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One of the most important parts of the cooling system is also the most invisible

A cylinder head gasket is required to effect a seal between the cylinder head and block of a gasoline or diesel engine. It is an integral component of the engine and is requires to perform many functions at the same time during engine operation.

The head gasket must maintain the seal around the combustion chamber at peak operating temperature and pressure. The gasket must seal against air, coolants, combustion and engine oil at their respective peak operating temperature and pressure. The materials used and design employed must be thermally and chemically resistant to the products of combustion and the various chemicals, coolants and oils used in the engine.

When assembled, the head gasket becomes an important part of the total structure of the engine. Cylinder head gasketsIt supports the cylinder head along with its operating components. It must be able to withstand the dynamic and thermal forces that are transmitted from the head and block. The type of engine application will be the determining factor in cylinder head gasket design. With engines ranging in size from one cylinder gasoline fired engines up to twelve cylinder, turbocharged or supercharged high-compression diesels, the material and design of the gasket is paramount to it’s functional life span.

Every application requires a unique cylinder head gasket design to meet the specific performance needs of the engine. The materials and designs used are a result of testing and engineering various metals, composites and chemicals into a gasket that is intended to maintain the necessary sealing capabilities for the life of the engine.(See Fig. 1).

The most widely used materials are as follows:

bulletSteel and stainless steel of various grades and forms.
bulletFiber based composite materials.
bulletGraphite in various densities.
bulletChemical formulations containing polytetrafluorothylene, silicone, nitriles, neoprene, polymeric resins and others.


Engines are designed to operate within a “normal” temperature range of about 190 to 220 degrees F. A relatively consistent operating temperature is absolutely essential for proper emissions control, good fuel economy and performance.

If the engine overheats and exceeds its normal operating range, the elevated temperatures can cause extreme stress in the cylinder head, which may result in a head gasket failure. This is especially true with aluminum cylinder heads because aluminum expands about two to three times as much as cast iron when it gets hot. The difference in thermal expansion rates between an aluminum head and cast iron block combined with the added stress caused by overheating can cause the head to warp. This, in turn, may lead to a loss of clamping force in critical areas and allow the head gasket to leak.

What else can happen when an engine overheats? Coolant can boil out of the radiator and be lost. Pistons swell inside their cylinders and can scuff or seize. Valve stems can swell in their guides and also scuff or seize. This, in turn, may damage valve train components (broken rocker arms, bent pushrods, etc.) or possibly result in damaging contact between the valve head and piston if the valve sticks open. Valve lifters can also stick, possibly causing a valve to remain open a little too long. Bearings can seize. Cylinder heads can crack (especially if someone dumps cold water into the radiator in an attempt to “cool off” the engine). Combustion chambers can become so hot that a spark is no longer needed to ignite the fuel, leading to a condition known as “preignition” where the engine misfires and runs erratically. Air/fuel mixtures are upset, and gasoline becomes less able to resist detonation. Oil thins out and is less able to protect the engine’s internal components against friction and wear.

When a localized hot spot forms, it causes the surrounding metal to swell excessively. This, in turn, can crush the head gasket causing the gasket to leak, erode and/or eventually burn through. Hot spots also create added stress in the head itself, which may cause the head to warp (go out-of-flat) and/or crack.

Aluminum cylinder heads with Siamese exhaust valves (such as the Chrysler 2.2L and Honda 1.3L and 1.5L) seem to be especially vulnerable to localized overheating in the area between adjacent exhaust valves. This is typical of head designs that restrict or limit coolant flow and circulation in critical areas. Some engine blocks with siamesed cylinders also provide minimal cooling between the cylinder bores. Even engines like smallblock Chevy V8s that have adjacent exhaust valves in the two center cylinders can experience hot spots if other factors are present, such as overheating, detonation and/or pre-ignition.

As long as the coolant level is okay and the cooling system is functioning normally, there should be no problems. But if there’s a loss of coolant due to a leak, an air pocket in the cooling system, a cooling problem that causes the engine to overheat or some other type of engine problem that causes normal combustion temperatures to soar (such as loss of EGR, incorrect ignition timing, vacuum leak, lean air/fuel mixture, exhaust restriction, etc.), the result can be the formation of localized hot spots and head gasket failure.

Head Gasket

A head gasket that has failed because of excessive crush created by a localized hot spot will be measurably thinner in the damaged area when checked with a micrometer. By comparison, a gasket that has failed due to detonation or pre-ignition will usually have cracked armor around the combustion chamber, which leads to burnthrough. (See Fig. 2).

The corresponding surface areas on both the head and engine deck where the gasket failed should be inspected for damage (erosion, pitting or cracks) as well as flatness. If either surface is damaged or is not flat, the head and/or engine block must be resurfaced otherwise the new head gasket may not seal properly. What’s more, the same conditions that caused the original hot spot to develop may still be present, which will only make matters worse.

One of the most common causes of localized hot spots is air in the cooling system. Air pockets can form when the cooling system is being refilled after a coolant change or when other engine repairs are being made (valve job, replacing a water pump, thermostat, etc.). As coolant is being poured into the radiator, the thermostat often blocks the venting of air from the engine leaving air trapped in the upper portion of the block and/or heads. Some thermostats have a small bleed hole or jiggle pin to prevent this from happening, but many do not. Some engines also have special bleeder valves on the thermostat housing or elsewhere to help vent trapped air from the system.

If the trapped air isn’t removed, it may cause localized hot spots to form when the engine is started. The trapped air may also prevent the thermostat from opening and cause the engine to overheat. That, in turn, may lead to additional damage such as head cracking or warping. Another symptom of air trapped in the cooling system would be little or no heat output from the heater when the engine is warm.