How is connecting rod and main bearing clearance measured, and what constitutes acceptable limits?

How is connecting rod and main bearing clearance measured, and what constitutes acceptable limits?

• A. Use plastigauge or micrometers per service data; compare to manufacturer limits.
• B. Visual inspection only; any apparent wear is acceptable.
• C. Measure with a standard ruler; any gap is acceptable.
• D. Use a dial indicator only with no reference data.

Answer: (A)

Measuring bearing clearance hinges on determining the oil gap between the bearing surface and the crankshaft journal, since that gap governs lubrication and wear. The reliable method is to use plastigauge or micrometers and compare the result to the manufacturer’s service limits. Plastigauge provides a direct clearance reading when you torque the cap to spec and then read the planted strip’s width against a scale; it shows the actual oil clearance in the assembled condition. Using micrometers, you can measure the crank journal diameter and the bearing shell thickness (or inner diameter) and compute the clearance, but you must reference the exact bearing size and journal dimensions from the service data. Acceptable limits come from the engine’s specification data; if the clearance is outside those limits, you select the proper bearing size or rework as needed. Visual inspection or using a ruler won’t give a precise or reliable clearance, and a dial indicator alone without reference data cannot determine if the clearance is within spec.

Measuring bearing clearance hinges on determining the oil gap between the bearing surface and the crankshaft journal, since that gap governs lubrication and wear. The reliable method is to use plastigauge or micrometers and compare the result to the manufacturer’s service limits. Plastigauge provides a direct clearance reading when you torque the cap to spec and then read the planted strip’s width against a scale; it shows the actual oil clearance in the assembled condition. Using micrometers, you can measure the crank journal diameter and the bearing shell thickness (or inner diameter) and compute the clearance, but you must reference the exact bearing size and journal dimensions from the service data. Acceptable limits come from the engine’s specification data; if the clearance is outside those limits, you select the proper bearing size or rework as needed. Visual inspection or using a ruler won’t give a precise or reliable clearance, and a dial indicator alone without reference data cannot determine if the clearance is within spec.