1. Inaccurate measurement of cylinder clearance
When measuring the cylinder clearance, it could not be measured in the direction of the piston skirt perpendicular to the piston pin hole, but in other directions.
The structural characteristics of aluminum alloy pistons are small up and down, a cone, and the skirt section is oval, so the cylinder clearance in the circumferential direction is not equal.
When measuring, the gap in the direction of the long axis of the ellipse is specified, that is, the gap of the piston skirt is measured perpendicular to the direction of the piston pin hole. This measurement is more convenient and accurate, and in the reciprocating motion, the direction of the piston skirt perpendicular to the piston pin hole is worn due to the lateral pressure. Therefore, when measuring the cylinder clearance, the piston skirt should be perpendicular to the piston Measure the direction of the pin hole.
Second, piston open flame heating
Since the piston and the piston pin have an interference fit, the piston should be heated and expanded when installing the piston pin. At this time, some maintenance personnel will directly heat the piston on an open flame. This is very wrong, because The thickness of each part of the piston is uneven, and the degree of thermal expansion and contraction will be different. Open flame heating will make the piston unevenly heated and easily cause deformation;
The surface of the piston will also be attached with carbon ash, reducing the service life of the piston. If the piston naturally cools after reaching a certain temperature, its metallographic structure will be damaged and the wear resistance will be greatly reduced, and its service life will be shortened significantly. When installing the piston pin, the piston can be heated evenly in hot oil to slowly expand it, and it should not be directly heated with an open flame.
3. Polishing the bearing bush with emery cloth
For some inexperienced repairmen, scraping the tiles is a difficult task. Because the technique of scraping the tiles is difficult to master, it is difficult for the bush to meet the technical requirements. For this reason, when replacing the bush, some The contact area of the crankshaft is polished with emery cloth instead of scraping shoes.
This method is extremely undesirable in actual maintenance, because the abrasive particles on the abrasive cloth are harder, but the bearing alloy is softer, so that the sand particles are easily embedded in the alloy when grinding, and the wear of the journal will accelerate when the diesel engine is working. Shorten the service life of the crankshaft.
