FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
A technician removes a main bearing shell from a reciprocating engine crankshaft and notices bright, copper-colored streaks showing through the bearing's babbitt overlay surface. What does this indicate?
- AThe bearing has been improperly torqued and needs re-installation only
- BNormal wear pattern typical of a properly broken-in bearing
- CExcessive oil pressure has polished the bearing surface
- DThe overlay material has worn through to the underlying bearing alloy and the bearing must be replaced
Show answer & explanationAnswer & explanation
Correct answer: D. The overlay material has worn through to the underlying bearing alloy and the bearing must be replaced
Bearing shells are manufactured with a thin overlay bonded to a stronger backing alloy. Visible copper or bronze-colored streaks through the overlay indicate the softer surface layer has worn through, exposing the harder base material, which signals the bearing has reached the end of its useful life and must be replaced.
Why the other options are wrong
- A. Torque issues cause different symptoms such as spinning or fretting, not overlay breakthrough.
- B. Uniform light wear is normal, but exposed base material is not.
- C. Oil pressure does not cause color changes in bearing overlay material.
Bearing Overlay Wear
Wear-through of the soft overlay plating on a main or connecting rod bearing shell, exposing the harder base alloy underneath, indicating bearing replacement is required.
- Bearings have a soft overlay bonded to a stronger backing material
- Copper/bronze color showing through overlay = base metal exposed
- Indicates bearing has reached end of service life
- Often caused by oil contamination, misalignment, or extended service
Memory trick: See copper shine? Time's up — retire the bearing.