Mark on the crown of the piston to ensure correct installation.Pistons generally have some sort of mark on their crown, to indicate which way the piston should be reassembled in an engine.

Depending on whether the engine is 2-stroke or 4-stroke, the marks serve slightly different purposes.

In 4-stroke engines, generally speaking, the mark will point toward the front of the engine, or toward one particular side.

For example, in the Lifan 5 HP engine, the mark on the crown of the piston points toward the camshaft side of the engine.

The purpose of the mark is to ensure the piston is installed the correct way around, thus ensuring that the oil squirt hole in the connecting rod is pointing toward, and lubricating the major thrust side of the engine.

In 2-stroke engines there is a slight difference. Again, generally speaking, the mark on the piston points toward the exhaust port. This ensures that the piston ring end gaps never align with a port. Because the rings exert great radial forces (in order to achieve a gas-tight seal), if they were to uncover a port, the ring would be inclined to spring into that port. As the piston continues its movement, the end of the ring would be snapped off.

For that reason, 2-stroke pistons have small pins which positively locate the piston ring end gaps. Then, once we observe the mark on the crown of the piston, and reassemble the piston accordingly, the end gaps will never be in line with a port, and the rings will never be damaged.

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