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How to replace the piston ring
The piston connecting rod group plays a critical role in the engine's operation. Among its components, the piston rings are subjected to extreme conditions—high temperature, high pressure, and high-speed motion—while often operating under poor lubrication. As a result, these rings experience rapid wear and potential failure. Over time, this wear leads to a reduction in ring elasticity, an increase in end gap and back clearance, and a loss of cylinder sealing. This can cause oil and air leakage, which ultimately reduces engine power and fuel efficiency. Therefore, maintaining the proper condition of piston rings is essential for optimal engine performance. When the maximum cylindrical error of the cylinder reaches 0.09–0.11 mm, it is recommended to replace the piston rings during engine overhaul.
First, when selecting new piston rings, they must be carefully inspected and tested to ensure they meet specific quality standards. The working surface of the ring should be free from nicks, scratches, or flaking, and the outer cylindrical surface finish must be appropriate. Light leakage around the ring’s circumference should not exceed two points, with the total central angle of light leakage not exceeding 45 degrees. The gap between the two sides of the gap should not be more than 30 degrees, and the maximum gap at any light leakage point should not exceed 0.03 mm. Additionally, the warping of the ring should not exceed 0.02 mm, and the standard sinking depth in the groove should not go beyond 0.15–0.25 mm. Finally, the end play, back clearance, and elasticity of the ring must comply with manufacturer specifications.
Next, the grinding process is crucial if the end play or back clearance of the piston ring does not meet the required standards. For example, if the end gap is too large, the sealing capability of the cylinder decreases, and the piston ring may need to be replaced. If the end gap is too small, thermal expansion could lead to seizing, so the ring mouth can be adjusted by trimming. Care must be taken to keep the end face flat and properly adjust the edge amount. Chamfering the outer end of the ring helps prevent damage to the cylinder wall.
When the back clearance is too large, air and oil leakage can occur, requiring replacement. If the clearance is too small, the ring might get stuck in the groove. In such cases, the upper and lower surfaces of the ring groove can be machined, or the piston ring itself can be ground using a special machine or 0# sandpaper.
During assembly, piston rings are fragile and should be handled with care. Special fixtures are ideal for installation, but if unavailable, the ring can be opened slightly with the thumbs while protecting the ring’s edges with the other fingers. Each ring must be installed according to its design and direction. For example, twisted rings should be oriented correctly: inner cut and chamfered rings should have their notches facing upward, while outer cut and chamfered rings should have notches downward. The first ring groove usually has a chamfered edge, while the second and third grooves use the outer groove and chamfer.
Some chrome-plated rings are designed for the first groove, and conical rings should be installed with the marked side up. To prevent gas leakage, the openings of each ring should be staggered. With four rings, each opening should be 180° apart; with three rings, 120° apart. Also, each ring’s opening should be 45° away from the piston pin centerline. After installation, applying lubricant to the rings ensures smooth operation.
In summary, selecting, grinding, and assembling piston rings require attention to detail and adherence to specific guidelines. Proper replacement and maintenance of piston rings are essential to ensure the engine runs efficiently and reliably. (Zhao Zhishui, Agricultural Mechanization Technology School, Xingtai City, Hebei Province)