Cylinder Head Bolts

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Cylinder Head Bolts
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Your vehicle's head gasket (HG) is sandwiched between the cylinder head and engine block. The part serves multiple purposes. First, it prevents coolant from seeping into each cylinder's bore. Second, it helps maintain pressure within the engine by sealing off the combustion chambers. The operation and performance of your car are heavily dependent on the HG's ability to keep the seal intact and free from leaks.

When the head gasket fails, it must be replaced. Unfortunately, the job is expensive - the price can climb past $1,000. The part itself is relatively inexpensive; a high-quality head gasket typically costs less than $70. The reason replacing it is so costly is due to its location. The mechanic needs to remove the engine head in order to get to it.

Below, you'll discover the main reasons the HG fails. Some you can avoid while others may be inescapable.

Stress Between Disparate Metals

Your car's cylinder head and engine block are made from different metals. The heads are usually comprised of aluminum. The block is made from cast iron. One of the reasons cylinder heads are made from aluminum is because the metal is lighter than iron. The problem is, aluminum has a higher thermal expansion rate. When the temperature around your engine rises (as it does during operation), the aluminum expands much more quickly than the block's iron.

This places stress on the HG. The head and block rub against each other because they are expanding at different rates. Even though the gasket sitting between them is normally designed with steel or copper, the constant rubbing over thousands of miles gradually tears it apart.

Many of today's vehicles are equipped with HGs made of graphite. This material is more capable of withstanding the stress caused by the varying expansion rates of the cylinder head and engine block. But they can still break down over time.

Bad Installation

There are a few ways in which a bad HG installation can cause problems down the road. One of the most prevalent issues involves the head bolts. These bolts hold the gasket securely to the engine block. If they're tightened down with the wrong torque, they will create seal problems.

Another issue involves the surface of the block or cylinder head. If either surface is less than smooth, the rubbing between them will accelerate the gasket's deterioration and eventual failure.

Occasionally, head gasket failure is due to the type of sealer used upon it. If the HG is made from a composite (e.g. graphite), the wrong sealer can cause a chemical reaction. This reaction can degrade the gasket's integrity.

Stress From Engine Problems

As I mentioned earlier, there are factors that impact your vehicle's head gasket that you can influence. These involve problems with your engine. The most common engine-related issue that leads to gasket failure is detonation (also known as spark knock). This occurs when the air-fuel mixture within a cylinder's combustion chamber is poorly calibrated. It can happen due to a malfunctioning exhaust gas recirculation (EGR) valve, bad (i.e. cheap) fuel, and carbon buildup. These circumstances can cause the mixture inside the chamber to ignite erratically, which places stress on the gasket.

Another engine-related problem is overheating (though this is a rarer cause of HG failure than spark knock). Your engine is already operating at high temperatures. When it overheats, the temperature rises dramatically. While the HG is designed to withstand heat, the significant rise can cause it to deteriorate.

There's obviously very little you can do about the type of metals that comprise your car's engine block and cylinder head. And you're unlikely to have much say over the HG's installation. But you can influence the long-term health of your engine and thereby, help protect the integrity of your vehicle's head gasket.

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Safety tips for vehicle repair

What do we mean here by "Safety"?

There are two types of safety involved here: safety when actually doing the job and the safety of the vehicle once the job is done. This article addresses both of these.

Use the right tool for the job.

Using make-do tools frequently ends up in tears because you hurt yourself, break a piece of the car or - more frequently - both.

Use a solid support

If you have to jack the car up, put a solid support - such as axle stands - under it. Gently lower the car on the jack until most of the weight is resting on the stands. You can leave the jack there to take some of the weight, thereby subjecting all the supports to less stress. Whilst we're here, make sure the jack has sufficient oil in it and if not, only top it up with oil meant for the job - car engine oil will not do.

Know what you're doing

For all but the simplest jobs, before you start, get the workshop manual and read through the details of what you're going to do. Make sure you have the right tools. Have a look at the bits you're going to be working on before even picking up a tool and make sure it matches what's in the manual. Those books aren't unfallible and your car may not exactly match the book.

