Valve Wear.
The different types of wear that take place on valves are abrasive, adhesive, corrosive and compressive fatigue.
ABRASIVE WEAR - On a valve that has abrasive wear, the valve’s surface exhibits a scratched appearance from hard particles digging into the valve as it moved across a surface. A possible source can be a poor air cleaner allowing grit from the air to get into the engine. Another likely sources is worn parts. These worn engine parts contain compounds (carbides) of carbon and metals such as iron, chromium, tungsten, These carbides are present to resist wear, but they can also cause it. The carbides are particles within the structure of the metal and as wear increase, the carbides are released due to the softer material being worn away around them. When released they become grit, which produces abrasive wear.
ADHESIVE WEAR - A significant aspect about a valve that has adhesive wear is that at the contact point where it touches another metal surface must be very hot in order for the micro welding to take place. Adhesive wear is microscopic welding, the heat produced at the contact point is very high almost near the melting point of the two metals touching each other. The heat comes from the stress of contact, not from the temperature of the environment. A certain amount of the temperature is from the exhaust gas temperatures, but adhesive wear can be as detrimental on an intake valve which is operates a lot cooler.
Tiny points of metal are sticking up on the valve seat, due to the fact that nothing is perfectly smooth and contacts similar points on the valve head as seating of the valve continues, these points are under very high stress because they are initially carrying all the load. The rubbing of these high points over one another is what creates the high surface temperature each time the valve seats. The rough surface is the result of the microscopic welds being torn apart as the valve and seat separate.
CORROSIVE WEAR - This is not as easy to identify as abrasive and adhesive. That is due to the reason that any products produced by the corrosion of a surface are usually carried away when the two parts come together which breaks them loose and they are then swept away by the exhaust gas or air stream. If pitting is formed on the worn surface it can be from corrosion.
COMPRESSIVE FATIGUE WEAR - When the tip of a valve wears it is caused by compressive fatigue. It can be identified as an irregularly pitted pattern on the tip of the valve. On Suzuki GS and EFE engines the rocker arm adjustment screw contacts the valve tip repeatedly and exerts a force beyond the compressive strength of the metal. A portion of the impact of the valve tip directly under the adjustments screw contact moves straight down, as it goes deeper in the metal of the valve tip it moves sideways. That sideways motion is in tension rather than compression and repetitions of this can cause a microscopic crack to form beneath the surface. As more of these micro cracks are formed, they allow a small chunk to break away forming a pit on the tip surface.
AVOIDING WEAR – Hi quality components, proper fuels, lubes, and maintenance are all necessary to realize the best durability and performance. The component factors that are essential to reducing wear are :
HARDNESS is the most important single material factor. It is a measure of both strength and of wear resistance.
MICRO FINISH if the metal is smoother, then the load is spread out more and each point is not as stressed.
CORROSION if parts of an engine are exposed to abnormal corrosives they will corrode quickly. You can't do a whole lot to avoid adhesive wear.
COMPRESSIVE FATIGUE can reduced by not exceeding an engine’s RPM redline which can create a lot of extra load on the valve tip and accelerate wear.