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Head Gasket

Head Gasket Published: 10th Aug 2016 - 0 Comments - Be the first, contribute now!

Head Gasket
Head Gasket
Head Gasket
Head Gasket
Head Gasket
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Few other mechanical maladies strike greater fear into a classic owner’s heart than the realisation of a failed cylinder-head gasket. Rob Marshall delves into the what, when, where and why

They joke that there’s only two certainties in life – death and paying taxes. But if you’re a motorist you can add another one – a blown cylinder head gasket! Years ago, rust was the main worry but on modern classics it’s that slither of seal that separates the engine block from the head and when it goes there’s big trouble ahead.

Even if you are a keen DIYer, the problem lies not just with expensive parts, because the time required to dismantle half of the engine can be considerable, especially when the additional cost and hassle of specialist engineering work is contemplated.

The cylinder-head gasket is one of an engine’s most stressed components. It must keep cooling, lubrication and combustion aspects separate, while resisting not only a wide range of temperatures and pressures but also different material expansion rates between the cylinder-head and engine block castings, especially if they are produced from different metals. Certain engines are particularly prone to blowing a gasket; on MGFs it’s almost a given.

Preventing failure

No matter how well you look after your classic, head gasket failure can be caused by simple old-age, although it may also be an indication of an underlying problem. Some all-iron engines are less vulnerable in this regard, as overheating is unlikely to weaken the gasket, or cause it to fail completely.

Aluminium cylinder-heads (and alloy engine blocks, such as those fitted to some classic Skodas and the Coventry-Climax engined Hillman Imp) warp slightly under extreme temperatures, causing an uneven clamping force that prevents the gasket from sealing adequately across its whole surface, thereby promoting failure. Renewing any suspect cooling system components is vital, as is replenishing the system’s anti-corrosion properties, by changing the anti-freeze mixture every two years (typically), which will help to reduce the chance of the head-gasket rotting. Should your engine possess a water pump that is driven by a cam-belt, changing it at the same time as a belt change, regardless of its condition, is also a wise preventative measure.

Especially with mixed metal engines, expansion rates differ between the head and engine block. Allowing the engine to warm-up gradually, instead of driving flat-out immediately following a cold start, will put the gasket under less stress. Go faster tuning work, especially that which increases the compression ratio, such as raising turbocharger boost pressure, will also stress the head-gasket but some higher-quality performance gaskets are available and are worth seeking out, dependent on your classic’s make and model.

When the gasket blows

Before we delve into this aspect deeper heed the words of experts, such as Benchsound of Corringham in Essex. Fixing head gasket failures forms a significant part of its business but says few motorists appreciate that something usually caused it to fail in the first place. In other words, it’s futile simply to replace the failed gasket before ascertaining whether some other outside factor lead to its demise, such as a faulty radiator, thermostat, poor hoses and so on, which must be attended to as well or you’ll be back to square one.

If you suspect that the cylinderhead gasket has failed, stop driving the car and ensure that you have arrived at the right diagnosis, because stripping the engine is neither inexpensive, nor a quick procedure. While a leak-down test will help, a typical sign of failure is coolant and lubricant mixing.

Should oil, or a white (or brown) emulsification be discovered within the cooling system, gasket failure is more likely than if a mayonnaise-type deposit is discovered within the rocker cover, because engine oil can simply mix with atmospheric moisture and collect at the top of the engine, especially if your classic is used only for short journeys in cool and damp weather.

A loss of compression, between a pair of adjoining cylinders, may also indicate a head-gasket problem. Any evidence of coolant entering a combustion chamber, also indicated by a falling radiator coolant level, must also be investigated. While significant steam output from the exhaust may be a clue, it could be normal; remove the spark plugs and see if any of them look less than immaculate, after being treated to an in-cylinder ‘steam-clean’. Note, however, that cracks can develop within cylinderhead water jackets, fooling you to think that the gasket is to blame.

Yet, combustion gases that are found to bubble-up from the expansion tank, or further evidence of the cooling system pressurising, is another potential sign of gasket failure. Should the leak be slight, tools are available that can ‘sniff’ the presence of engine combustion gases within the anti-freeze mixture. On air-cooled engines, you may notice oil leaks and the sound of escaping air, as the engine is cranked over on the starter motor. You may consider trying using a quick-fix additive that can be poured into the cooling system, to seal the head gasket. More on this later.

