ASE A1 Engine RepairEngine Block AssemblyHard
A technician is inspecting a set of main bearings during an engine overhaul. They notice that the bearings show wear patterns that are heaviest at the edges and lighter in the center. What is the MOST likely cause of this wear pattern?
- AContaminated engine oil.
- BExcessive crankshaft end play.
- CMain bearing housing bore distortion (out-of-round or taper).
- DInsufficient oil pressure.
Show answer & explanationAnswer & explanation
Correct answer: C. Main bearing housing bore distortion (out-of-round or taper).
Wear patterns heaviest at the edges of a main bearing, and lighter in the center, are indicative of main bearing housing bore distortion. This distortion (out-of-round or taper) causes the bearing to not conform perfectly to the crankshaft journal, leading to concentrated pressure and wear at the edges rather than even distribution across the bearing surface.
Why the other options are wrong
- A. Contaminated engine oil usually causes abrasive wear, characterized by scratches or embedded particles, not specific edge wear patterns.
- B. Excessive crankshaft end play would primarily affect the thrust bearing, causing wear on its thrust surfaces, not necessarily the journal bearing edges.
- D. Insufficient oil pressure typically causes even wear across the bearing surface, or complete failure due to lack of lubrication.
Main Bearing Edge Wear
Main bearing edge wear, where wear is concentrated at the outer edges of the bearing shell, is a sign of main bearing housing bore distortion.
- Indicates the bearing bore is not perfectly round or straight.
- Can be caused by improper torquing of main caps, block overheating, or manufacturing defects.
- Requires machining (line boring/honing) of the main bearing bores to correct.
Memory trick: Edge wear means 'Distorted Home' for the bearing.