What You Should Know About Cylinder Heads

The head of the engine is what turns the cylinders into combustion chambers and directs the airflow into and out of the engine.

In straightforward terms, the block does all the spinny stuff while the head controls the input and the output. These two components work with the other bits to turn gasoline or diesel into horsepower and torque.

What’s Inside the Cylinder Head

Unless you are building a high-performance engine, the cylinder heads are generally left alone. You simply service the components that mount to it. However, when things go sideways, you may need to address the cylinder heads themselves. V-type engines have two heads, while inline engines have one.

Cylinder heads cap off the engine block and cylinders, so that the pistons can compress the air, allowing for explosive combustion. (Photo: Jefferson Bryant)

Inside the cylinder head are chambers for each cylinder that the head services. Each piston inside the cylinders has an intake port, an exhaust port, two or four valves per cylinder, water jackets for cooling, and the valvetrain component mounts.

Types of Cylinder Heads

There are three main cylinder head types, though only two are commonly used today.

Flathead – Look at an original Ford Model T, and you will find a flathead four-cylinder, whose head is just what it sounds like—flat. It is not much more than a glorified flat plate of iron that caps off the engine block. The valves are in the block, and only the spark plugs are in the head. This type is not used much anymore.

Overhead valve (OHV) – Also referred to as a “pushrod” engine, the OHV head moves the valves to the head, which is much more efficient. It also allows for larger displacement engines and much more airflow. These heads use rocker arms actuated by pushrods that run into the engine block. These connect to lifters, which ride on the camshaft lobes to operate the valves.

Overhead cam (OHC) – Instead of having one camshaft centered in the engine block, the OHC engine uses one or two camshafts at the top of the head to operate the valves. Depending on the design, the cams may or may not use rocker arms, but they do not use lifters or pushrods.

OHC engines rev quickly because they don’t have as much mass to move, nor do they have issues, such as valve float, common in high-revving OHV engines. OHC engines are even more efficient and allow designers to develop unique cam profiles to make maximum horsepower. The main drawbacks of OHC engines are that the head is larger, the cam timing is complicated, and they use long timing belts or chains that must be changed regularly.

Common Cylinder Head Problems

In most cases, the cylinder heads don’t require servicing, only the parts that bolt to them. However, there are a few instances where it is necessary to address the heads. The main problems that damage a cylinder are overheating, freezing, and physical damage.

The intake ports (4 rectangular holes on head) bring air and fuel from the intake and direct it into the combustion chamber. (Photo: Jefferson Bryant)

Overheating is the main issue for cylinder heads, and it is by far the most common form of damage. While there are any number of causes of overheating, the results are the same. Get above 250 degrees Fahrenheit, and you have a very real problem. You might be thinking, “But the exhaust gets way hotter than 250 degrees!” You would be correct, but that is in a controlled area. The cylinder head itself should not exceed 230 degrees Fahrenheit for normal operation. What the head is made of makes a difference too.

Aluminum cylinder heads, which comprise the vast majority of modern engine cylinder heads, are more susceptible to warping but resist cracking better. Aluminum naturally dissipates heat much faster than other materials. While the heads get hotter quicker, they cool off more quickly. If your engine has a cooling issue and the temps have spiked, a quick shut off and cool down will often save the engine from blowing a gasket or cracking the head. If allowed to overheat repeatedly, the mating surface to the block can warp, breaking the seal to the block and thereby blowing the head gasket.

Cast iron heads take longer to heat up, so they take much longer to cool down. One 250-degree overheat will probably be fine, but the head can warp quickly if it happens more often. Because cast iron is less malleable than aluminum, it can quickly crack. Also, cast iron is less likely to come back into shape after even a slight warping event, leaving you with—you guessed it—a blown head gasket.

Air flow is controlled via the intake and exhaust valves. The intake valves are larger than the exhaust valves. (Photo: Jefferson Bryant)

• 260 degrees is considered seriously overheated regardless of the material, as both iron and aluminum can deform at that temperature.
• Above 280, any head will likely see permanent damage unless it was for a very brief moment.

Older engines with an iron block and iron heads can suffer catastrophic failure at 230 degrees because the block expansion squeezes the pistons, and the heads warp and crack. Modern engines have better metallurgy than they did 50 years ago. Today’s iron blocks are far superior to those in the olden days.

Cylinder Heads, Air-Cooled Engines, and Freezing

Keep in mind that this information applies to water-cooled engines. Air-cooled engines run much hotter than their water-cooled brethren, and 235 degrees is the maximum safe temperature for these engines. A cylinder head temperature kit for VW, Porsche, and GM air-cooled engines is a good way to monitor your air-cooled engine’s temperature. Over 235 degrees, an air-cooled engine can warp enough to pull the head studs out of the block.

Conversely, if your engine’s coolant does not have enough antifreeze, it can freeze and expand. That will crack the water jacket and ruin the head. Mechanical damage is typically caused by a handful of issues, including broken timing belts, broken pistons, foreign objects in the combustion chamber, detonation, and broken valve springs or valves.

Without a properly running engine, your vehicle is nothing more than an inconveniently shaped camper. The cylinder heads are a vital component. Don’t ignore signs of overheating, coolant leaks, or other cooling system failures. They can turn your sweet ride into a sour yard ornament.