ASE A1 Engine RepairEngine Block AssemblyHard
A technician is inspecting a piston for potential reuse. They notice that the piston skirt has collapsed, indicated by a smaller diameter at the bottom of the skirt compared to the top, and excessive clearance when measured against the cylinder bore. What is the MOST likely cause of this condition?
- AExcessive heat and insufficient cooling.
- BNormal wear from long-term engine operation.
- CInsufficient piston-to-cylinder wall clearance.
- DImproper piston ring installation.
Show answer & explanationAnswer & explanation
Correct answer: A. Excessive heat and insufficient cooling.
Piston skirt collapse is typically caused by excessive heat, often due to insufficient cooling or prolonged high-temperature operation. This excessive heat causes the piston material (aluminum alloy) to lose its structural integrity and deform, leading to the skirt compressing inward, reducing its diameter. This results in excessive piston-to-cylinder wall clearance and potential piston slap.
Why the other options are wrong
- B. While normal wear occurs, significant skirt collapse is usually indicative of an abnormal thermal event, not just standard wear.
- C. Insufficient piston-to-cylinder wall clearance would cause scuffing, seizing, or scoring, not collapse; it would make the piston too tight, not too loose due to a smaller skirt.
- D. Improper piston ring installation primarily affects sealing and oil control, not the structural integrity and collapse of the piston skirt.
Piston Skirt Collapse
Piston skirt collapse is the deformation and reduction in diameter of the piston skirt, typically caused by exposure to excessive heat.
- Results in excessive piston-to-cylinder wall clearance and can cause piston slap.
- Often a symptom of engine overheating, poor lubrication, or detonation.
- Requires piston replacement; cannot be repaired.
Memory trick: Collapsed skirt signals 'Hot Trouble' inside.