ASE A1 Engine RepairEngine Block AssemblyHard
A technician measures a crankshaft main journal diameter as 2.498 inches. The service manual specifies a main bearing clearance of 0.001 to 0.003 inches. If the main bearing bore in the block is 2.502 inches, what size main bearing is required to achieve the correct clearance?
- AOversize (e.g., +0.010 inch)
- BIt is impossible to determine without bearing shell thickness.
- CStandard size
- DUndersize (e.g., -0.010 inch)
Show answer & explanationAnswer & explanation
Correct answer: C. Standard size
The maximum allowable bearing shell thickness for a standard bearing can be calculated as (Main Bearing Bore - Crankshaft Main Journal Diameter - Minimum Clearance) / 2 = (2.502 - 2.498 - 0.001) / 2 = 0.003 / 2 = 0.0015 inches. Since this is a typical thickness for a standard bearing, and the resulting clearance (2.502 - 2.498 - (2 * 0.0015)) = 0.001 inch falls within the specification, a standard size bearing is appropriate.
Why the other options are wrong
- A. Oversize bearings are used when the main bearing bore in the block has been machined larger.
- B. Bearing shell thickness is implied by the bearing size designation (standard, undersize, oversize) and the desired clearance.
- D. Undersize bearings are used when the crankshaft journal has been ground down (smaller diameter).
Engine Bearing Clearance
The small space between a journal (crankshaft or connecting rod) and its bearing, filled with a film of oil, which is critical for lubrication and preventing metal-to-metal contact.
- Measured as the difference between the bearing bore (with bearing installed) and the journal diameter.
- Too little clearance causes friction and overheating; too much causes oil pressure loss and knocking.
- Achieved by selecting appropriate bearing sizes (standard, undersize, oversize) after measuring components.
Memory trick: Journal, bore, and clearance, calculate the shell, then choose with care.