1、 Inaccurate measurement of cylinder clearance
When measuring the cylinder clearance, it cannot be measured in the direction that the piston skirt is perpendicular to the piston pin hole, but in other directions.
The structural feature of aluminum alloy piston is that the upper part is small and the lower part is large, which is a cone, and the skirt section is oval, so the cylinder clearance along the circumferential direction is not equal.
When measuring, the clearance in the direction of the long axis of the ellipse shall prevail, that is, the clearance of the piston skirt perpendicular to the piston pin hole shall be measured. 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 greatly worn due to the action of lateral pressure. Therefore, when measuring the cylinder clearance, it should be measured in the direction of the piston skirt perpendicular to the piston pin hole.
2、 Piston open flame heating
Due to the interference fit between the piston and the piston pin, when installing the piston pin, the piston should be heated and expanded first. At this time, some maintenance personnel will directly heat the piston on an open fire, which is very wrong, because the thickness of each part of the piston is uneven, the degree of thermal expansion and cold contraction will be different, and the open fire heating will cause uneven heating of the piston, which is easy to cause deformation;
Carbon ash will also be attached to the piston surface to reduce the service life of the piston. If the piston cools naturally after reaching a certain temperature, its metallographic structure will be damaged, the wear resistance will be greatly reduced, and its service life will be significantly shortened. When installing the piston pin, heat the piston evenly in hot oil to make it expand slowly. Do not heat it directly with open fire.
3、 Grind the bearing bush with emery cloth
For some inexperienced repairmen, scraping is a difficult job. Because the scraping technology is difficult to master, it is difficult for the bearing bush to meet the technical requirements. Therefore, when replacing the bearing bush, someone polished it with emery cloth to increase the contact area between the bearing bush and the crankshaft.
This method is extremely undesirable in actual maintenance, because the abrasive particles on the abrasive cloth are hard, while the bearing bush alloy is soft. In this way, the abrasive particles can easily be embedded into the alloy during grinding, which will accelerate the wear of the journal and shorten the service life of the crankshaft when the diesel engine is working.
