Cracked Cylinder Head: Can it be Repaired?

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.

What is a cylinder head?

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.

What causes cylinder head failure/damage?

Cylinder heads can fail in a manner of ways:

Most common

Cracking
Internal parts of the cylinder head suffer mechanical failure

Less common

Warped beyond economical repair or warp repaired too many times before, i.e. over-skimmed
Pre-combustion chambers (diesel engines) sink in or protrude and the head cannot be repaired due to the material of the chambers (ceramic chambers cannot be skimmed and the head must be replaced) or replacement chambers are unattainable
Corrosion of the cylinder head due to over-fuelling or coolant impotence

What causes cracking?

Commonly the engine overheats to the point of cracking
Overheats and cold water is immediately poured into the radiator causing a difference in temperature, like putting a cold cup into the microwave
Metal fatigue due to sustained variation of stress

What causes mechanical failure?

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:

Timing component failure resulting in piston valve contact (interference engine)
Dropped valves resulting in damage to the cylinder head from the valve embedding back into the cylinder head and damage to the pistons,
Dropped valve seats resulting in debris lodging into the face of the cylinder head or worse
Mechanical components coming loose, for example nuts or bolts loosening, shims dislodged
Failure or incapacity of auxiliary engine parts, like insufficient oil lubrication resulting in seizure
Poor maintenance for example oil turning into sludge

What causes warping or sinking of combustion chambers?

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.

What causes corrosion?

Over-fuelling, burning too rich, or old coolant can result in corrosion or pitting of the metal
Over-fuelling is when there is too much fuel being consumed by the engine caused by a malfunctioning fuel regulator or leaking injectors (the part that injects the fuel into the engine during operation) resulting in the cylinder head being corroded.
Coolant has anti-corrosion properties and if not changed on time or being of poor quality can result in corrosion

Can a cylinder head be repaired?

Depending on the severity of the damage, a cylinder head can be repaired, however, you have to consider whether it is economical to repair.