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A valve guide is a tube that runs through an engine's cylinder head in which the exhaust and intake valves are positioned. The valve guide is typically made of a material, such as bronze, and guides the valve, keeping it straight. As the valve guide wears, the valve stem begins to become loose and oil is able to leak into the combustion chamber. This creates smoke and a loss of power from the engine.
The valve guide is only used for engines using a push rod-type camshaft. For the overhead valve type of engines, the valve guide is not utilized as the cam actually actuates the valves and push rods are not used. The typical valve guide is designed to outlive the engine and thus will never require repair. With proper lubrication and scheduled oil change intervals, the valve guides will remain in peak operational form.
The valve guides are designed to fit the valve stems with a minimal amount of clearance and they use oil that is seeped through the valve guide seal for lubrication. This oil is tasked with the job of controlling the high temperatures of the valve stem through the process of combustion. In aluminum head applications, the valve guides also aid in the prevention of heat damage to the head. The ultra-high heat generated by the combustion chamber is radiated away from the valve stem by the guides.
Perhaps the most expensive method for repairing a worn valve guide is to drive the old guides out of the head with a press and install new guides. This repair method is risky and many heads are damaged beyond repair. Once successfully driven out, the old guide is replaced with a new guide that is pressed into place. Once the new valve guide is in place, it is trimmed to fit and dressed to receive a new valve. This procedure is used primarily on aluminum heads.
Knurling is perhaps the most popular method of repairing a worn guide. This procedure build up the guide and then reams it back to the proper size. Once the guide is repaired with whichever method is chosen, the guide seal is replaced. This is a small rubber cup-like component that pushes down over the valve stem. The seal is pushed onto the guide boss cast into the head and prevents excess oil from entering the guide.