What is a Cylinder Head?
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes at the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket.
A cylinder head is usually located on the top of the engine block. It serves as housing for components such as the intake and exhaust valves, springs and lifters, and the combustion chamber. This page covers the main function and various designs of cylinder heads, and their causes and symptoms of failure.
The passages in the cylinder head allow air and fuel to flow inside the cylinder while permitting the exhaust gases to flow out of it. The passages are otherwise called ports or tracts. The cylinder head also channels the coolant into the engine block, thereby cooling down the engine components. The cylinder head uses a gasket that aids in preventing water or oil from leaking into the combustion chambers.
Most original equipment manufacturers are made cylinder heads from cast iron. A cylinder head made of cast iron is more durable and less expensive. However, cast iron is heavy and provides a lesser efficiency in dissipating heat.
For this reason, some manufacturers prefer using cylinder heads made of aluminum. These cylinder heads are much lighter than cast iron cylinder heads. Performance cars and race cars commonly have aluminum cylinder heads.
Cars with inline (straight) engines have one cylinder head and automobiles with V engines have two cylinder heads, one for each cylinder bank. In some vehicles, where cylinder banks are very close to the V engine, one cylinder is all that is needed. Large industrial vehicles may have one head per cylinder. This makes replacing a cylinder head much more affordable.
Types of Cylinder Heads
- Flathead Cylinder Heads: These were the first type of cylinder heads. Flathead cylinder heads simply protect the cylinder block and have no moving parts. These cylinder heads do not allow for an efficient air flow, and thereby provide a poor engine performance.
- Overhead Valve Cylinder Heads: These cylinders heads are superior to flathead cylinder heads. Overhead valve cylinder heads have the camshafts above them. These heads have their pushrods and valves connected to provide a smooth airflow.
- Overhead Camshaft Cylinder Heads: These are the most advanced designs of cylinder heads. Overhead camshaft cylinder heads have the camshafts inside the cylinder head, eliminating the need of pushrods. This provides a better airflow, and in turn, increases the efficiency of the engine.
How Does a Cylinder Head Work?
The cylinder head allows the engine to perform at maximum capacity by connecting to the intake and exhaust manifolds to allow the quick burn of fuel and air that moves the engine’s pistons.
The intake manifold feeds air into the head, where it is combusted with fuel to produce exhaust gases, which are expelled through the exhaust manifold.
It’s important that the intake and exhaust valves, which allow the entry and exit of relevant gases, open and close at the correct times to keep the engine running smoothly.
Why Do Cylinder Heads Fail?
The most common reason for cylinder head failure is overheating caused by, for example, coolant loss, head gasket failure, or restricted flow of coolant.
In extreme cases, a cracked or damaged cylinder head can result in terminal damage to the entire engine! You must deal with any problems as soon as possible to try and prevent this from happening.
Cylinder heads can also warp because of the tremendous pressure placed on them by extreme temperature changes. This means that it is no longer providing a flat surface to hold the head gasket in the right position, which can lead to expensive problems later on in the form of major repairs or total engine replacement being required.
Warping is a particular problem for cylinder heads made of aluminum. Different engines have different levels of tolerance for cylinder head warping – some can withstand a high amount of warping while still working effectively. If your cylinder head does warp, you may be able to take it to a mechanic who specializes in realigning them – if not, you will probably need a replacement.
5 Symptoms of Cylinder Head Failure
The symptoms of cylinder head failure are varied – it is important that drivers are able to recognize them as quickly as possible so any damage can be limited. If the cylinder head fails, it will lead to the engine (and by extension the vehicle) breaking down.
The most common reason for cylinder head failure is overheating caused by coolant loss, head gasket failure, or restricted flow of coolant. In extreme cases, a cracked or damaged cylinder head can result in terminal damage to the entire engine.
Internal combustion engines heat up very quickly. As the name implies, they are designed to contain small, controlled explosions.
