ASE A1 Engine RepairEngine Block AssemblyHard
A technician is measuring a crankshaft main journal for wear. The service manual specifies a standard diameter of 2.500 inches with a wear limit of 0.001 inch. The technician measures the journal at several points and finds readings of 2.499 inches, 2.4985 inches, and 2.499 inches. What is the MOST appropriate action for the technician?
- APolish the crankshaft journal and reuse it with standard bearings.
- BMachine the crankshaft to the next undersize and use undersized bearings.
- CReplace the crankshaft, as it is beyond repair.
- DInstall new standard-sized bearings, as the journal is still within tolerance.
Show answer & explanationAnswer & explanation
Correct answer: B. Machine the crankshaft to the next undersize and use undersized bearings.
The wear limit is 0.001 inch from the standard 2.500 inches, meaning the minimum acceptable diameter is 2.499 inches. The technician measured 2.4985 inches at one point, which is below the wear limit. Therefore, the crankshaft journal needs to be machined to the next undersize (e.g., 0.010 or 0.020 undersize) and paired with corresponding undersized bearings.
Why the other options are wrong
- A. Polishing would not restore the diameter to within limits; it only smooths the surface.
- C. Replacing the crankshaft is an option, but machining to undersize is a common and cost-effective repair for journals worn within typical undersize ranges.
- D. The journal is NOT within tolerance, as 2.4985 inches is below the 2.499-inch wear limit.
Crankshaft Undersize Machining
Crankshaft undersize machining is the process of regrinding crankshaft journals to a smaller, precise diameter when they are worn beyond service limits, requiring the use of undersized bearings.
- Common repair for worn crankshafts, extending their service life.
- Requires matching undersized bearings (e.g., -0.010, -0.020 inches).
- Must maintain proper journal concentricity, taper, and runout after machining.
Memory trick: Worn journals need an 'Under-Size' solution.