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Head gasket guide: what it does and blown gasket repair

Head gasket guide graphic

Head gaskets sit between a car engine's cylinder head and block – discover all about it (and how to avoid a blown head gasket) with this expert guide for DIY mechanics.

What is a head gasket?

A head gasket is the gasket that sits between the cylinder head and the block. Its job is to seal the two mating surfaces of the top and bottom halves of the engine. This means, unlike any other gasket in an engine, the head gasket has to seal oil, coolant, and compression from the cylinders simultaneously. This makes it the most stressed gasket in an engine, and therefore one of the most likely to fail.

How does a car head gasket work?

A head gasket needs to seal the oilways between the cylinder head and block, so that oil can freely circulate around the entire engine. It also needs to do the same with the waterways, so that coolant can also circulate freely around the engine. 

But what makes a head gasket different from any other gasket is that it also needs to seal the cylinder to prevent the engine losing compression. Inside an engine you have a piston going up and down within a cylinder. 

On top of the cylinder you have the cylinder head, which allows the fuel/air mixture to enter, and the exhaust gases to leave via inlet and exhaust valves. Without a good seal between the cylinder head and the cylinder, when the piston rises on the compression stroke some of the air/fuel mixture will be able to escape, resulting in a loss of compression. 

In order to form a good seal around the top of the cylinder, the head gasket features steel rings, known as fire rings. These are actually crushed when the cylinder head is tightened onto the engine block to provide a seal capable of dealing with the high temperatures and pressures involved. This is why you must always tighten to the head bolts to the required torque settings; too tight and the gasket will be crushed too much to form a proper seal, too loose and it won’t seal enough.

A head gasket removed from a car engine

What is a head gasket made of?

Most modern head gaskets are made from multiple layers of steel (MLS). Typically these use three steel layers; the centre layer is slightly thicker, while the two thinner outer layers are coated in rubber-like high temperature and chemical resistant polymers (most commonly Viton) which helps the contact faces seal against the cylinder head and engine block.

Earlier head gaskets were often made from composite material (often referred to as fibre head gaskets), typically graphite-based coated in a wax-like finish, and with beads of silicone around the waterways and oilways to aid sealing. 

These, older, composite head gaskets are more prone to failure than modern MLS gaskets, but many MLS replacements are now available for older engines that would have been produced with a composite gasket when new.

Why does a head gasket fail?

The most common cause of a blown head gasket is engine overheating. When the engine gets too hot, the cylinder head expands (thermal expansion), which can crush the head gasket and cause failure.

Once a head gasket has failed it can cause all manner of problems, including:

1. An overheating engine

The most common cause of engine overheating is a low coolant level. This may be because of poor maintenance, or more likely to be because of a coolant leak.

Water conducts heat 13 times better than air. So once the coolant level drops, the engine will quickly overheat.

Other likely causes of an overheating engine are a faulty thermostat, faulty electric cooling fan, or blocked coolant passages in the engine.

Once an engine has overheated, it’s possible that the cylinder head will be distorted by the heat. This could get expensive…

Get more free car maintenance help with the Haynes Home Mechanic Guides

2. Poor starting

If the head gasket fails in such a way it allows the compressed air/fuel to escape, the compression of that cylinder is reduced. This loss of compression results in a rough-running engine and a notable reduction in engine power.

With a head gasket that is only slightly leaking, very small quantities of coolant can leak into the combustion chambers when the vehicle is parked up and left overnight.

The next time the engine is started, it can misfire on one or more cylinders. This may only last a few seconds until the coolant is cleared, but should be taken as a sign – it can only get worse!

3. Oil contamination

Often the first sign of contamination is a milky sludge or ‘mayonnaise’ on the underside of the oil filler cap or dipstick, caused by water mixing with the oil.

Be careful though, as this symptom is more commonly a sign of condensation mixing with the engine oil. The condensation is caused by frequent short journeys, which don’t allow the engine oil to get hot enough for long enough to evaporate off the water.

