One of the advantages of an overhead cam cylinder head is that it eliminates the need for certain valvetrain components. Mounting the camshaft in the cylinder head rather than the engine block eliminates the need for lifters and pushrods. On some OHC engines, the cam lobes run directly atop the valve stems. On others, cam followers or rockers and hydraulic lash adjusters are used to operate the valves.
Getting rid of the lifters and pushrods reduces the reciprocating weight of the valvetrain and theoretically allows the engine to rev higher and produce more power. But it also requires a more complex cam drive system and limits the amount of metal that can be milled off the face of the head if the head needs to be resurfaced.
Excessive milling changes the installed height of the camshaft with respect to the cam drive, which in turn retards valve timing and ignition timing (if the cams also drive a distributor or crankshaft/camshaft position sensor).
Mounting a camshaft in the cylinder head also increases the risk of cam bore misalignment, which can lead to cam binding and breakage. The heads are usually the hottest part of an engine, so if the engine overheats the heads distort more than the block. Aluminum isn
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