By Mike Bumbeck, automedia.com

Difficulty: Difficult

Estimated time: 300 minutes

Understanding why a relatively inexpensive head gasket costs so much to
replace means first realizing what it sits between. In the engine block are
pistons that travel up 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 to 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. Head gasket
repairs mean undertaking the major operation of removing an engine's head.

Boiling over

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.

Replacing a head gasket

Head gasket replacement is not a job for the average do-it-yourselfer.
While a head gasket expiring due to old age is rare, it usually means
everything else is worn out to the point of replacement as well. Cracking
the head off the engine and peering inside may reveal that everything else
got fully cooked, too. The best way to prevent head gasket failure is to
keep the cooling system in top working order and make sure the engine is in
tune and running correctly. The failure of a $5 thermostat or $3 coolant
hose can cause thousands of dollars in engine damage in a hurry.

How-to tips

The following are a few general tips to changing a head gasket. For
vehicle-specific head gasket replacement procedures, always consult a
service manual.

Step 1:
Getting down to the head gasket can be an involved process. Always mark and
index everything before removal. In this case, it was easier to remove the
exhaust manifold, head and intake as a unit. A service manual is the first
and most important tool in the toolbox.

Step 2:
Check the head and block for flatness. A machined straight edge across the
surface and feeler gauge will reveal all. The service manual will provide
specifications. Blocks or heads out of specification must be sent out to
the machine shop and be repaired. The machine shop will also be able to
check for cracks.

Step 3:
Prepare the surface. Never use anything that will remove metal. Be careful
not to scratch the deck of the block or mounting surface of the head. While
it may seem like a good idea to use a scrubber pad connected to a power
tool, the surface irregularities created can cause sealing problems down
the road.

Step 4:
Chase the head bolt or stud holes with a tap or thread chaser to remove any
crud and prepare the threads. Use compressed air to blow out any
stragglers. Correct head bolt torque is of the utmost importance. Any
interference can throw off the torque readings.

Step 5:
Prefit the head gasket in the correct orientation. Never use sealant unless
specifically required by the manufacturer. Head gaskets that require
sealant will usually come so equipped. The service manual will outline
sealant locations and requirements.

Step 6:
Correct bolt tightening sequence and torque is key to proper head gasket
sealing. Use new bolts if required. Coat head bolt threads with some engine
oil before installing. Always follow tightening sequence and torque
instructions to the last detail.

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