Cylinder heads, like most parts, eventually, they can crack or fail in other manners. Before we get into the repair side, we will touch on what a cylinder head is, what causes failure, without going into too much technical detail, but feel free to skip ahead if you are already familiar.
An engine, without the hanging parts (starter, alternator, fuel pump, manifolds, etc.) consists of 2 important parts, the cylinder head and the engine block.
There different types of engines, namely over-head-valve (OHV) and over-head-camshaft (OHC), we'll use the OHC engine for reference. The engine block, or cylinder block, contains the crankshaft, connecting rods and pistons, the pistons are housed in, and travel, in what are known as cylinders. The cylinder head fits on top of the block and covers the cylinders, but note, that between the head and block, a cylinder head gasket is used to ensure the combustion process is sealed and coolant and oil do not leak as they travel through the engine for cooling and lubricating.
Can be caused by various factors and can result in such catastrophic damage that may require the sum, if not most, of the mechanical parts to be replaced, or worse the engine to be replaced due to pistons hitting the valves, bending them, breaking camshafts/rocker shafts, lifters or rockers, destroying pistons. Factors include:
Warping of the cylinder head, particularly referring the face of the head (the part that makes contact with the head gasket and block), is caused by overheating. The cylinder head may not crack but the face is gets warped when it should be flat. The cylinder head can be repaired by skimming, however, if it has been skimmed before then there may not be sufficient material to skim, therefore a new cylinder head is needed.
Sinking of the pre-combustion chambers, occurs when the engine overheats and if the chambers and the cylinder head are made of two different materials, for example, a cylinder head is made of aluminium whereas the pre-combustion chambers are ceramic, as they expand and contract at different temperatures, the combustion chambers may be smaller than the hole it was made for and sink in. Since ceramic chambers cannot be skimmed, it is more viable to replace the cylinder head.
Depending on the severity of the damage, a cylinder head can be repaired, however, you have to consider whether it is economical to repair.