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). Generally, if there is only a single camshaft on a head, it is just referred to as an OHC head. Also, an engine with an OHV cylinder head is also known as a pushrod engine.
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 (sometimes called keepers). Aluminum heads will also have a valve spring cup to prevent the spring from wearing away the aluminum of the cylinder head.
An ideal method of rebuilding the cylinder head would involve replacing all of the valves, guides, seats, springs, etc. with new ones. Depending on how the engine was maintained, this is typically not necessary. A major cause of valve, guide and seat wear is an improperly tuned engine. An engine that is running rich will often wash the lubricating oil from the valve guide with gasoline, causing the guide to wear rapidly. Conversely, an engine which is running too lean will cause valves to burn because of the higher combustion temperatures. Springs can fall victim to the driving habits of the individual. A driver who often runs the engine speed to the "red-line" will wear out or break the springs faster then one that stays well below it. Unfortunately, mileage takes it toll on all engine parts. Generally, the valves, guides, springs and seats in a cylinder head can be machined and re-used. However, if a valve is burnt, it may be wise to replace all of the valves, since they were all operating under the same conditions. The same goes for any other component in a cylinder head. Think of it as an insurance policy against future problems related to that component.
Unfortunately, the only way to find out which components need replacing, is to disassemble and carefully check each piece. After the cylinder head(s) are disassembled, thoroughly clean all of the components.