The cylinder head sits on top of the engine and holds everything that controls the gas flow in and out. Use the buttons to look inside it and follow the camshafts, the valves, the spark and the coolant through one cycle.
The cylinder head is the block of aluminium that sits on top of the engine block and caps the cylinders. It holds the combustion chambers, the valves, the camshafts, the spark plugs and the intake, exhaust and cooling passages, all in one casting. On a four-stroke engine it is where every stroke of the cycle is controlled.
Because it carries so much, the head is a crowded place. Two camshafts run along the top, eight or more valves hang beneath them, a spark plug drops into the middle of each chamber, and passages for air, exhaust and coolant thread around all of it.
From the outside the head is just a flat aluminium lid with pipes and wires attached. Cut away, it shows how the camshafts, valves, springs and ports are packed together around the combustion chamber, and why the shape of that chamber matters.
One idea holds it together: the camshafts turn in step with the crankshaft, so the valves open and close at exactly the right moment in the four-stroke cycle. Everything else in the head exists to support that timing.
The camshafts are geared to the crankshaft and turn at half its speed, so one full turn of a camshaft matches two turns of the engine. As a camshaft rotates, its egg-shaped lobe comes round and pushes the intake valve down against its spring.
With the intake valve open, the downward stroke of the piston draws air and fuel along the intake port and into the combustion chamber. The port is shaped to smooth the flow in, and the valve closes again once enough charge has entered.
With both valves shut, the rising piston compresses the mixture. At the top of the stroke the spark plug fires, and the flame spreads quickly through the chamber, raising the pressure sharply.
That pressure drives the piston down on the power stroke. Then the exhaust camshaft opens the exhaust valve and the piston rises again, pushing the burnt gas out through the exhaust port and on to the exhaust system.
A cam lobe can only push a valve open; it can never pull it shut. That job belongs to the valve spring, which snaps the valve back onto its seat the moment the lobe passes, keeping the seal gas-tight until the next opening.
All that burning makes the head very hot, especially around the exhaust valve and the spark plug. The cooling jacket is a web of passages cast through the head; coolant circulates through it and takes the heat to the radiator, where it is released to the air.
The head gasket sits between the head and the engine block. It seals the joint so the high-pressure combustion gas cannot escape and the coolant and oil cannot leak out or mix, which is why a failed head gasket is such a serious fault.
The cylinder head is the block of aluminium that sits on top of the engine block and caps the cylinders. It holds the combustion chambers, the valves, the camshafts, the spark plugs and the intake, exhaust and cooling passages, all in one casting.
A dual overhead camshaft head puts one camshaft above the intake valves and another above the exhaust valves. Each cam works its own set of valves, so the intake and exhaust can be timed separately and the engine breathes better at high speed.
The camshafts do. As each camshaft turns, its lobes push down on the valves and open them against the valve springs. Once a lobe passes, the spring pushes the valve back onto its seat and seals the combustion chamber.
The spark plug sits in the combustion chamber and ignites the compressed air and fuel mixture with an electric spark. That spark starts the burn that pushes the piston down and turns the engine.
Burning fuel makes the head very hot, and the valves, spark plug and combustion chamber all need to stay within their working temperature. The cooling jacket is a network of passages cast through the head; coolant flows through them and carries the heat away to the radiator.
The head gasket is the seal between the cylinder head and the engine block. It stops combustion pressure, coolant and oil from leaking out of the joint, and it also keeps the coolant and the oil from mixing with each other.