Cylinder Head and Valvetrain

The cylinder head bolts to the top of the engine block and forms the roof of the combustion chamber. It contains the intake and exhaust valves, the valve springs, and on overhead cam engines, the camshafts. The head is where the spark plug threads in and fires into the combustion chamber. The cylinder head is one of the most complex castings on the engine — it has coolant passages, oil passages, intake and exhaust ports, and precisely machined valve seats all in one piece.

Intake valves open to let the air-fuel mixture into the cylinder. Exhaust valves open to let burned gases out. Most modern engines have four valves per cylinder — two intake and two exhaust — because four smaller valves flow more air than two large ones. Each valve has a head — the flat disc that seals against the valve seat — and a stem that slides in a guide pressed into the head. The valve seat in the head must be perfectly smooth and the valve face must seal against it with zero leakage for proper compression. A burnt valve — one with damage to its sealing face — leaks compression and causes a misfire on that cylinder.

Each valve has a spring that holds it closed. The camshaft pushes the valve open against spring pressure, and the spring pushes it closed when the camshaft lobe rotates past. A weak or broken valve spring fails to close the valve quickly enough at high RPM, which causes a misfire and possible valve-to-piston contact. On overhead cam engines with hydraulic lash adjusters, a ticking noise at startup that goes away after a few seconds is usually an adjuster that bled down overnight — normal. A tick that persists after warmup may indicate a collapsed adjuster.

The camshaft is a shaft with egg-shaped lobes on it. As the camshaft rotates, each lobe pushes a valve open at exactly the right time and for exactly the right duration. The shape of the lobe determines how far the valve opens, how long it stays open, and how quickly it opens and closes. The camshaft is driven by the crankshaft through a timing chain or timing belt at exactly half crankshaft speed — the camshaft turns once for every two turns of the crankshaft. This is because the four-stroke cycle takes two crankshaft revolutions. The camshaft must be timed precisely to the crankshaft — if the relationship is off by even a few degrees, valves open and close at the wrong time, the engine runs poorly, and in severe cases the pistons can strike the valves.