ASE A1 Engine RepairEngine Block AssemblyMedium
During engine disassembly, a technician notices that the main bearings have a shiny, polished appearance with no signs of scoring or pitting, but they are significantly worn thin. Which of the following is the MOST likely cause of this type of wear?
- ANormal wear from high mileage.
- BInsufficient lubrication due to low oil level.
- COver-torqued main bearing caps.
- DExcessive clearance between the journal and bearing.
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive clearance between the journal and bearing.
A shiny, polished appearance with significant thinning of bearings, without scoring or pitting, suggests 'fatigue' or 'normal' wear exacerbated by excessive clearance. Excessive clearance allows for increased movement and impact, leading to faster wear and polishing.
Why the other options are wrong
- A. While high mileage causes wear, this specific 'polished and thin' appearance points more strongly to excessive clearance as a primary accelerator.
- B. Insufficient lubrication typically results in scoring, overheating, and eventual seizure, not just polished thinning.
- C. Over-torqued caps would reduce clearance, leading to overheating, scoring, and premature failure, not polished thinning.
Bearing Fatigue/Polished Wear
Bearing fatigue wear, appearing as polished or thinly worn surfaces without scoring, often results from excessive clearance allowing impact.
- Indicates excessive bearing clearance.
- Shaft 'hammers' the bearing surface.
- Accelerated by high mileage and lack of oil film strength.
Memory trick: Polished and thin? Too much space within!