The function of a motorcycle cylinder sleeve includes:

1. Protecting the cylinder: The cylinder sleeve is a component installed inside the engine cylinder to protect it from direct wear or corrosion, thereby extending the engine's lifespan.

2. Repairing wear: When the internal walls of the engine cylinder wear out or become damaged due to prolonged use, the cylinder sleeve serves as a repair method. By replacing the cylinder sleeve, the precision and sealing performance of the cylinder can be restored to ensure proper engine operation.

3. Improving sealing performance: After installation, the cylinder sleeve effectively enhances the sealing between the cylinder and the piston, reducing gas leakage and improving the engine's compression ratio and combustion efficiency.

4. Enhancing performance: Some high-performance cylinder sleeve designs can also modify the engine's displacement or enhance its performance characteristics, such as increasing power output or adjusting the engine's operating characteristics.

In summary, the motorcycle cylinder sleeve plays a crucial role in maintaining and repairing the engine, protecting the cylinder, and improving engine performance and reliability.