A valve shim is a device used to set the height of valve springs in an internal combustion engine. It is a steel washer-like disc that sits under the valve spring or on top of the valve stem, and is used to adjust the lash or clearance of a valve. The valve shim is made of high carbon or spring steel and is an invaluable tool for tuning high performance engines. Without a valve shim, the engine would suffer from poor starting, exhaust problems, and reduced power.
A valve shim, or valve spring shim or cap as it is more commonly called, is a device used to set the height of valve springs fitted to an internal combustion engine. This is also the name of a steel disk used to adjust the lash or clearance of a valve on an overhead cam engine. Made of a steel washer-like disc in a variety of thicknesses, the valve shim sits under the valve spring or on top of the valve stem. The installed height and spring pressure of the valve spring or valve are then measured and compared to the recommended specifications. The amount and thickness of valve shim used on each valve and spring will 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 rebored if the gap between the stem and cam lobe is too tight. The difficulty comes in adding length to the valve stem when the clearance between the valve stem and cam lobe is too great. In this scenario, the proper valve shim thickness is placed between the valve stem and cam lobe to take up the excess clearance distance. Mounted in a machined cup within the cylinder head, the valve shim is held in place by tension from the valve spring.
Without the addition of a valve shim, the engine would suffer from poor starting, exhaust problems, and reduced power. When the clearance between the valve and the cam lobe is too large, the valve does not open as far as it is intended resulting in less fuel being allowed into the cylinder. The valve also closes a little too early, allowing some of the exhaust to remain in the cylinder, thus contaminating the air/fuel mixture and creating reduced power.
By placing a valve shim at any out-of-tolerance valve spring location, all cylinders can receive and remove the same amount of material. Many engine manufacturers find the valve shim an invaluable tool when tuning a high performance engine. The highest quality valve shim is made from high carbon or spring steel to last in harsh cylinder head conditions without losing its fit and finish. Some lower quality bargain shims are made from mild steel that can actually be ground thinner as the valve spring pushes against the shim, ultimately causing poor engine performance and damage.
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