Valve Spring
A high-tension coil spring that holds an engine valve closed against the cylinder head until opened by the camshaft.
A valve spring is a high-tension steel coil spring that sits on top of each intake and exhaust valve in the cylinder head. Its primary job is to hold the valve tightly closed against its seat in the combustion chamber. When the camshaft rotates, its lobe pushes the valve open to let air in or exhaust out. As the lobe moves past, the valve spring instantly pulls the valve shut, maintaining cylinder compression.
Valve springs must operate at incredibly high speeds—compressing and extending thousands of times per minute at highway engine speeds. They must have sufficient tension (spring rate) to pull the valve closed fast enough to keep up with the camshaft. If the spring is too weak or worn, the valve may fail to close quickly enough, a dangerous condition known as valve float.
During valve float, the valve remains slightly open when it should be closed, resulting in a sudden loss of engine power, misfires, and potentially catastrophic damage if a piston strikes the floating valve. In extreme cases, a valve spring can fatigue, crack, or break completely. A broken valve spring will cause the valve to drop into the cylinder, leading to severe piston and head damage.
Replacing a valve spring requires removing the valve cover and compressing the spring using a specialized tool. In performance engine building, upgraded dual or honeycomb valve springs are installed to support higher valve lift and prevent valve float at high RPMs.