Head gasket
Encyclopedia
Gasket
thumb|sright|250px|Some seals and gaskets1. [[o-ring]]2. fiber [[Washer |washer]]3. paper gaskets4. [[cylinder head]] [[head gasket|gasket]]...
that sits between the engine block and cylinder head
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
in an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
. Its purpose is to seal the cylinders to ensure maximum compression
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
and avoid leakage of coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
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
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
, it shares the same strength requirements as other combustion chamber components.
Types
- MLS or Multiple Layers Steel – Most modern head engines are produced with MLS gaskets. These typically consist of three layers of steel. The contact faces are usually coated with a rubber-like coating such as VitonVitonViton is a brand of synthetic rubber and fluoropolymer elastomer commonly used in O-rings and other molded or extruded goods. The name is a registered trademark of DuPont Performance Elastomers L.L.C.....
that adheres to the cylinder blockCylinder blockA cylinder block is an integrated structure comprising the cylinder of a reciprocating engine and often some or all of their associated surrounding structures...
and cylinder head while the thicker center layer is left bare. - 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 cylinderCylinder (engine)A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
to bite into the copper. When this is performed copper gaskets are extremely durable. Recently companies have started producing copper gaskets with integral sealing wires permitting their retrofit into engines without the removal of the engine block for machine work. - Composite – an older technology. Typically these are made from asbestosAsbestosAsbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
or graphiteGraphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
but are more prone to blowouts than newer gaskets. Asbestos gaskets are becoming increasingly rare due to health concerns. - Elastomeric – a gasket type used by RoverRover GroupThe Rover Group plc was the name given in 1986 to the British state-owned vehicle manufacturer previously known as British Leyland or BL. Owned by British Aerospace from 1988 to 1994, when it was sold to BMW, the Group was broken up in 2000 with the Rover and MG marques being acquired by the MG...
on their K Series enginesRover K engineThe K-Series engine is a series of engines built by Powertrain Ltd, a sister company of MG Rover. The engine was built in two forms: a straight-four cylinder, available with SOHC and DOHC, ranging from 1.1 L to 1.8 L; and the KV6 V6 variation....
. It utilised a steel core plate with molded in place silicon rubber beads to seal oil and coolant passages. The bores were sealed by rolled steel fire rings in a more conventional manner. The idea behind the gasket design was to produce a production version of the technology used in F1 engines at the time. The original application of the gasket on the smaller versions of the K Series was very successful. However, an engine redesign resulted in persistent problems with the design – the joint interface became unstable and the gasket could not cope.
The cost of a replacement gasket is usually not extreme, but the price of total repair is significantly high. This is because removing/replacing an engine head is very time consuming - around 75% of cost will be labour. Furthermore, untreated blown gaskets usually seriously damage the engine, then requiring even more expensive work.
Gasket problems
The usual internal combustion engine is a water-cooled four-stroke petrol design. It uses three circuits of working fluids:- Combustible air/fuel mixture
- Water based coolant, usually with glycol admixed
- Motor oil for lubrication
These are all vital, but must never be allowed to intermix. Apart from sealing the cylinder, the head gasket also seals water and oil conduits between the head and block. Any connectivity between them will cause engine failure, or significant problems like burning oil (smoke from the exhaust). Blue exhaust smoke may be indicative of oil burning. White exhaust smoke is an indicator of coolant burning, another sign of head gasket wear.
Occasionally, the compression in the cylinder will cause a leak to form in the gasket and the gasket will have to be replaced, or severe damage can take place (a "blown" head gasket). This problem has been exacerbated by the use of aluminum rather than iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
cylinder heads; while lighter than iron, aluminum has a much greater thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
rate, which in turn causes a great deal more stress to be placed on the head gasket. Engine makers have responded to this by adding a non-stick coating such as Teflon to the surface of the head gasket.
If the gasket fails, a variety of problems can occur, from compression loss (leading to power reduction, or a rough engine), to exhaust gases being forced into the cooling system, leading to the engine overheating and increased engine wear due to the motor oil
Motor oil
Motor oil or engine oil is an oil used for lubrication of various internal combustion engines. The main function is to lubricate moving parts; it also cleans, inhibits corrosion, improves sealing, and cools the engine by carrying heat away from moving parts.Motor oils are derived from...
being mixed with antifreeze
Antifreeze
Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....
. Coolant can leak into the cylinders, causing the exhaust to issue steam and the catalytic converter
Catalytic converter
A catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...
to be damaged. If a very large amount of coolant does this, hydrolock
Hydrolock
Hydrolock is an abnormal condition of any device which is designed to compress a gas by mechanically restraining it; most commonly the reciprocating internal combustion engine, the case this article refers to unless otherwise noted...
can occur, causing 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, until all the coolant is gone and the engine overheats. In other occurrences, compression leaks into small spaces between the gasket and either the cylinder head or engine block trap air that is then released when the engine is turned off, which then escape into the coolant and create air pockets. Sometimes, these air pockets can get trapped in the engines coolant thermostat, causing it to stay closed and further the overheating issue, thereby creating still more voids between the gasket and the engine. Air pockets can also cause the engine to expel coolant into the overflow or expansion tank
Expansion tank
An expansion tank or expansion vessel is a small tank used to protect closed water heating systems and domestic hot water systems from excessive pressure...
, thereby reducing the amount of coolant the engine has available to stay cool.
Leak-down tester
A leak-down tester is a measuring instrument used to determine the condition of internal combustion engines by introducing compressed air into the cylinder and measuring the rate at which it leaks out....
, 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
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
or hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
gases in the expansion tank
Expansion tank
An expansion tank or expansion vessel is a small tank used to protect closed water heating systems and domestic hot water systems from excessive pressure...
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
Mayonnaise
Mayonnaise, , often abbreviated as mayo, is a sauce. It is a stable emulsion of oil, egg yolk and either vinegar or lemon juice, with many options for embellishment with other herbs and spices. Lecithin in the egg yolk is the emulsifier. Mayonnaise varies in color but is often white, cream, or pale...
in the oil, often to be seen on the dipstick
Dipstick
A dipstick is one of several measurement devices.Some dipsticks are dipped into a liquid to perform a chemical test or to provide a measure of quantity of the liquid....
, 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.
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
Black light
A black light, also referred to as a UV light, ultraviolet light, or Wood's lamp, is a lamp that emits ultraviolet radiation in the long-wave range, and little visible 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.