Nobody wants engine problems such as oil consumption, a compression leak, valvetrain noise or an outright valve failure. So every effort should be made to make sure everything that is worn or damaged is replaced or reconditioned when rebuilding a cylinder head. But sometimes valve problems occur anyway and lead to expensive comebacks. How can you avoid such woes? By understanding the causes of valve failure and making sure you have not overlooked anything when replacing and reconditioning valves, seats, guides and the rest of the valvetrain components. Valve work can be a tricky aspect of engine rebuilding for several reasons. For one, it requires absolute precision. Close enough is not good enough. If tolerances are not exact and the valvetrain geometry is not right on you are going to have problems. Guaranteed. Valve work also requires attention to detail. The kind of details we are talking about here are worn parts that appear to be okay but are not and need to be reconditioned or replaced. The best advice here is, "If in doubt, toss it out." Not being too particular about the condition of the valve stems, guides, keepers, retainers, springs, rocker arms and pushrods can lead to trouble. Not checking details like installed valve stem height, installed spring height, stem-to-guide clearance, seat width and contact, rocker arm alignment, and so on will get you every time. What is more, valve work often requires a certain amount of detective work. To fix a valve problem, you first have to figure out what caused it in the first place. Replacing a broken valve, for example, won't fix the problem if he underlying cause is misalignment between the valve guide and seat. Unless the misalignment is corrected, the new valve will fail too as flexing causes it to fatigue and break. Replacing a burned valve won't fix a compression problem if the underlying cause is a hot spot in the cylinder head. If the hot spot is not eliminated, the new valve will run hot and burn too. Replacing a worn guide by installing a new one, a liner or a valve with an oversize stem won't fix an oil consumption problem if guide wear is the result of excessive side scrubbing of the valve stem due to rocker arm misalignment. Unless the stem height is corrected, the guide repair won't last. That is why analyzing what caused a problem before you try to fix it is so important. Broken or burned valves as well as worn or loose guides, cracked or loose seats and similar valvetrain damage is often the end result of a chain reaction of events. One problem leads to another and eventually a valve failure. So replacing parts without understanding what made them fail is no fix at all. To avoid valve related problems down the road, do the following: 1. Analyze the amount of wear as well as wear patterns in the head and valvetrain components when the head is disassembled. A careful inspection should reveal any abnormal conditions or wear patterns that would indicate additional problems. 2. Inspect each and every component in the valvetrain and head so all worn or damaged parts can be identified and replaced or reconditioned. 3. Keep a close watch over production quality so the parts that are being reconditioned are done so correctly. 4. Pay attention to specs, critical dimensions and rocker arm geometry to assure proper reassembly.   ENGINE VALVE DEFECTS Many things can make a valve fail. Defective valves are one thing nobody talks much about, but it ranks as the number two cause of valve failures. Thermal and mechanical overstress is number one. One study that was conducted by a leading valve manufacturer found that as many as 1 out of every 5 (20.7%) valve failures resulted from defects in the valves themselves! This particular study was published over a decade ago, and even though the same basic alloys and manufacturing processes that were used then are still used today quality control has come a long ways. CNC (Computer Controlled Numeric) production machinery and statistical process control have done much to eliminate human error in the manufacturing process. But like any other mass produced component, defects occasionally slip through. So do not rule out bad valves as a possible cause of a premature valve failure. Defects include the presence of metallurgical impurities and inclusions in the metal that weaken the valve, forging defects that leave microscopic cracks, pores or separations in the metal that lead to breakage, faulty welds between stems and heads in two-piece valves that can allow the head to separate, faulty welds in hollow stem valves that can lead to breakage, improper heat treatments that fail to fully harden or anneal a valve resulting in rapid wear, machining errors that produce the wrong dimensions or surface finish which can cause all kinds of problems if not detected prior to installation, and poor adhesion of chrome plating that allows the protective plating to flake off the stem. The best way to make sure the new valves you are using are