cylinder head

The cylinder head is a cast iron or aluminum alloy component bolted to the top of the engine block, forming the upper boundary of each combustion chamber. It serves as both a structural seal and a functional housing for the valvetrain, fuel injectors, and spark plugs (in gasoline engines). Without it, the compression and heat generated during combustion would have nowhere to go, and the engine block itself would be exposed to extreme pressures and temperatures.

The underside of the head features a carefully machined surface, called the deck, that makes contact with the engine block's surface. This joint is sealed by a gasket, typically a multi-layer steel or graphite composite roughly 1 to 1.5 mm thick, that prevents combustion gases and coolant from leaking between the two surfaces. Modern gaskets are formidable pieces of engineering: they must withstand pressures exceeding 1500 psi and temperatures above 300 degrees Fahrenheit while maintaining an airtight seal indefinitely.

Variants and materials

Overhead valve (OHV) engines have a relatively simple head with the valves and pushrods sitting directly on top of the block. Overhead cam (OHC) designs integrate the camshaft directly into the head, requiring more complex internal passages and bearing surfaces. The choice between cast iron and aluminum reflects a trade-off: iron dissipates heat more evenly and tolerates thermal cycling, but aluminum saves roughly 30 percent of the weight while offering better thermal conductivity, a critical advantage in high-performance and turbocharged applications.

The head contains numerous internal galleries, small passages through which engine coolant flows to regulate temperature. These passages must be designed to avoid air pockets and dead zones; trapped air creates hotspots that can cause detonation or valve recession. Port design on the intake and exhaust sides of the head also governs flow efficiency: straighter ports with minimal sharp corners reduce turbulence and improve volumetric efficiency.

The most common failure mode is gasket blow-out, where combustion pressure ruptures the seal and coolant or combustion gases escape into the oil or cooling system. This occurs after tens of thousands of miles of thermal cycling, especially in engines with high compression ratios or poor cooling system maintenance. A warped deck surface (caused by overheating or uneven bolt torque) guarantees gasket failure. Proper reassembly requires following the manufacturer's bolt torque sequence and specifications exactly, often in multiple passes, to ensure even clamping force across the entire head.