Engine cylinder head is one of the most important parts in the engine. It has high precision requirements and complex processing technology. The processing quality directly affects the overall performance and quality of the engine. Therefore, the processing of engine cylinder head is particularly important, and the processing of the key parts of valve seat ring and guide hole is the most important. The intake and exhaust valve seat rings of an engine are important working parts to control the gas intake and exhaust emission. They will be subjected to the erosion of air flow and the impact and friction of valves under high temperature. The working conditions are very bad: during normal operation, the valve seat rings are in the high temperature of (600 ~ 800) ℃ for a long time, High temperature gas corrosion and parts deformation will aggravate the damage of valve guide and seat ring cone, resulting in the valve seal is not tight, a lot of energy is wasted with the discharge of high temperature gas, thus greatly reducing the power of the engine. Therefore, the valve seat ring and guide hole should have good high temperature wear resistance, corrosion resistance, heat transfer, high temperature strength, high temperature creep resistance and thermal expansion coefficient matching with the cylinder head. At the same time, if the center of the valve and the center of the valve seat ring deviate too much when the engine is working, the wear of the valve and the guide hole will be accelerated while the engine power decreases and the fuel consumption increases. Therefore, the machining accuracy of the valve seat ring and the guide hole, especially the runout of the guide hole on the working cone of the valve seat ring, is strictly limited by the tolerance. The machining accuracy of valve seat ring and guide hole of engine cylinder head is generally required as follows: the concentricity tolerance of gasoline engine is required to be (0.015 ~ 0.025) mm; However, for diesel engine, it is only (0.01 ~ 0.015) mm, the cone runout is (0.03 ~ 0.05) mm, and the hole accuracy grade is generally H7.