Feeling and sealing gasket pressure

Jan. 1, 2020
A head gasket's job is simple. It is designed to seal combustion gases within the cylinder/combustion chamber and prevent coolant and/or oil from escaping the engine at the head-to-block joint. Today's head gaskets have to provide a perfect, lock-tig

In order for the head gasket to do its job, you need to know where to make sure sealants stick.

underhood head gaskets head gasket seals engine repair repair shop training technician training A/C training automotive aftermarket A head gasket's job is simple. It is designed to seal combustion gases within the cylinder/combustion chamber and prevent coolant and/or 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 continued trend toward lighter-weight engine castings and reduced head clamp loads all have resulted in operating conditions that cause much higher levels of horizontal and vertical motion between the head and block than in the past.

After only a few thousand miles of operation, the finishes and flatness of engine surfaces are no longer "virgin" castings, and the engine has heat cycled from hot to cold and back, making the job of sealing much more difficult.

Another challenge facing modern day head gaskets is the industry's move toward zero tolerance for even minor combustion and/or fluid leaks, primarily because of emissions requirements. As a result, today's head gaskets have to provide a perfect, lock-tight seal in spite of ever increasing pressures, temperatures and other dynamic forces.

"The shift toward higher pressures and temperatures will continue over the long-term as engine manufacturers strive for an optimal burn of the air-fuel charge," according to Jim Daigle, product manager, Fel-Pro® gaskets. "This tight seal and optimum burn is key to improving fuel efficiency and minimizing emissions."

History

The original head gasket technology was known as a sandwich-style gasket. This technology combined metal-faced materials with a filler material in between. Because engines of the day were low horsepower/low compression, these low-tech gaskets sealed engines adequately for the time period. This technology continued into the late 1950s.
As engines grew bigger and more powerful, a limited number of manufacturers started using embossed steel shim-style gaskets in the early 1960s. While the steel shim gaskets were even lower tech than their predecessors, they lead the way to today's multi-layer steel head gaskets.

The later 1960s saw the rise of a new type of composite gasket that featured a perforated steel core with a composite face material applied to each side. These were the first head gaskets that could be installed without a secondary (spray-on or brush-on) coating to create a micro-seal between cylinder head and block, and they required no re-torquing.

As engine technology progressed, it was obvious that conventional composite-style head gaskets couldn't stand up to the increased casting motion encountered in the new smaller, lighter and more powerful engines. The 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.

The late 1970s and early 1980s saw gaskets utilizing expanded graphite facing material being developed. This material was mechanically clinched to a steel core and offered excellent conformability to surface finish variations. The natural lubricity of expanded graphite also accommodated some casting motion, which occurred frequently in the early days of bi-metallic engines.

Then in the early 1990s, Multi Layer Steel (MLS) head gaskets were introduced to the industry. 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.

Because an MLS gasket is comprised of stacked and embossed multiple steel layers of full-hard stainless steel, it acts as a spring between the head and block to maintain sealing stress against the castings. MLS technology is now the industry standard.

Diagnosing

It is easy to diagnose a cylinder head gasket that is leaking externally, as you will see the coolant, oil or carbon soot. But what if no external leak turns up? Then it is time to check for an internal one.

The first and easiest check is to use a dye test where a special tool that looks like a turkey baster is used to draw air from the top of the radiator. The air passes through a dye that changes color if exhaust gases are present. This usually is a reliable test as long as the dye is fresh.

A variation on this theme is to use an exhaust analyzer to sniff the same air. But be careful to not allow any coolant to enter your exhaust analyzer.

The most accurate test is the cylinder leak-down test. Each cylinder is brought to top dead center (TDC) and compressed air is pumped into the cylinder via a leak-down tool while observing the coolant level in the radiator. When the offending cylinder is pressurized, it's hard to miss the reaction of the coolant.

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. While this test will diagnose failures that involve the coolant passages, what about failure between cylinders?

Compression leakage between cylinders and the poor-run condition that results from this failure are hard for a customer to ignore. Yet some customers 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 and just a few miles of driving will ruin a casting and/or 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 to show 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 the engine is in a proper state of tune, the turbocharger control is functioning properly, the EGR ports are clear and the system functions properly, the cooling system is functioning properly and that exhaust backpressure is within limits. All of these checks are to ensure that cylinder temperature and pressure are not abnormally high.

Servicing

Despite the fact that castings have gotten thinner and engines are smaller and more highly stressed than ever before, today's crop of high tech, overhead cam bi-metal engines have turned out to be much longer lived than their heavy, cast iron predecessors from the 1970s and '80s.

Today's OEM 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.

Before the advent of engine controls, both detonation and pre-ignition were frequent causes of head gasket failure. While these issues still arise today, they are less common because head gaskets are more capable now of withstanding much higher combustion pressures and temperatures.

Once head gasket failure has occurred, the opportunity for some investigative work to determine the cause of the failure rises. When the old head gasket is removed, it's important to measure the bore diameter from the intake side to the exhaust side of the gasket. According to some gasket makers in the industry, if the gasket bore has grown .010 inch 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.

A great deal of information can also be gained by looking at the witness marks on the face of the gasket and the corresponding deck surface. These marks are a direct reflection of surface finish and should be read carefully before installing a new gasket. Having 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 prep. Be sure to clean and chase threads (remember that there is a difference between a tap and a thread chaser), lubricate bolts (and washers, if applicable) and the underside of bolt heads. When it comes to torquing the bolts, always follow the OEM's recommended torque procedures to achieve the correct clamp load and most importantly, have your torque wrenches calibrated regularly.

Jim Marotta is a freelance writer with more than 17 years' experience in the automotive industry. He currently works as a technical writer for ChiltonPRO.com.

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