Inform me if I'm being stupid and there is an abvious reason. But why is there no cylinder and cylinder head all in one's?  If you need a good seal between the cylinder and head, with some people glueing them in, why isn't there a single solid piece that is the cylinder and the head, that way the air can't possibly escape or the head come loose?     Edit: just realised it would be pretty hard to manufacture out of a sturdy material like metal. Would that be the only reason?