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A valve shim, or valve spring shim or hat as it more commonly called, is a device used to set the height of installed valve springs in an internal combustion engine. This is also the name of a steel disk used to adjust the lash or tolerance of a valve on an overhead cam engine. Made of a steel washer-like disc in a variety of thicknesses, the valve shim is placed underneath the valve spring or on top of the valve stem. Then, the installed height and spring pressure of the valve spring or valve are measured and compared to the recommended specifications. The amount and thickness of valve shim used on each valve and spring typically vary, however, most valves and springs will commonly require at least one shim.
In the case of an overhead cam engine, the valve stem can be ground off if the clearance between the stem and the cam lobe is too tight. The difficulty comes in adding length to the valve stem when the clearance between the valve stem and the cam lobe is too great. In this scenario, the proper thickness of valve shim is placed between the valve stem and the cam lobe to take up the distance of the excess clearance. Mounted in a machined cup within the cylinder head, the valve shim is held in place by the tension of the valve spring.
Without the addition of a valve shim, the engine would suffer from poor ignition, exhaust issues and reduced horsepower. When the clearance between the valve and the cam lobe is too great, the valve is not pushed open as far as it is intended to be resulting in less fuel being allowed into the cylinder. The valve is also closed slightly too soon, which allows some of the exhaust to remain in the cylinder, thereby contaminating the air/fuel mixture and creating reduced power.
By placing a valve shim in any out-of-tolerance valve spring location, all cylinders are able to receive and exhaust the same amount of material. Many engine builders consider the valve shim to be an invaluable tool when tuning a performance engine. The highest quality valve shim is made of very high carbon or spring steel to last in the harsh conditions of the cylinder head without losing its fit and finish. Some lesser-quality bargain shims are made of mild steel that can actually be hammered thinner as the valve spring pushes against the shim, eventually causing poor engine performance and damage.