Selection of Piston and Piston Ring

1.Selection of piston

The repair size of the piston is compatible with the repair size of the cylinder, and it is divided into standard size and enlarged size.

When repairing the engine, the piston should be selected according to the wear condition of the cylinder, and the repair size of the cylinder should be determined.

When replacing the piston, the following points should be paid attention to:
1) On one engine, the same brand, the same grade, and the same group of pistons should be selected to make the material, performance, quality and size consistent.
2) The diameter difference of the pistons in the same group should not exceed 0.025mm, and the quality difference should generally not exceed 8g. Otherwise, re-select.
3) The diameter difference between the piston head and the skirt, the roundness of the skirt and the cylindricity tolerance should meet the technical requirements.

2. Selection of piston rings

The piston ring should have the repair size corresponding to the piston and cylinder. When the engine is overhauled, the piston ring with the same repair size as the cylinder and piston should be selected. At present, some accessories manufacturers have supplied pistons, piston rings and piston pins as a complete set according to different specifications, which not only facilitates the repair, but also improves the repair quality.

In order to ensure the good cooperation between the piston ring and the piston ring groove and the cylinder, the following inspections should be done in the selection of the piston ring:

1) End gap

The piston ring has an end gap to prevent the piston from being stuck in the cylinder after it is heated and expanded. When checking the end clearance of the piston ring, put the piston ring into the cylinder flat, push it flat with the top of the piston, and then measure the clearance at the opening with a thickness gauge. When the end clearance is too large, the piston ring should be re-selected: Hours, the end of the ring mouth should be repaired. When filing and repairing, attention should be paid to the smoothness of the ring mouth, and the burrs should be removed from the outer mouth after filing to prevent the sharp ring mouth from pulling the cylinder liner.

2) Backlash

Generally, the first ring is 0.05~0.09mm, and the other rings are 0.03~0.07mm. If the side clearance is too large, the sealing effect of the piston ring will be affected; if the side clearance is too small, the piston ring may be stuck in the ring groove after thermal expansion. When checking the side clearance of the piston ring, put the piston ring into the ring groove and measure with a thickness gauge.

3) Backlash

The backlash is the gap between the inner circular surface of the piston ring and the bottom of the ring groove after the piston ring is installed on the piston and placed in the cylinder. Therefore, it is difficult to measure the clearance directly, and the backlash is usually expressed by the difference between the groove depth and the ring thickness. Generally, it should be 0~0.75mm lower than the groove bank. If it is too low, air leakage and oil channeling will occur, so it should be re-selected.

The empirical method for checking the side clearance and backlash of the piston ring is: put the piston ring into the groove of the piston ring, and it can rotate freely without feeling of looseness. It is appropriate that the ring is lower than the groove bank. For the assembly clearance of piston rings of common models, please refer to the standard parameter table.

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