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A valve guide is a small tube, usually made of metal, that is designed to cover, protect, and align one or more valves in a combustion engine. They’re perhaps most commonly seen in cars, but are an important part of the engines for many vehicles and gas-powered devices. The main idea is to keep the valves running to the exhaust and intake chambers supported and properly positioned, but the guides usually also play an important role when it comes to regulating temperature and preventing things like oil leaks. Most guides are designed to outlast the valves they protect, and are usually very durably created. With proper engine care and regular oil changes, these devices should go very much unnoticed. Should the casing wear down or age prematurely, though, big problems can result — engines often overheat, car hoods may begin smoking, and the whole system can grind to a halt until things are fixed. Repair options are usually pretty straightforward, though the process can be somewhat complicated for untrained individuals.
Combustion engines are usually made up of many different parts, and are typically designed to shuttle energy and power pretty quickly from one place to the next in order to facilitate seamless movement. Most engines contain a series of valves, which are basically connections between parts that allow or, alternatively, control the passage of fluids, air, or steam. Over time, valves can wear down; they’re normally made of several interlocking parts and they have a tendency to fall out of place if not held firmly. This is the main idea behind the valve guide: hold the valve, protect it, and make sure it doesn’t have an opportunity to shift or lose traction.
In general, guides are designed to fit over the valve stems with a minimal amount of clearance and they use oil that is seeped through the 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 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.
Guides are only used for engines using a push rod-type camshaft. For the overhead valve type of engines, the guide is not used, as the cam actually actuates the valves and push rods aren’t necessary. 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 guides should remain in peak operational form.
No construction is foolproof, though, and in some instances the guides will wear down. As the 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 problem is usually fairly easy for a mechanic to diagnose, but repair can be more of a challenge.
Perhaps the most comprehensive method for repairing a worn 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; it also tends to be quite expensive. 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.
A method known as knurling is usually a more popular option, though it isn’t always as accurate. This procedure build up the guide with extra metal and materials, 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.
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