Cylinder Head Gaskets - Sealing Under Pressure

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.

Dye Tests

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 exhaust analyzer.

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 leak is.

As a final set of checks be certain that:

  1. The engine is in a proper state of tune,
  2. The turbocharger control is functioning properly,
  3. The EGR ports are clear and the system functions properly,
  4. The cooling system is functioning properly, and
  5. Exhaust backpressure is within limits.

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.

Replacing Gaskets

When replacing gaskets, pay special attention to head bolts and block preparation:

  1. Be sure to clean and chase threads. (There is a difference between a tap and a thread chaser:  Using a tap to chase the threads on your cylinder block will actually cut new threads instead of cleaning them.)
  2. Lubricate bolts (and washers, if applicable) and the underside of bolt heads.
  3. When it comes to torquing the bolts, always follow the OEM-recommend torque sequence and torque specifications to achieve the correct clamp load.
  4. Most importantly, have your torque wrenches calibrated regularly. Since torque specifications are so important, an uncalibrated torque wrench could leave cylinder head bolts too loose or could possibly overtighten and break them.


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 or www.ChiltonDIY.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.


Posted 21 Dec 2010 10:14 AM by Jim Marotta

Comments

TiLlaves wrote re: Cylinder Head Gaskets - Sealing Under Pressure
on 21 Feb 2014 9:07 AM

Great article.

Chilton_Editor wrote re: Cylinder Head Gaskets - Sealing Under Pressure
on 21 Feb 2014 9:13 AM

Thank you!