The principal function of the cylinder head is to ensure that the upper part of the engine’s combustion chamber remains tightly closed to maintain pressure.
Sitting on top of the engine block, the cylinder head closes off the combustion chamber apart from a minor gap sealed by the head gasket that aids in preventing water or oil from leaking into the combustion chambers.
A secondary task for the cylinder head is to maintain a constant lubrication level within the cylinder. If these oil levels are not maintained, engine performance will begin to suffer.
These joint functions combine to make the cylinder head an essential part of the internal combustion engine.
Produced using either heat resistant aluminium alloys or similar light metals, the cylinder head must withstand consistently high heat levels.
This, and its complex structure, makes the cylinder head one of the most expensive parts you will find in any vehicle.
The cylinder head is fixed at the bottom of the engine to the crankshaft housing and sealed at the top with a valve cover, housing such components as the exhaust and intake valves, springs and lifters, the combustion chamber and spark plugs.
The structure of the cylinder head differs if the engine is gasoline or diesel, while virtually all gasoline engines are fitted with an aluminium cylinder head. Diesel engines are usually fitted with cast iron cylinder heads to support their increased weight and strength.
The cylinder head is designed with a series of passages that allow air and fuel to flow inside the cylinder while exhaust gases flow.
These passages, known as either ports or tracts, are constructed using a process known as lost-foam casting.
Lost-foam casting is formed using a polystyrene model of the cylinder head through gluing together layers, which creates a mould around the polystyrene.
The mould is then removed, and molten aluminium poured in its place. Once the metal has solidified, the casting is machined.
The shape of the combustion chamber has developed over the history of the engine, and nowadays, most chambers are either Hemi shaped, Pent roofed or wedge-shaped.
Most of the chambers in popular cars of the Fifties and Sixties were wedge-shaped. in which compression stroke forces the mixture from the narrow end of the wedge into the broader end, generating turbulence that causes the fuel/air mixture to concentrate around the spark plug.
More powerful vehicles are fitted with a Hemi-chamber, a dome-shaped recess particularly suitable for use on single valve crossflow heads. As a result of the spherical shape of the Hemi chamber, it is difficult if not impossible to fit in the multiple valve combination commonplace in most modern engines.
A more reasonable alternative is the Pent-roof combustion chamber. The Pent-roof is similar to the Hemi chamber but constructed using non-spherical surfaces, making it possible to fit four valves per cylinder. In recent years Pent-roof chambers have become commonplace in engines.
In Pent-roof chambers heads, one side of the block is connected to the air intake and the other to the exhaust manifold. This combination, known as the crossflow head, while a cylinder head where its valves are found on the same side is known as Reverse-flow, a combination regarded as being considerably less efficient than a crossflow design.
In vehicles powered by an overhead cam (OHC) engine, the camshaft is situated in the head, while in the over-head valve (OHV) combination, the camshaft is in the block.
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A guide to acquiring, restoring and maintaining UK or European Classic Cars of the Fifties and Sixties- as well as a recollection of the iconic cars of the era and the visionaries that produced them.