Much of the energy in the combustion process is actually lost to thermal energy, instead of being converted into kinetic energy to power the vehicle. This is a byproduct of their design that can be mitigated but never eliminated entirely.
The number one cause of a cracked cylinder head is overheating. When an engine overheats, its components may be stressed far beyond the heat threshold it was designed to withstand.
Since most heads are made out of aluminum, they may warp or crack when the engine gets hot enough.
An engine may overheat for a number of reasons, most of which are due to a failed component in the cooling system. Some possible causes are outlined below.
2. Air in the Cooling System
Air in the cooling system may allow hot spots to form, where one part of the engine is much hotter than the portion measured by the temperature sensor. This happens because air cannot transfer heat as effectively as a liquid can.
If air is trapped in one spot away from the coolant temperature sensor, the temperature sensor may not pick up the hot spot. You may not know your car is running hot until the damage is already done.
Additionally, an air leak in the cooling system may reduce the pressure in the cooling system. Liquid coolant is more likely to boil at a lower pressure.
3. Failed Water Pump
If the water pump fails, the coolant won’t be able to circulate through the engine properly. The coolant in the radiator may be nice and cold, but the coolant surrounding the water jackets in the head and block will be extremely hot.
Without a pump, coolant will only be able to circulate via the natural convection process, which is much too slow to remove excess heat before the engine overheats.
4. Failed Thermostat
Combustion engines are most efficient when they are running at the operating temperature they were designed for.
An engine’s normal operating temperature is still plenty hot enough to scald you – typically around 190 to 210 degrees Fahrenheit (85 to 99 degrees Celsius). Running the engine below this temperature increases fuel consumption, emissions, and wear.
A thermostat blocks off coolant from the radiator until the coolant in the engine reaches the target temperature. Once the target temperature is reached, the thermostat opens to allow coolant at ambient temperature to enter the engine, cooling it until the thermostat is cold enough to close. The cycle continues as the engine runs.
If the thermostat is stuck open, your car will probably run pretty cold. This is because the coolant from the entire system is cycling through the radiator 100% of the time and rarely gets a chance to fully warm up.
If the thermostat is stuck closed, your engine will overheat pretty easily. The hot coolant has nowhere to go to cool off. In a pinch, some engines allow you to use the heater as a tiny radiator.
The effectiveness of this method depends on the size of the heater core and how the cooling system is routed.
5. Coolant Leak
If too much coolant leaks out, eventually you won’t have enough to properly cool the engine. You can lose a decent amount of coolant before overheating, but it’s a good idea to regularly check the overflow reservoir and radiator to make sure you’re topped off.
If you notice a coolant leak, it’s a great idea to check your radiator cap. It’s a cheap part with some serious repercussions if it fails.
Cylinder Head Repair Cost
A good garage should be able to repair and recondition a cylinder head for as little as $75, but it can also cost hundreds of dollars to do the same job. This will depend on the value of the cylinder head and the price you are quoted to repair it if it develops a crack or stops working as effectively as it should. In many cases, it may be worth simply buying a new one.
The cost of a cracked cylinder head repair job will vary depending on the make and model of the vehicle that it’s in. You can be sure that it will cost at least $500, which includes labor and parts costs.
If you were to replace the entire cylinder head, it would only cost $200 to $300 on average for parts. With labor at about $90 to $100 per hour, this comes out to roughly $500 for the job. However, this is assuming the cylinder head is made of aluminum like they are in most cars these days.
If the cylinder heads are hard to access (as they are on Subarus, since they use a boxer engine layout), labor costs may be significantly higher.
If you have an older vehicle or a more expensive vehicle, you probably have a cylinder head made of iron. Since iron is a more expensive material than aluminum, you might be looking at $500 just for the parts.
The labor may also require more hours for this repair job to be done correctly. Therefore, you could be looking at $1,000 to repair a cracked cylinder head made of iron.