What is a Cylinder Head? How Do They Work?

What Is a Cylinder Head and What Does It Do?

Your cylinder head or heads are located, as you might expect, at the top of your engine. Inline or straight engines have one cylinder head, while V-shaped engines and boxer engines have two, one for each bank of cylinders. Cylinder heads are usually made of cast iron or aluminum. Many of the moving parts that make your engine work are found inside the cylinder head. It houses the intake and exhaust valves, rocker arms, and the spark plugs.

In engines with overhead cams (OHC), the camshafts are inside the cylinder head. The cylinder head also contains inlets that allow coolant from the radiator and oil to flow through the engine. Depending on the design of your engine, the layout of your cylinder head will differ from other cylinder heads. The material can also differ. If all of this info has got your "head" spinning, don't worry; we'll walk you through how the cylinder head works, and all the different kinds of cylinder heads out there.

How Does A Cylinder Head Work?

The cylinder head is where a great deal of the action for running your engine happens. It contains the combustion chambers where the quick burn of fuel and air that moves the pistons occurs. For this to take place, the cylinder head must be connected to the intake and exhaust manifolds. The intake manifold feeds air into the head through the intake ports, past the intake valves and into the combustion chamber. Once the air and fuel have been combusted, exhaust gasses exit the combustion chamber past the exhaust valves, through the exhaust ports, to the exhaust manifold. The intake and exhaust valves are located inside the cylinder head. These allow fresh fuel and air to enter, and combusted exhaust gasses to exit the chamber, respectively. These must be precisely timed to open and close at the right times to keep the engine running smoothly. This is achieved through a system of parts known as the valvetrain.

The exact parts used in the valvetrain will vary with engine layout, but can contain, in addition to the intake and exhaust valves, rocker arms or a camshaft or two camshafts. Rocker arms push on the valves to open and close them. They in turn are moved by push rods or lifters which respond to the camshaft located in the block. Some engines, instead, have the camshaft located above the valves where it can operate them directly. What parts are present and how exactly they work depends on the configuration of your head and valvetrain.

What Are Flathead Engines?

In the early days of automotive engineering, flathead or sidevalve engines were the norm. In these, valves are on the side of the combustion chamber, in the engine block, and fuel and air enter from the side and exhaust exits out the same side. A cross section of this type of combustion chamber looks like an upside down letter L. So, flatheads are sometimes known as L-head engines. The valves in a flathead engine are operated directly by the camshaft. This made flatheads easy to produce and to work on. They also were very reliable. A flathead engine could still run with a broken valve.

The flathead design did have its flaws, though. The exhaust gasses had to follow a circuitous path to get out of the combustion chamber which inhibited the engine's speed and caused it to hold on to a lot of heat. New engine designs came along to solve these problems and eventually supplanted the flathead engine. Some of these, early on, were variations on the sidevalve idea.
In T-head engines, fuel and air enter the chamber on one side and exhaust exits on the opposite side. If flathead engines have an upside down L cross section, T-heads have a T-shaped cross section. This keeps the simple valve on the side design, but gives the gasses a crossflow, allowing the engine to exchange gasses or "breathe" better. T-head engines still had the valves operated directly by the camshafts, but required two camshafts: one for the intake and one for the exhaust valves. Sidevalve engines are no longer the most effective design for cars, but they are still used in farm equipment and other simple machinery due to their simplicity and reliability.

Different Types of Cylinder Heads

Cylinder Head on Intake Over Exhaust Engines

Intake over exhaust engines represent a further improvement over the flathead design and the first time that valves were positioned in the cylinder head. The exhaust valve remained in the block, while the intake valve was located in the cylinder head. In early examples, the intake valve was operated by suction from the chambers. Later versions used mechanical valvetrains to operate the intake valves. The camshaft moves push rods that push rocker arms that operate the valves. Although intake over exhaust was more common, some exhaust over intake engines were built. Intake over exhaust engines can still be found in some motorcycles.

Cylinder Head on Overhead Valve Engines

Overhead valve engines pick up where the intake over exhaust engine left off, by putting both the intake and exhaust valves in the cylinder head. The camshaft is located in the block, or between the V in V configuration engines. Because of this, they are sometimes called cam-in-block engines. The cam can be driven by the crankshaft via a short chain or belt or even directly by a gear. Like in the intake over exhaust engine, push rods push rocker arms to open and close the valves.

Cam-in-block engines have largely been succeeded by overhead camshaft engines, but still have enough advantages that some are still in use. The cam-in-block design is strongly associated with General Motors and is still used in some Chevy V8s. Placing the cam in the block reduces the overall size of the engine. The cam-in-block design is also simpler and easier to produce. The biggest disadvantage of the cam-in-block design is that it has more parts in the valvetrain (with the push rods), which means more valvetrain inertia. This limits how many revolutions per minute the engine can run at. The greater number of moving parts also makes these engines noisier.

