There are two basic types of cylinder heads used on today’s automobiles: the Overhead Valve (OHV) and the Overhead Camshaft (OHC). The latter can also be broken down into two subgroups: the Single Overhead Camshaft (SOHC) and the Dual Overhead Camshaft (DOHC).
Most cylinder heads are made of an aluminum alloy because it is light weight, highly durable and transfers heat well. However, cast iron was the material of choice in the past, and is still used on many vehicles today. Whether made from aluminum or cast iron, all cylinder heads have valves and seats. Some use two valves per cylinder, while the more hi-tech engines will utilize a multi-valve configuration using three, four and even five valves per cylinder. All cylinder heads have a valve guide for each valve. The guide centers the valve to the seat and allows it to move up and down within it. The clearance between the valve and guide can be critical. Too much clearance and the engine may consume oil, lose vacuum and/or damage the seat. Too little, and the valve can stick in the guide causing the engine to run poorly if at all, and possibly causing severe damage. When the valve contacts the seat, sealing the combustion chamber, it does so with precision-machined surfaces. The last component all cylinder heads have are valve springs. The spring holds the valve against its seat. It also returns the valve to the closed position after the valve has opened. The spring is held in place around its corresponding valve by a retainer and valve locks. Aluminum heads will also have a valve spring cup to prevent the spring from wearing away the aluminum of the cylinder head.