ASE A1 Engine RepairEngine Block AssemblyHard
During engine disassembly, a technician notices that the connecting rod bearing shells have a dull, dark gray appearance with some localized flaking. The crankshaft journal appears scored. What is the MOST likely cause of this bearing failure?
- AContamination by abrasive particles in the oil.
- BLack of proper assembly lubrication.
- COil starvation due to a clogged oil passage.
- DExcessive bearing clearance.
Show answer & explanationAnswer & explanation
Correct answer: A. Contamination by abrasive particles in the oil.
A dull, dark gray appearance with localized flaking on the bearing shells, combined with a scored crankshaft journal, is characteristic of bearing failure due to contamination by abrasive particles. These particles embed in the soft bearing material, then abrade the harder journal surface.
Why the other options are wrong
- B. Lack of assembly lube would cause immediate seizure or severe damage on initial startup, not typically flaking and scoring after operation.
- C. Oil starvation typically results in extreme heat, discoloration (blueing), and complete melting or welding of the bearing to the journal, not primarily flaking and scoring from embedded particles.
- D. Excessive bearing clearance typically causes a knocking noise and oil pressure drop, with bearing wear showing as uniform erosion or fatigue, not usually flaking and scoring from abrasives.
Abrasive Contamination Bearing Failure
A type of bearing failure caused by hard foreign particles (e.g., dirt, metal shavings, carbon) entering the oil supply, embedding in the bearing material, and then scoring the crankshaft journal.
- Characterized by dull, dark gray bearing surfaces with embedded particles.
- Leads to scoring or grooving of the crankshaft journal.
- Often results in localized flaking or erosion of the bearing material.
Memory trick: Flakes and scores, from grit in the pores, oil was dirty, now engine deplores.