Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in any engine, and, as part of the combustion chamber, it shares the same strength requirements as other combustion chamber components.
- Air/fuel mixture in each cylinder
- Water-based coolant in the coolant passages
- Lubricating oil in the oil galleries
Correct operation of the engine requires that each of these circuits do not leak or lose pressure at the junction of the engine block and the cylinder head. The head gasket is the seal that prevents these leaks and pressure losses.
- Multi-Layer Steel (MLS): Most modern engines are produced with MLS gaskets. These consist of two to five (typically three) thin layers of steel, interleaved with elastomer. The contact faces are usually coated with a rubber-like coating such as Viton which adheres to the engine block and cylinder head while the inner layers are optimized for resilience.
- Solid copper: a solid sheet of copper, and typically requires special machining called O-ringing that places a piece of wire around the circumference of the cylinder to bite into the copper. When this is performed copper gaskets are very durable.
- Composite: an older design which is more prone to blowouts than newer designs. Composite gaskets are traditionally made from asbestos or graphite but asbestos gaskets are becoming increasingly rare due to health concerns.
- Elastomeric: uses a steel core plate with molded in place silicone rubber beads to seal oil and coolant passages. The bores are sealed by rolled steel fire rings in a more conventional manner. This type of gasket was used in the Rover K-series engine.
- O-ring: these gaskets are typically built from steel or copper. They are reusable and if used between correctly prepared flat surfaces will yield the highest clamping pressure, due to their much lower surface area compared with other gasket types.
Occasionally, the compression in the cylinder will cause a leak to form in the gasket thus needing replacement, otherwise severe damage can take place and eventually become a "blown" head gasket. This problem has been exacerbated by the use of aluminum rather than iron cylinder heads; while lighter than iron, aluminum has a much greater thermal expansion rate, which in turn causes a great deal more stress to be placed on the head gasket. Engine manufacturers have responded to this problem by adding a non-stick coating such as Teflon to the surface of the head gasket.
Blue exhaust smoke may be indicative of oil burning. White exhaust smoke is an indicator of coolant burning, another sign of head gasket wear.
If the gasket fails, then the engine is either facing compression loss issues, leakage issues or a combination of both:
Compression loss from a damaged head gasket can lead to power reduction, or a rough engine. This is because exhaust and intake gases are either leaking out of the cylinder or being forced into the cooling system, which leads to the engine overheating and can also lead to increased engine wear due to the motor oil being mixed with antifreeze.
In other occurrences the gases can leak into small spaces between the gasket and either the cylinder head or engine block traps those gases, and then released when the engine is turned off. These gases then escape into the coolant and create air pockets. Sometimes these air pockets can get trapped in the engine's coolant thermostat, causing it to stay closed and cause further overheating, thereby creating more voids between the gasket and the engine. Other times these air pockets can also cause the engine to expel coolant into the overflow or expansion tank, thereby reducing the amount of coolant, the engine has available, to stay cool.
A damage head gasket can cause coolant to leak into the cylinders thus tampering with the combustion cycle and causes the exhaust to blow out steam, damaging the catalytic converter. If a very large amount of coolant leaks into the cylinders, then the engine can suffer from hydrolock, which can cause extensive engine damage. Sometimes, all that may happen when a head gasket is blown is excessive steam erupting from the tailpipe, yet the engine may act and drive like normal (keep in mind that the steam is still damaging the catalytic converter), until all the coolant is gone and the engine starts to overheat.
The condition of a head gasket is typically investigated by checking the compression pressure with a pressure gauge, or better, a leak-down test, and/or noting any indication of combustion gases in the cooling system on a water-cooled engine. Oil mixed with coolant and excessive coolant loss with no apparent cause, or presence of carbon monoxide or hydrocarbon gases in the expansion tank of the cooling system can also be signs of head gasket problems.
A good sign of head gasket failure on water-cooled engines is the presence of a substance that resembles mayonnaise in the oil, often to be seen on the dipstick, or oil filler cap. However, the presence of this substance is not conclusive proof of head gasket failure, since oil could mix with the coolant via other routes. Likewise, it is entirely possible for a head gasket to fail in such a way that oil never comes in contact with coolant. Because of this, oil that has NOT become contaminated with coolant should not be taken as a definitive sign that the head gasket is okay.
A leaking head gasket can be classified as either external or internal. An external leak can be identified as oil and coolant accumulating underneath the engine. The presence of coolant can be detected by shining a black light on what appears to be an oil leak; the appearance of coolant will show up under the black light. External leaks can also appear as previously described in the oil. An internal leak can usually be diagnosed by excessive coolant accumulating in the expansion tank along with the presence of hydrocarbons in the form of foam. The possibility of vapors or condensation and/or water (from the road or rain) building up (in aftermarket product installation) from an external breather or catch tank from the head (rocker cover) can also cause a buildup of froth or foam in the oil but is highly unlikely.
The cost of a replacement gasket is usually not extreme, but the price of total repair is significantly high. This is because the process of removing/replacing an engine head is very time-consuming—around 75% of cost will be labor. Furthermore, untreated blown gaskets usually seriously damage the engine, then requiring even more expensive work.
Effect of engine knocking
Engine knocking (detonation) can be caused by poor quality fuel, an engine fault or if inappropriate fuel and/or ignition settings are trialled chosen while engine tuning is taking place. If the detonation is severe, the cylinder pressure can be increase to 8 times above normal pressures, which can cause the cylinder head to lift away from the engine block, disrupting the seal between the two. Most gaskets used in standard production engines can be critically damaged by severe detonation.
- Bickford, John H. (1997). Gaskets and Gasketed Joints. CRC Press. p. 57. ISBN 0-8247-9877-5.
- "Best Head Gaskets – Choosing the Right Gasket for you car". Auto Accessories Performance. 15 March 2017. Retrieved 2017-05-23.
- "Multi-layer Steel (MLS) Cylinder-Head Gaskets". Victor Reinz. Retrieved 2017-05-23.
- Nunney, M. J. (1998). Light & Heavy Vehicle Technology. Elsevier. p. 23. ISBN 0-7506-3827-3.
Since exposure to asbestos is now recognized as being a health hazard [...] the manufacture of non-asbestos cylinder head gaskets has now become established.
- "How to Repair or Replace a Head Gasket". www.do-it-up.com. Retrieved 24 October 2020.
- "Head Gasket Sealers". www.headgasketsealer.com. Retrieved 24 October 2020.