What is a head gasket?

February 01, 2023 by

Don’t lose your head or blow a gasket: find out all you need to know about head gaskets here

As you might be able to tell from the two rather obvious jokes in the summary above, gaskets are such important things (as are heads) that their failure has entered our common language and become an idiom for losing one’s temper.

Head gaskets are one type of gasket, and they feature on all cars with internal combustion engines. Their potential failure is, as you might imagine, a relatively serious issue, so it’s worth knowing what a head gasket is, what symptoms a failing head gasket might exhibit, and how much a replacement head gasket costs.

What does a head gasket do?

A car’s engine is made up of countless parts, but the two largest are the engine block and the cylinder head, often just called the block and the head. .

The block is the main body of the engine, and in it the cylinders, pistons and crankshaft sit.

The head contains the inlet and exhaust valves, plus the camshaft that causes these to open and close, and the head sits on top of the block, forming the top of the combustion chambers, or cylinders.

The head and the block are two separate components, and they are joined together with head bolts – long screws that go through the head and tighten it to the block.

An engine block, on top of which will be placed a head gasket, followed by the cylinder head; note the channels for oil and coolant surrounding the cylinders, and holes for the head bolts; this is a six-cylinder engine, whereas the gaskets we picture are for four-cylinder engines

Combustion takes place at enormous pressure and temperature inside the cylinders, and the valve head forms the top of the cylinders, so it’s of crucial importance that the seal between the block and the heat is airtight.

Added to this complex picture is the fact that oil and coolant flow (in separate channels and passages – they do not mix) from the block to the head, meaning both these components have channels that allow the passage of these liquids.

A portion of a cylinder head; note the exhaust and inlet valves, and the channels for oil and coolant; this head is upside down, and would be flipped over before being placed on the block

Now the engine block and head are made of metal, and while metal is strong and capable of withstanding high temperatures and pressures, it is not good at creating a seal when two pieces of metal join.

Enter the head gasket: this is a thin (1.5mm or so) seal that is generally made of three layers of steel interweaved with elastomer for flexibility. The head gasket sits between the head and the block, and ensures that oil, coolant, exhaust gases, fuel and air cannot escape from the joint between the block and the head.

As well as holes for the cylinders, the head gasket has holes through which oil and coolant flow through as they pass from the block to the head.

A head gasket and head bolts; note the large holes for the cylinders, and the smaller holes for oil and water passages (ignore the component on the right – this is an oil rail, and not typically part of a head gasket kit)

What happens if a head gasket fails?

Bad things. Bad, expensive things.

You know how we said earlier that the oil and coolant in an engine don’t mix? Well, they’re certainly not supposed to mix, but because the oil and coolant passages run close to each other, a failed head gasket can cause them to mix. If the coolant (which is mainly water) gets into the oil its lubricative properties are ruined, so before long you have metal grinding against metal as the engine turns.

A blown head gasket can also cause hot exhaust gases to get into the coolant, or coolant to leak into the combustion chambers, with engine overheating a likely outcome of this, and resultant damage also likely.

Why do head gaskets fail?

Head gasket failure is actually pretty rare, but there are two key reasons why they might fail.

The first of these is poor design of the gasket or engine by the car manufacturer. New materials might be used in gaskets, for example, with these materials not being up to the tough job head gaskets have to perform.

Engine overheating is the other most common reason. If you don’t change the engine’s coolant when it is due, or if you have a leak in the coolant system you don’t attend to, overheating may occur, with the gasket being asked to handle higher temperatures than it was designed to, and failing as a result.

Unsympathetic driving can also be a factor: gaskets are designed to swell slightly as the engine warms up, and if you regularly force the engine rev highly when it is cold, the extra stress this places on the gasket can cause it to fail.

Signs of head gasket failure

Head gasket failure symptoms vary depending on what has got into what.

  • If coolant gets into the combustion chambers it can be emitted from the exhaust as white smoke, often with a slightly sweet smell as a result of the glycol in coolant.
  • If coolant gets into the oil it can cause a sticky sludge to form, which stops the coolant from doing its job. A telltale of this is a browny-yellowish mayonnaise-like sludge forming on the underside of the oil filler cap, or dipstick.
  • If oil gets into the combustion chambers blue smoke may be emitted from the exhaust as this is burned off.
  • If air, fuel or exhaust gases are escaping as a result of head gasket failure, you may notice a loss of engine power, or possibly rough running if one cylinder is failing to ignite properly or at all.
  • If a leak occurs around an oil or water passage close to the exterior of the engine, and these liquids are not making their way into the combustion chambers or each others’ channels, you may simply notice a leak in the engine bay, or underneath the car.

If head gasket failure causes the engine to overheat and you continue to drive, the cylinder heads and other engine components can warp, causing thousands of pounds of damage.

A garage will be able to diagnose head gasket failure by analysing the coolant to see if it contains things it should not, such as residue from exhaust gases. A mechanic may also take off the radiator cap to see if bubbles appear in the coolant, which also indicates gasket failure.

How much does a new head gasket cost?

Head gaskets themselves are relatively cheap – think £50 or so – but significant labour and skill are involved in replacing them, so a total bill of £1,000 would not be unheard of.

The cylinder head must be removed from the engine block – a big job in itself – before the old gasket is carefully and completely removed, while care is taken to not to scratch the surfaces of the head or block; this could lead to further failure of the gasket in future.

The new gasket must be mounted with precision and care, while new head bolts will also be required, as most are designed only to be tightened once, lest they stretch and fail to hold the block and head together to the required tightness.

Also bear in mind that in engines designed in a ‘V’ formation (EG V6, V8 and V12 engines), where there are two banks of cylinders will have two head gaskets to replace; same goes for horizontally opposed ‘boxer’ engines where the cylinders lie horizontally, rather than vertically.

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