Make sure you can see

Especially if you're working under the car, use a powerful light but make sure you aren't going to get a hot piece of worklight dripping on your head! Eve in bright sunlight, it can be very dark working under a car or behind an engine. The sunlight makes your eyes' pupils close down, letting less light into your eyes and so you can't see the dark parts as well as you expect to. So you need the worklight! Wear eye protection when working under the vehicle (or upside down in a footwell!!) - dirty, gritty oil in your eye stings and can cause blindness.

Don't force things

If a fixing won't move when you try and remove it with the right tool, check the torque that should be used to tighten it. Mostly, the torque to undo it won't exceed two or two and a half times that (those figures are from my experience and there may be better ones out there somewhere). Use WD40 or a similar unlocking fluid before you try and undo what may be an obstinate nut or bolt. Leave it on for at least an hour - overnight is better - and apply some more before actually attacking it with the tool.

Beware of putting so much force on a tool that you break the fastener. The suddenly-released force when the fixing breaks can cause you to hit something hard with a part of your hand or arm. This can result in nasty gashes, perhaps needing stitches, and even broken fingers and limbs. It's more common than you may think. In some cases, you can put a folded towel where you might hit your hand if the fixing "lets go", or wrap it around your hand. Wear work gloves as well.

Use barrier cream and work gloves or rubber gloves

Many of the oils used in motor vehicles are very bad for your skin - they can cause all sorts of unpleasant reactions and, in extreme cases, cancer. A good barrier cream will pretty much prevent such effects. Wear rubber gloves as well when doing precision work, heavy leather work gloves for the brute force stuff.

Use the right fixings

If you need to replace a nut or bolt - or any other fixing - make sure you use the right one. This is particularly true of highly stressed bolts such as those used on cylinder heads or some suspension components. If the manual says that new fixings should always be used then put the old ones in the recycling bin and get the new ones. Ignoring this will probably lead to expensive damage and can be fatal - especially true of disc brake calliper bolts!

Use a torque wrench

If you over-tighten a nut or bolt then it may fracture under the stress of normal use. Again, this can be expensive but can also be fatal. It is imperative that, for most fixings on a motor vehicle, they are tightened to the correct torque. Buy or rent a torque wrench and get the correct settings for the fixings you are dealing with.

When tightening some bolts, such as on a cylinder head, not only must they be tightened in the correct sequence but it often has to be done in stages. That will sometimes involve not only a torque specification but also turning the bolt through a particular angle at some stage. Failure to observe such instructions when working on modern high-performance engines can easily lead to expensive damage. If you get an engine blowout in the middle of an overtaking manoeuvre, for example, the results are generally not pleasant!

Use thread sealant

This is especially true when dealing with safety-critical fixings and those subject to high levels of vibration. Examples are brake fixing bolts, engine Main Bearing bolts and some major suspension components. It can also be used even when not specified to give an added margin of safety - but be careful as the manual may specify that, in some places, it must not be used.

About the Author

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My photography site and my blog, Ablogation

Married, two grown kids. Many years in IT.

Worked for several years with a Complementary Medicine practitioner on methods for self improvement. Together we developed some interesting ideas, many of which were tested on his (consenting!) patients. Unfortunately he died a year ago but I have decided to publish what we came up with as it can help people and it's what he would have wanted me to do.

Which bolts on a cylinder head do you always tension first?

Usually start in the middle and work your way out to the ends in an X pattern. Not always the case because of the different ways that heads are cast and/or machined. It is best to try and find info for your exact make and model engine. Doing it wrong can warp or crack your head. Also a good idea to "sneak" up on the torque value. Go through the sequence 2-3 times, a little tighter each time till you hit the recommended torque.

1997 Ford F250 XL from North America - Comments
What things have gone wrong with the car? Oil leak from transfer case at about 50,000 K.m. Oil leak from front axle at about same time.

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