full article here.