FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
During overhaul, a technician installs a new plain bearing insert into a connecting rod housing bore. Before the rod cap is torqued, the bearing half protrudes slightly above the parting surface because its circumference is slightly greater than the housing bore. This designed characteristic is known as bearing crush, and its main purpose is to:
- AEnsure the bearing seats firmly against the housing bore and prevents bearing rotation in service
- BProvide a reservoir for extra oil at the parting line to reduce oil consumption
- CAllow the bearing to expand outward and increase running clearance as it wears
- DCompensate for taper in the crankshaft journal so oil clearance remains constant
Show answer & explanationAnswer & explanation
Correct answer: A. Ensure the bearing seats firmly against the housing bore and prevents bearing rotation in service
Bearing crush creates a slight interference fit so that when the cap bolts are torqued, the bearing shell is forced tightly against the housing bore wall over its entire surface. This ensures good heat transfer, prevents the bearing from spinning (rotating) in its bore, and keeps the shell seated under operating loads.
Why the other options are wrong
- B. Bearings are not designed to trap oil pools at the parting line.
- C. Crush does not increase clearance over time; that would be abnormal wear, not a design feature.
- D. Taper compensation is unrelated to crush; it is addressed by journal grinding tolerances.
Bearing Crush
A slight excess in bearing shell circumference over the housing bore that creates radial pressure, holding the bearing firmly in place once the cap is torqued.
- Prevents bearing rotation (spinning) in the housing
- Ensures full metal-to-metal contact for heat transfer
- Distinct from bearing spread, which holds the shell in place before final torque
Memory trick: Crush it tight so it won't spin in flight!