Understanding why a relatively inexpensive head gasket costs so much to replace
means first realizing where it's located. In the engine block are pistons that travel
and down in cylinder bores. The pistons are connected to rods, which in turn are
connected to a spinning crankshaft from which the vehicle takes its power. Bolted
the top of the engine is the cylinder head. Inside the cylinder head are valves that
open and close to let air and fuel into the cylinders and expel the spent exhaust
gas. The head gasket is compressed
between the engine block and the cylinder head. The head gasket seals in the internal
combustion process and also keeps
coolant and oil from mixing together as the two fluids travel from the engine block
to the cylinder head. Head gaskets
themselves are not very expensive. What ends up costing big money is getting to it.
Removing and replacing the head
gasket means undertaking the major operation of removing an engine's head.
A head gasket can fail for a number of reasons, none of them good. Engine overheating
is the most popular culprit. The metal of the engine block and head expands beyond
limits during overheating. This expansion can pinch the head gasket to the point
where it will fail. The next major causes of head gasket failure are engine pre-ignition
or detonation. The crux of these two conditions is out-of-time combustion occurring
in the cylinders. Internal combustion is supposed to occur with both valves closed
and with the piston at the top of its power stroke. Out of time combustion puts extra
strain on the valves, pistons, and head gasket. Ironically, engine overheating can
also increase the likelihood of pre-ignition and detonation. The sound of an engine
pinging or knocking is the insidious sound of damage.