By Jim Marotta
A head gasket's job is to seal combustion gases within the combustion
chamber and to prevent coolant and oil from escaping the engine at the
head-to-block joint. These requirements have not changed over the years, but
the engine operating environment has. Increased cylinder pressures and
temperatures, the use of dissimilar block and head materials, and the trend
toward lighter-weight engine castings and reduced head clamp loads result in
operating conditions that cause much higher levels of horizontal and vertical
motion between the head and block.
After only a few thousand miles of operation the finishes
and flatness of engine surfaces are no longer virgin castings: the engine has the engine has heated and cooled repeatedly, making the job of sealing much more difficult.
Another challenge facing modern-day head gaskets is zero
tolerance for even minor combustion or fluid leaks, primarily due to emissions
requirements. Today's head gaskets have to provide a perfect, lock-tight seal
in spite of ever increasing pressures, temperatures and other dynamic forces.
Evolving Head Gasket Technology
The original head gasket technology is known as a sandwich-style
gasket. Sandwich gaskets combine metal-faced
materials with a filler material in between. Since engines of the day were low
horsepower and low compression, these gaskets sealed engines adequately into
the late 1950s.
As engines grew bigger and more powerful in the early 1960s,
some manufacturers started using embossed steel shim-style gaskets. While the
steel shim gaskets were even lower tech than their predecessors, they led the
way to today's multi-layer steel head gaskets.
The later '60s saw the rise of a new type of composite
gasket featuring a perforated steel core with a composite face material applied
to each side. These were the first head gaskets that didn't need a secondary
(spray-on or brush-on) coating to create a micro-seal between cylinder head and
block, or re-torquing.
As engine technology progressed, conventional
composite-style head gaskets couldn't withstand the increased casting motion in
the new smaller, lighter, and more powerful engines. Lateral motion between the
aluminum head and iron block was destroying the facing material through a
shearing action, and the vertical motion - in which the head was actually
moving away from the block - was over-compressing the gasket.
During the late 1970s and early 80s gaskets contained
expanded graphite facing material mechanically clinched to a steel core, offering
excellent conformability to surface finish variations. The natural lubricity of
expanded graphite accommodated some casting motion, which occurred frequently
in the early days of bi-metallic engines.
In the early 1990s, manufacturers introduced Multi-Layer
Steel (MLS) head gaskets. Engines using these gaskets featured higher
combustion temperatures and pressures, lighter-weight castings, and reduced
head-to-block torque loads to prevent bore distortion and combustion blow-by.
An MLS gasket is composed of stacked and embossed multiple
layers of full-hard stainless steel, which allow it to act as a "spring"
between the head and block to maintain sealing stress against the castings. MLS
technology is now the industry standard.
Diagnosing Cylinder Head Gasket Leaks
When a leak occurs externally it is easy to diagnose, as you
will see the coolant, oil or carbon soot. But what if no external leak turns
up? It is time to check for an internal one.
The first and easiest check is a dye test. Use a special
tool that looks like a turkey baster, to draw air from the top of the radiator.
The air passes through a dye that changes color if exhaust gases are present.
This is usually a reliable test, as long as the dye is fresh.
Alternatively, use an exhaust analyzer to
sniff the same air. Be careful to not allow any coolant to enter your
Cylinder Leak-Down Test
The most accurate test is the cylinder leak-down test. Bring
each cylinder to TDC (top dead center) and pump compressed air into the
cylinder via a leak-down tool while observing the coolant level in the
radiator. When you pressurize the offending cylinder, it's hard to miss the
reaction of the coolant.
The most accurate way to test for cylinder
head gasket failure is with a cylinder leak down tester.
If there is no reaction in the radiator, remove the
pressure, turn the engine until the next cylinder in the firing order is at TDC,
and perform the leak-down test again. This test will diagnose failures that
involve the coolant passages, but what about failure between cylinders?
An engine with compression leakage between cylinders will run poorly, yet some will
continue to drive their vehicles, resulting in a very expensive engine repair.
Aluminum cylinder heads have little tolerance for hot gases and extreme
pressures: just a few miles of driving
will ruin a casting and the cylinder block.
Once again, connect a leak-down tester, and bring the
cylinder to TDC. Adjust the regulator for 100 psi going into the cylinder and
read the second gauge showing the pressure held in the cylinder. Two adjacent
cylinders that show abnormally high leakage will tell you where your potential
As a final set of checks be certain that:
All of these checks are to ensure that cylinder temperature
and pressure are not abnormally high.
Why Do Head Gaskets Fail?
Today's OEM (original equipment manufacturer) design
standard is 100,000-miles-plus durability, which requires improved
manufacturing techniques, better quality control, and closer assembly
tolerances to rack up the miles without incident.
Prior to the advent of engine controls, detonation and
pre-ignition were frequent causes of head gasket failure. While these issues
still arise, they are less common since head gaskets are capable of
withstanding much higher combustion pressures and temperatures.
When a head gasket fails, investigate to determine the cause of the failure. Remove the old head
gasket and measure the bore diameter from the intake side to the exhaust side
of the gasket. If the gasket bore has grown .010" over standard, there's a good
chance the engine has seen some detonation. Burning and carbon tracking can
also be a good indicator of detonation.
Burning and carbon tracking on this cylinder
head may indicate detonation.
Burning and carbon tracking from the
cylinder head gasket side.
A damaged sealing ring on a gasket from an engine in which the cylinder head torque was not maintained.
You can gain a great deal of information from the witness
marks on the face of the gasket and the corresponding deck surface. These marks
are a direct reflection of surface finish. Read them carefully before
installing a new gasket. The correct surface finish on the heads and block is
critical to a good seal.
When replacing gaskets, pay special attention to head bolts
and block preparation:
Always use the OEM-recommended torque sequence and specifications when replacing a
cylinder head gasket. Tap into your data 24/7 at: www.ChiltonPRO.com
A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 20 years experience in the automotive industry.