The cylinder head holds all the valvetrain in place. Camshafts (2), valves (16V DOHC, 4 valves per cylinder), valve springs (16), rocker arms (16), lifters (16), etc. A lot of power can be gained by upgrading the valvetrain. In fact, most of the power we can gain out of our engine comes from the top end.
Machine work can also be done to gain power as well. 3 angle valve jobs, porting, polishing, etc.
The cylinder head is bolted to the top of the engine block by either using factory head bolts or aftermarket ARP head studs. If you ever plan to make any kind of power out of your engine, you need to upgrade the head bolts to ARM head studs. Between the head and engine block is a head gasket. This can also be upgraded, but the OEM composite head gasket, or even the cheap Felpro head gaskets will handle a lot of power.
