When the piston works, the high speed reciprocating movement is done in the cylinder, and the relative speed of the cylinder wall can be as high as 1000 m/min, and it always bears the effect of high temperature and high pressure, and the factors such as heat dissipation and bad lubrication make the piston damage to varying degrees after a period of use. The common damage of piston is: crack, serious burning on top, ring groove has been trapezoid or side clearance exceeds the specified value. If the piston is damaged, the piston will not be repaired. When the above phenomenon is not available, the new piston ring can be used continuously. The piston and cylinder head and cylinder together constitute cylinder volume and combustion chamber. Under the action of high temperature and high pressure gas, the piston performs high-speed reciprocating linear motion in the cylinder, and completes the work cycle process of suction, compression, work and exhaust. The piston is subjected to the combined action of inertia force, gas pressure and lateral pressure due to its periodic change in speed. When the piston works in the cylinder, the maximum temperature of the gas can reach 2000 degrees C. The piston top also bears the effect of transient high temperature, and the temperature distribution is very uneven, thus causing serious heat load. At the same time, the working conditions of difficult lubrication are bad. The piston can be divided into 4 parts along the height direction from top to bottom: the above part of the first ring groove is called the top, which forms a part of the combustion chamber together with the cylinder; the piston ring groove area is called the ring groove, which is used to install the piston ring; the piston pin seat is called the pin seat and is used to install the plug pin; the lower part of the piston pin is called the skirt. It can guide the reciprocating motion of the piston and bear lateral pressure. The wear of piston ring groove usually occurs in the height direction, and the first piston ring groove wears most seriously. When the piston ring groove is worn, the side gap of the piston ring is increased. If the piston is not repaired or replaced in time, the engine oil and the cylinder pressure will decrease when the engine works. Piston ring backlash refers to the clearance between piston ring and piston ring groove in height direction. When measuring, put a new piston ring into the ring groove and measure the backlash of the ring with a feeler gauge. If the side gap is too small after the replacement of the new piston ring, the piston ring can be lapping on the fine sand cloth. If the side gap is too large, the ring groove is worn, the ring groove turning can be widened and replaced with the thickened piston ring, and the combined oil ring can be added above the piston ring, but the piston of the common engine is cheap, and the piston can generally be made of the piston. The ring is replaced with the piston. The contact between the piston and the cylinder wall is only in the skirt part, and is perpendicular to the piston pin axis, which determines the clearance between the piston and the cylinder. The piston skirt is subjected to high pressure gas, and the side force acting on the piston wall is tightly attached to the cylinder wall, resulting in friction and wear. For normal work, this part of the wear is usually 0.01 to 0.02 mm wear per 10 thousand km. Check the wear of the piston skirt. In the vertical direction of the piston pin, the diameter of the piston skirt is measured with a diameter micrometer. If the measured value exceeds the limit value, the whole piston should be replaced when the engine is overhauled. When the roundness or cylindricity of the piston skirt is less than the technical requirements, the original elliptical axis of the skirt stretches after the heat expansion, making the gap between the piston and the cylinder wall smaller, producing a bulging cylinder, pulling the cylinder and piston, and making the local high temperature to form melting wear. When bad lubrication or lubricating oil and combustible mixture contain abrasives, the piston and cylinder will be drawn. When the heat is serious, the melting will be caused by the local high temperature. If the piston is pulled out, only a slight straight line stripe is formed, it can be used to repair the light burrs. The piston pin hole is worn by the piston pin and the seat hole. The gap between the piston pin and the seat hole is increased, and the abnormal noise is produced during operation. The inner diameter dial gauge and inner diameter micrometer are measured in the direction and horizontal direction of the pin hole, and the roundness and wear volume are calculated. According to the standard value, determine whether the piston pin hole wear is out of limit. The cause of the wear of the piston pin hole is caused by the reciprocating impact of the piston pin and natural wear, forming an ellipse. When the piston is assembled, the piston is unheated, and the pin hole is bruised. The gap between the hole and the pin is large and the impact wear is serious. Piston pin hole wear is mostly repaired by reaming. The piston scraping is generally marked, and the mildly scraped piston, if it does not affect the clearance between the cylinder and the cylinder, is allowed to continue to use after the grinding of fine sand cloth. The serious scraping of the piston must be replaced, and the cause of the failure is ascertained according to the following conditions. (1) scratches on both sides of the piston skirt are usually caused by a small gap between the new piston and the cylinder. (2) one side of the vertical piston pin of the piston skirt is scratched in the direction of the vertical piston pin. Usually, the idle speed of the idle speed is too low to make the cylinder wall lubricated or the engine work for a long time, and the piston is scratched on one side of the side with a larger side pressure. (3) scratches on both sides of the piston skirt part, usually due to the tight fit between the piston pin and the seat hole, and the excessive expansion along the piston pin after being heated. (4) too much clearance between the piston and the cylinder will cause the upper part of the first ring groove to be worn or scratched. (5) the location of the piston scratch occurs on the side of the piston pin seat, usually caused by the deformation of the connecting rod. (6) wear and tear of the piston skirt's short axis. The top of the piston is directly subjected to high temperature and high pressure gas. Because the ignition timing is incorrect, the fuel composition does not meet the requirements and so on, it will produce detonation, or because the engine is overloaded for a long time, all will cause the piston to burn the top phenomenon. There are loose holes in the top of the piston, and some of them are localized. The top surface ablation of the piston will lead to the high temperature gas channeling into the crankcase, accelerating the oxidation and deterioration of the lubricating oil, the deterioration of the cylinder seal, the reduction of compression ratio, the deterioration of the fuel combustion process, the power and economy of the engine, and the breakage of the piston, the damage of the cylinder liner, the connecting rod, the crankshaft and the body.