Gasket renewal

If you buy a new old-stock gasket, ensure that it has never been bent, or damaged, from being lugged around numerous autojumbles. With new parts, various different constructions of cylinder-head gaskets exist and it is possible that the original gasket specification, especially of the type that contained asbestos, are no longer available. Check with your owners’ club, if there are any known problems with modern construction gasket alternatives. Consider also that you may need to order not only the gasket but also additional parts that may include other gaskets and a set of cylinder-head bolts, if appropriate, because some engines are fitted with bolts that stretch, which must not be reused.

With the engine dismantled, check that neither its block, nor cylinder-head castings are cracked and obtain specialist advice, if any of them are found. Even if a cylinder liner has moved only slightly, the life of a replacement gasket is likely to be reduced. In most cases, especially with aluminium types, the cylinder-head’s surface should be measured for flatness and skimmed by a specialist engineer. Note that some classic heads cannot be skimmed beyond a certain tolerance; check your car’s service history to ensure that the head has not been machined before. Skimming also raises the compression ratio slightly, so be wary, if your car suffers from ‘pinking’ on super-unleaded petrol then a higher compression ratio will make matters worse.

Obviously, both block and head surfaces must be clean and flat, before fitting a new gasket and ensure that all of the relevant oil and water holes match the castings’ galleries. In most cases, the cylinderhead retaining bolts/nuts will require tightening in a specific order, to ensure an even clamping force, prior to being tightened fully to the specified torque. After the engine has been run, most engines require the retaining nuts/bolts to be re-torqued again, after the engine has cooled. While it is a good idea to check them for tightness occasionally (as in the case of Stag and Dolomite Sprint engines where they are known to ‘relax’), consider that over-tightened fixings are another cause of gasket failure.

Preventive maintenance

Should you then renew a working gasket, as in the case of a K-Series engine? Given that the extreme likelihood of the gasket failing, and perhaps taking the cylinder head with it, then it sounds like good policy. A typical garage charge is in the region of £300-£400 and that’s a cheap insurance as it may not only save the engine but also the car.

What about one of the new wave of pour-in fixers? Many motorists (usually mini cab drivers who need to keep their car mobile) report success with their use (check out the testimonials on the respective websites), but is unlikely to fix a gasket blow between cylinders. Their make up may also affect other cooling system components negatively – like the heater matrix but for £20-£30, what have you got to lose? We’re running sealer in a BMW 323i, a car prone to such ailments, and it’s working fine, including the heater but you need to follow their specific instructions, including initial running procedures, to the letter.

10 ways of spotting one

1. All mixed up

Can you spot oil droplets or sludging in the radiator? Another indication is an exhaust acting like a hose pipe when hot

2. Fancy a leak?

Look for seeping (antifreeze stains) along the cylinder head joints. Gasket may not have blown yet but it will

3. Hot and bothered

Is the engine running hotter than normal or the temp becomes erratic when running? It needn’t boil over though

4. You dipstick!

Apart from oily radiator, is there water on the engine’s dipstick or are you finding the oil level requires little replenishing?

5. Follow your nose

It’s bad news if you can smell fuel or general combustion fumes in the rad’s header tank, rather than the whiff of anti-freeze

6. Salad days

A creamy mayonnaise in the rocker cover or its cap can be a bad sign as it indicates oil and water are mixing together

7. Under pressure

A large compression test drop across two adjoining cylinders is a sure sign that it’s blown at its smallest point

8. Gurgle give-away

A gurgling within the bowls of cooling system when you switch off is another sign but can also signify a major air lock. Can feel pretty forceful at times.

9. Piling on the pounds

Have a garage carry out an engine pressure test. This typically costs around £20 upwards and it will soon be tidings of bad news. The newest high-tech method is by way of a colour gas tester that acts like litmus paper

10. A poor performer

A general lack of go, poor starting and that elusive misfire you can’t rectify can all be put down to a gone gasket. Remember this when buying a new classic car…

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