free from defects, therefore, is to (1) inspect the valves to make sure tolerances are within specs (stem diameter, stem taper, overall length, etc.) and there are no obvious defects (nicks, pits, hairline cracks, etc.), and (2) source your valves from a reliable supplier. One valve looks pretty much like another, so you cannot judge quality by appearances alone. A cheap price may be attractive, but if the valve does not hold up where is the savings? So don't take chances on poor quality valves from questionable suppliers that might end up costing you far more than what you saved on the valves themselves. Buy from a reputable supplier who stands behind their product. WHY ENGINE VALVES FAIL Any valve will eventually wear out if driven enough chilometri. But many valves call it quits long before they should because of burning or breakage. Let's talk about burning first. Exhaust valves are the ones most likely to burn because they run hotter than the intakes. The intake valves are cooled by the incoming air and fuel, and consequently operate at about 800 degrees F. Exhaust valves, on the other hand, receive little such cooling and are blasted by the hot combustion gases as they exit through the exhaust port. Exhaust valves run at 1200 to 1350 degrees F. on average, which makes them much more vulnerable to erosion and burning than intakes. The higher operating temperature requires a tougher alloy, so exhaust valves are usually made of stainless steel or have stainless steel heads (typically 21-2N or 21-4N alloy with a high chromium and nickel content). For heavy-duty gasoline and diesel applications where heat is even more of a problem, a tough Stellite facing (cobalt alloy) may be needed on the exhaust valve face to control wear. The intake and exhaust valves rely on physical contact with the valve seat and guide for cooling. About 75% of the combustion heat that is conducted away from the valve passes through the seat, so good seat contact is essential to prevent burning. The remaining 25% of the heat is dissipated up through the valve stem and out through the guides. Sodium filled hollow valve stems in heavy-duty applications are sometimes used to draw even more heat up through the stems to aid cooling. If the valve does not receive adequate cooling, it can overheat, burn and fail. The exhaust valve (the smaller of the two valves) in this engine has overheated and lost a chunk of metal, causing the cylinder to lose compression. Also note the failed exhaust valve is the same color as the intake valve. A "good" exhaust valve that is sealing and holding compression will usually have white or tan colored ash deposits on it. Anything that interferes with valve cooling or creates extra heat in the valve or head can lead to premature valve failure. A buildup of deposits on the valve face and seat can have an insulating effect that slows cooling and makes the valve run hot. So too can poor contact between the valve and seat if the seat is too narrow, nonconcentric or off-square. If deposits build up in one spot or flake off in another, it can allow leaks that create hot spots on the valve and result in "channeling" (grooves eroded or burned into the valve). Weak springs or insufficient valve lash can also prevent good valve-to-seat contact and allow excessive heat to build up in the valves. A loose seat or poorly fitting guide can also hinder heat transfer to the head and contribute to burning. Not paying attention to the installed valve height when doing a valve job can lead to burning. When valves and seats are ground or cut, the valves sit deeper in the head than before. This causes the stems to stick up higher which changes the rocker arm geometry and may lead to a loss of valvelash when the engine gets hot. Two engines where this particular problem has been turning up are the Ford 2300 OHC engine and the rear-wheel drive version of the Mitsubishi 2.6L (which has hydraulic lash adjusters). If the proper geometry cannot be restored by grinding the tips of the valve stems (no more than about .010 maximum or you run the risk of grinding through the case hardened layer), the seats should be replaced to correct installed height (an expensive fix but cheaper than a comeback). Another option is to install valves with slightly oversized heads (.030 in.) that ride higher on the seat to compensate for seat machining. Valve recession can cause the same kind of problem. As the seats wear away and the valves recede into the head, valvelash is lost. Eventually there is little or no lash left and the valve makes poor contact with the seat, overheats and burns. Valve recession tends to be more of a problem on older engines that lack hard valve seats and are used in heavy-duty truck, marine, agricultural or industrial applications. The cure here is to install hard seats. Stellite or hard faced valves may also be necessary if the valves show evidence of erosion. Cooling problems in the engine itself can lead to valve sticki…

Fonte: AA1Car.com