Cylinder Head on Overhead Camshaft Engines

Overhead camshaft engines are technically still overhead valve engines. The valves remain in the cylinder head. Rather than using push rods to connect the camshaft to the valves, though, the camshaft or camshafts are moved to a position above the valves, where they can activate the valves directly. The camshaft is connected to the crankshaft by a belt or a chain known as the timing belt or timing chain.

Some engines use one camshaft for both the intake and exhaust valves, while others use a separate camshaft for each set of valves. These are called single overhead cam engines and dual overhead cam engines, respectively.
Although overhead cam engines are more complex and larger than cam-in-block engines, they offer some advantages. Putting the camshaft above the valves gives engine designers more flexibility in where the valves can be placed. This allows for more precise valve timing, and for the use of multiple ports per cylinder. Dual overhead cams allow for even more precise timing. These engines are the most complex, but make up for the complexity with their improved performance.

Has That Thing Got a Hemi (Hemispherical Head)?

Another factor of cylinder head design that can affect the performance of an engine is the shape of the combustion chambers. The shape of the combustion chamber can affect how completely the air fuel mixture burns and how well gasses flow through the engine. The shape affects the placement of the valves and spark plug, as well.

Hemispherical (known for their use in Chrysler's Hemi engines) and pent-roof chambers are the most common. In these, the valves are at an angle, on either side, providing crossflow for the gasses. They also have a greater surface area than other combustion chambers of the same volume. That gives a more complete burn of the fuel. Hemispherical heads are also easy to produce.
Pent-roof engines maintain many of the advantages of hemispherical engines, but have flattened, roof like tops. This allows for a greater number of valves to be used per cylinder, which enhances the flow of gasses. Pent-roof chambers are the most common design in use today.
Wedge shaped chambers are, as you might expect, shaped like a wedge, with the spark plug at the wide end of the wedge. The valves are at an angle, perpendicular to the roof of the chamber. The wedge shape compresses the gasses leading to a powerful combustion.
Bathtub shaped chambers have an oval, upside-down bathtub shape. The spark plug is mounted into the side, and the valves are mounted straight into the top of the chamber. That allows for a simple valvetrain to be used. This makes the bathtub shape common in cam-in-block engines. Bathtub chambers have a short flame path, making for quick combustion.

What Cylinder Heads Are Made Of

Cylinder heads are usually made of one of two materials, cast iron or aluminum. Iron cylinder heads are easier to produce and less expensive, but they are heavier than aluminum heads. Aluminum cylinder heads, in addition to being lighter, also dissipate heat better which makes engine overheating less likely. In some engines, an aluminum head sits atop a cast iron engine block. One disadvantage of this design is that the two metals expand at different rates when heated. That puts a strain between the two parts which lead to cracking or warping of the cylinder head.

Signs of a Bad Cylinder Head

As mentioned above, the most prevalent problem with cylinder heads is that they can crack or warp. Since combustion occurs inside them, they are exposed to high heat. Of course, they have coolant flowing through them, but coolant leaks and other cooling problems may cause the engine to overheat. Overheating makes the metal of the engine expand. When the cylinder head and the engine block are made of different metals, the different metals expand at different rates, which put strain on that metal. That can lead to cracks or warping in the cylinder head.

A cracked or warped cylinder head will often leak coolant, which exacerbates your problem. Frequent overheating or lost coolant might indicate a cracked cylinder head. Cylinder head cracks can also hinder your engine's performance. You may experience engine misfires. If left untreated, a cracked cylinder head can lead to further engine damage. Some cracks might be visible to the naked eye. A pressure test can also be used to identify cracks in the cylinder head.

Do-It-Yourself Cylinder Head Replacement

Although an experienced do-it-yourselfer could replace a cylinder head, this might be a job you'd prefer to leave to the professionals. We would not advise that you attempt replacing your cylinder head without having a lot of prior mechanical experience. Improper installation could lead to serious damage to the engine and necessitate even bigger repairs.

Replacing a cylinder head is a lengthy process with many steps. Your vehicle's oil and coolant need to be drained, since they flow through the cylinder head. The intake and exhaust manifolds need to be separated from the cylinder head. The valve covers and timing chains need to be removed, as do any accessories, like the power steering pump, that get between you and the cylinder head.
With all that done you can remove the cylinder head. Be very careful not to scrape or scratch the engine block as you remove the cylinder head. Doing so will make any leak problems you had before even worse. To avoid warping the metal, there is a specific pattern you are meant to remove and install the cylinder head bolts in. This can be found in a service manual. When you are installing the new cylinder head, you will want to consult the service manual for the appropriate torque to tighten the bolts to. You can then reverse all the previous steps to put everything back together.

Need a New Cylinder Head Replacement?

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