A faulty head gasket can also allow engine oil to leak into the coolant passages in the cylinder head. This often shows up as an oily/milky residue in the coolant expansion tank, as the oil floats to the surface of the coolant.

Again, be careful though, as this can also be caused by a leaking engine oil cooler or transmission oil cooler.

4. Smoking

If engine oil leaks past a faulty head gasket, blue smoke may come out of the exhaust pipe, as it’s burnt during the combustion process. This can be very bad news for the catalytic converter. If this is left to happen for an extended period, the catalytic converter can be ‘poisoned’ and require replacement.

5. External leaks

If a head gasket has failed between the waterway or oilway and the outside of the engine, the result can be a simple coolant or oil leak.

This may not manifest itself as an immediate problem (other than causing a mess) but if the coolant or oil levels are allowed to drop too far it can lead to serious engine issues.

A failed head gasket can allow combustion gasses into the coolant passages, which may pressurise the coolant system, and cause an external coolant leak from any weak spot in the system. It’s quite possible that the coolant leak may only be noticeable once the engine is up to normal working temperature.

To help diagnose a failed head gasket, special test equipment is available which can ‘sniff’ for exhaust gasses in the coolant.

Head gasket being held by a mechanic

How can a head gasket be repaired?

At the very least, the cylinder head must be removed along with the old gasket. Then the cylinder head and cylinder block mating surfaces must be thoroughly cleaned before a new gasket and the cylinder head can be refitted. It’s also advisable to flush the coolant system to remove any gunge that could cause a blockage.

However, whilst it’s removed, the cylinder head must be checked for distortion, and it’s common to have the mating surface skimmed to ensure it’s perfectly flat. In some cases, the vehicle manufacturers insist that the cylinder head cannot be skimmed, but must be renewed. If the vehicle is old, the cost could write the whole car off.

Are head gasket sealers any good?

We don't recommend head gasket sealers because they only delay the inevitable and can cause complications elsewhere in the coolant system. How does head gasket sealer work? It's introduced into the radiator or coolant reservoir. The engine is then started and run for around half an hour with the ventilation heat and fan speed turned up to max. The sealant works its way through the engine and into the faulty parts of the head gasket, and then sets, creating a temporary seal. However, it's just that – temporary – and could fail at any time, leaving you (quite literally) in more hot water than you were in before.

Sealers also have a tendency to block small coolant passages, so you could end up with more blockage problems.

How long does it take to fix a head gasket?

Replacing the gasket can take anything from six hours to a few days, depending on the severity of the failure. A blown head gasket is one of the biggest failures your car can suffer, and to fix it properly takes time.

How much does a new head gasket cost?

The head gasket itself is a comparatively cheap part – as little as £20-£40 in most cases. However, the labour involved by getting a garage to do it for you will land you with a final bill that's likely to exceed £500.

Every Haynes manual walks you through the process of head gasket replacement as part of the 'cylinder head removal and refitting' procedure. Find your print and online manual here and save yourself hundreds of pounds.

How to prevent head gasket failure

Prevention is much better than cure when it comes to head gaskets. Head gasket failures are usually the result of an engine overheating, so the best way to prevent a problem is to ensure your cooling system is in good condition.

Ensure the system has no leaks, the radiator is working efficiently and the coolant is topped up to the correct level. Also, make sure electric fans are working correctly, and that the thermostat opens at the temperature it should.

Most manufacturers advise the coolant is changed on a regular basis to maintain its anti-corrosion properties. Check the service schedule for the correct coolant renewal interval.

If you suspect a head gasket failure you can test for combustion gasses in the cooling system. This test will show if the combustion gasses have leaked into the cooling system, and therefore if the head gasket has blown. However, it won't show if there are any other problems with the head gasket, so the absence of combustion gasses in the cooling system does not guarantee a healthy head gasket.

Some head gaskets can just fail because they are of a poor design and are not robust enough for the application. Thankfully MLS (multiple layer steel) replacement gaskets are available for most applications and offer improved reliability over the original gasket design.