FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
During overhaul, a technician measures a crankshaft main journal and finds it worn beyond the maximum allowable service limit but otherwise structurally sound. The crankshaft is sent out and reground to the next standard undersize dimension. What additional step is required to properly complete this repair before the crankshaft can be reinstalled?
- ANo bearing change is required as long as the oil clearance is checked with Plastigage after installation
- BInstall new standard-size main bearings and shim the bearing saddle to compensate for the reduced journal diameter
- CInstall undersize main bearings matched to the reground journal diameter, as marked by the undersize designation
- DIncrease the oil pump relief valve setting to compensate for the reduced journal diameter
Show answer & explanationAnswer & explanation
Correct answer: C. Install undersize main bearings matched to the reground journal diameter, as marked by the undersize designation
When a crankshaft journal is reground undersize, standard bearings will no longer provide correct oil clearance because the journal diameter has been reduced. Undersize main bearings, manufactured with additional material and marked with the specific undersize designation (e.g., 0.010 under), must be installed to restore correct running clearance.
Why the other options are wrong
- A. Standard bearings on an undersized journal would produce excessive oil clearance; a bearing change is mandatory, not optional.
- B. Standard bearings with shims do not properly restore correct clearance and are not an approved method.
- D. Adjusting the relief valve does not correct improper bearing clearance from a reground journal.
Crankshaft Journal Regrinding
A repair process where a worn crankshaft journal is machined to a smaller standard undersize diameter, requiring matching undersize bearings to restore correct oil clearance.
- Undersize bearings are marked (e.g., 0.010 under) to match reground journals
- Only performed within manufacturer-approved undersize limits
- Oil clearance is verified with Plastigage or micrometer measurement after installation
Memory trick: Shrink the journal, shrink the bearing — they must always match in size!