As is well known, the piston ring is one of the important parts of the engine's moving friction pair. It moves up and down reciprocally in the ring groove along with the rise and fall of the piston.

If the pressure of the gas is not taken into account, under normal circumstances, when the piston rises, the piston rings should be in contact with the lower side of the ring groove.

When the piston moves down, the piston rings should be in contact with the upper side of the ring groove.

When the piston is at the top and bottom dead centers, its contact surface should alternate from one side to the other.

However, during the combustion stroke, due to the effect of the combustion gas pressure, even when the piston is moving down, the piston rings are still pressed against the lower side of the ring groove. Therefore, in a four-stroke engine, except for the intake stroke, in each stroke of compression, expansion and exhaust, as well as in each stroke of a two-stroke engine, the piston rings are mostly in contact with the lower side of the ring groove.

From the above, it can be seen that during the up and down movement of the piston ring along with the piston, except for the suction stroke when the piston ring comes into contact with the upper plane of the ring groove, the other three strokes all come into contact with the lower side of the ring groove.

Therefore, if the flatness of the lower side of the piston ring or the lower side of the ring groove exceeds the tolerance, the gas in the combustion chamber will leak out through this channel. According to relevant data, if the contact area between the lower end face of the piston ring and the ring groove is reduced from 100% to 90%, the air leakage of the piston ring will increase by approximately five times accordingly, resulting in an oil leakage and leakage fault.

For this reason, it is recommended that when you conduct the inspection, you should hold a magnifying glass and carefully examine the contact marks between the lower end face of the ring and the piston ring groove. If the contact marks on the lower end face of the piston ring are good, but the contact marks in the piston ring groove are uneven, the piston part should be replaced. The piston rings can also be placed on a platform with a feeler gauge to measure the flatness of the rings (when the ring diameter is less than 100mm, its deflection should not exceed 0.03mm). Piston rings with deflection exceeding the usage limit value should be replaced.

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