A common issue with the Alto mini car is the occurrence of a rhythmic, metallic knocking sound from the lower part of the engine. The noise is heavy and persistent, resembling a "clunking" sound, and can cause noticeable vibrations in the vehicle body when severe.

During testing and analysis, it was found that the abnormal noise originated from the lower section of the engine. The sound becomes more pronounced during rapid acceleration or deceleration, and it remains consistent regardless of the engine's temperature. This suggests a potential problem with the crankshaft bearings.

Typically, the rear main bearing produces a dull and heavy sound, while the front bearing tends to be lighter and more brittle. However, in this case, the noise was unusually loud and rhythmic, indicating a possible fault in the crankshaft bearings.

The causes of abnormal noise in the crankshaft bearings may include:

• Excessive radial clearance in the main bearing
• Loose main bearing cap bolts
• Bent or deformed crankshaft
• Burned main bearing shells
• Loose or cracked main bearing bushings
• Excessive wear on the thrust washer
• Improperly smooth main bearing surfaces

If the issue stems from excessive wear on the axial thrust washer, the resulting noise may not be rhythmic but rather irregular due to increased axial play, causing the crankshaft to tilt slightly. In such cases, the noise tends to be more noticeable during cold starts when the engine is at a lower temperature.

When the abnormal noise occurs, the cylinder head of the affected cylinder may vibrate in sync with the sound. Testing by cutting off ignition in a single cylinder does not significantly reduce the noise. However, if two adjacent cylinders are disabled together, the noise may decrease or become less noticeable, indicating a problem with the main bearing between those two cylinders.

Upon removing the crankshaft, it was discovered that the axial thrust washer had worn excessively. After replacing it with a new one and reassembling the engine, the abnormal noise was resolved, and the vehicle performed normally during testing.

Understanding the root cause of such issues is crucial for proper diagnosis and repair. Regular maintenance and inspection of the crankshaft bearings can prevent serious mechanical failures and ensure smoother engine operation.

Related Bearing Knowledge:

Defects in rolling bearings often manifest through oscillations in the anti-friction layer of sliding bearings, which can lead to vibrations in NTN bearings. Proper installation and maintenance are essential for optimal performance. For more information, refer to "The Correct Device Method Collection" by KOYO Rolling Bearings and explore the concept of "Quality Misunderstanding" in bearing technology.

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