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For Trcuk Expert Only---Four Safety DON'TS about Truck Maintenance

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Date: Aug 24,2020 View:3410



1. Avoid installing incompatible new cylinder liners and pistons

When replacing the cylinder liner and piston, it is considered that the new cylinder liner and piston are standard parts, and they are interchangeable, and can be used after they are installed. In fact, the size of cylinder liner and piston has a certain tolerance range. If the largest size cylinder liner is fitted with the smallest size piston, the clearance will be too large, resulting in weak compression and difficulty in starting. Therefore, it is necessary to check the size grouping code of the standard cylinder liner and piston when reinstalling. The installed cylinder liner and piston must make the size grouping code of the standard piston and the size grouping code of the standard cylinder liner the same. Only in this way can the two be guaranteed. Has a standard fit clearance. In addition, when each cylinder is installed with liners and pistons of the same group code, attention should be paid to the inspection of the cylinder plug gap before installation. In order to ensure the assembly standard, a test should be carried out before installation to prevent fake and inferior products from being installed. 





2. Avoid not checking the plunger stroke margin

In the ironic test of the plunger fuel injection pump, many maintenance personnel did not pay attention to checking the stroke margin of the plunger. The so-called stroke margin of the plunger refers to the amount of movement that the plunger can continue to move upward after being pushed to the top dead center by the cam on the camshaft. After adjusting the fuel supply start time, the reason why the stroke margin is checked is because the stroke margin of the plunger is related to the wear of the plunger and sleeve. After the plunger and the sleeve are worn, the plunger has to move up for a while to start oil supply, which delays the start of oil supply. When the adjusting bolt is unscrewed or a thicker adjusting pad or gasket is used, the lowest position of the plunger moves up, so that the stroke margin of the plunger is reduced. Therefore, when repairing and debugging the fuel injection pump, the stroke margin should be checked first to determine whether the fuel injection pump is still allowed to be adjusted. During the inspection, the following different methods should be adopted according to the different structure of the fuel injection pump: a) Rotate the camshaft, push the plunger to the top dead center, remove the delivery valve and valve seat, and measure with a depth vernier. b) After the plunger is pushed to the top dead center, use a screwdriver to pry the spring seat of the plunger spring to raise the plunger to the highest point. Then insert a thickness gauge between the lower plane of the plunger and the tappet adjustment bolt to measure. The standard stroke margin of the plunger is about 1.5mm, and the limit stroke margin after wear should not be less than 0.5mm.




3. Avoid inaccurate cylinder clearance measurement

When measuring the cylinder clearance, it failed to measure in the direction of the piston skirt perpendicular to the piston pin hole, and measured in other directions. The structure of the aluminum alloy piston is characterized by a small top and a large bottom. It is a cone, and the partial cross-section is elliptical, so the cylinder clearance along the circumferential direction is not equal. When measuring, it is stipulated that the clearance in the direction of the major axis of the ellipse shall prevail, that is, the clearance of the piston skirt perpendicular to the direction of the piston pin hole is measured. This measurement is more convenient and accurate. In the reciprocating motion, the direction of the piston skirt perpendicular to the piston pin hole is greatly worn due to the side pressure. Therefore, when measuring the cylinder gap, the piston skirt should be perpendicular to the piston. The direction of the pin hole is measured.



4, Avoid heating the piston with open flame 

Since the piston and the piston pin have an interference fit, when installing the piston pin, the piston should be heated and expanded. At this time, some maintenance personnel will directly heat the piston on an open flame. This approach is very wrong, because If the thickness of each part of the piston is uneven, the degree of thermal expansion and contraction will be different. Open flame heating will cause uneven heating of the piston, which is easy to cause deformation; the surface of the piston will also adhere to carbon dust, reducing the service life of the piston. If the piston cools naturally 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 significantly shortened. When installing the piston pin, the piston can be evenly heated in hot oil to make it expand slowly. Never use an open flame to heat it directly.


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