ASE A1 Engine RepairEngine Block AssemblyMedium
A technician is inspecting a set of pistons during an engine rebuild. They notice a shiny, polished appearance on the thrust side of the piston skirts, accompanied by some scuffing. Which of the following is the MOST likely cause?
- AExcessive piston ring end gap
- BOver-tightened connecting rod bolts
- CIncorrect piston pin offset
- DInsufficient piston-to-cylinder wall clearance
Show answer & explanationAnswer & explanation
Correct answer: D. Insufficient piston-to-cylinder wall clearance
Insufficient piston-to-cylinder wall clearance causes the piston to expand and make excessive contact with the cylinder wall, especially on the thrust side, leading to a polished appearance and scuffing due to friction and heat.
Why the other options are wrong
- A. Excessive piston ring end gap primarily affects compression and oil control, not piston skirt wear patterns.
- B. Over-tightened connecting rod bolts would typically lead to bearing damage or rod distortion, not piston skirt scuffing.
- C. Incorrect piston pin offset can affect piston thrust and noise (piston slap) but is less likely to be the primary cause of widespread polishing and scuffing on the skirt.
Piston Skirt Scuffing/Polishing
Shiny, polished areas and scuff marks on the piston skirt, particularly on the thrust side, indicating excessive friction due to insufficient piston-to-cylinder wall clearance.
- Caused by inadequate clearance
- Leads to increased friction and heat
- Often on the thrust side of the piston
Memory trick: Tight skirt, polished friction, bad condition.