A valve job typically begins by disassembling, cleaning and inspecting the cylinder head. Cast iron heads are "Magnafluxed" to check for hairline cracks. This involves applying a strong magnetic field to the head and sprinkling iron powder on it. Cracks disrupt the magnetic field and attract the iron powder, making invisible cracks easy to see. If your engine has been overheated it is possible that the entire head could be warped so we routinely check for all engine defects when performing a valve job.
Overhead cam aluminum cylinder heads are often found to be warped (usually the result of overheating). If the condition cannot be corrected by resurfacing, the head can often be replaced by a new or used cylinder head and then we resume our process.
Next come the valves, guides and seats. The guides are checked for wear. In most cars, valve guides are almost always worn, so will most likely need replacement. We usually install new guides, guide liners or bore out the old guides to accept new valves with oversized stems. Aluminum heads have cast iron or bronze guides that can be replaced but most cast iron heads do not.
If the valves are to be reused, they will be inspected, checked for straightness then refaced. Many shops automatically replace all the exhaust valves to reduce the risk of failure (exhaust valves run much hotter than intakes and are much more likely to fail).
The seats in the head are either cut or ground to restore the sealing surface. If a seat is cracked or too badly worn to be refaced, the seat must be replaced. If that isn't possible (as is the case on many late model cast iron heads because the casting is too thin), then the entire head must be replaced. All aluminum heads have hardened steel seats that can be replaced.
The valve springs are all inspected and tested to make sure they are still capable of maintaining proper pressure. The spring retainers, keepers and other hardware is likewise inspected. Any worn or damaged components are replaced. New valve guide seals are always used.
The valves are then installed in the head and shimmed to restore proper valve height. This is necessary because machining the valves and seat alters their dimensions. Valve height is important because it affects valvetrain geometry and guide wear. If it is an overhead cam engine, the cam is also installed and the valve lash adjusted prior to returning the head to you. For more information,please visting http://www.bossgoo.com