Crankshaft Main Bearing: Testing and Inspection

Checking Crankshaft Clearances

Part 1 Of 2:

Checking Main Bearing Diameter:

Clean the crankshaft and use a micrometer to measure the crankpins and main journals. Measurements should be taken at several points round the circumference. Main journal and crankpin out-of-round (ovality) should not exceed 0.05 mm. If the measurements are close to or in excess of this limit, the crankshaft should be ground down to the specified undersize. Crankpins can be ground down to the first undersize using a straight grinding wheel. If they are ground down beyond that, a grinding wheel capable of grinding radii must be used since undercutting only suffices to the first undersize. Check that the shaft is straight to within 0.10 mm, using a dial gauge. Place the shaft with crankpins No. 1 and No. 5 in V-blocks on a flat and level bench. Apply a dial gauge to the middle crankpin and rotate the shaft.

Checking Bearing Clearances
Main Bearings





Before the bearing clearance is checked in connection with fitting of new bearings, the crankshaft journals ovality and taper must be checked. The clearances are checked with Plastigage, part number 390 06 558, available in three dimensions. Type PG-1 (green) must be used.

IMPORTANT: When using the Plastigage to measure play, be careful not to get any in the oil holes.

Measure ovality with a micrometer at two positions 90� from each other. Use Plastigage to measure clearance as follows:
1. Turn the engine upside down on the stand so that the weight of the crankshaft will not affect the measurements.
2. Clean the parts to be measured so that they are dry and free from oil and dirt. Place a strip of Plastigage along the shaft.





3. Fit the bearing cap.
Tightening torque 20 Nm + 70° (15 ft. lbs. + 70°).
The crankshaft must remain absolutely still while measurement is in progress.
4. Remove the bearing cap. The strip of Plastigage should now have adhered to either the bearing cap or the main journal.





5. Measure the width of the Plastigage strip using the scale that is printed on the package and read off the clearance. One side of the package gives mm measurement, the other side in 1000ths of an inch. Measure the strip at the widest point but do not touch it with your fingers. See Specifications regarding measurements.

Big-End Bearings





Before the bearing clearance is checked in connection with fitting of new bearings, the crankshaft journals ovality and taper must be checked. The clearances are checked with Plastigage, part number 30 06 558, available in three dimensions. Type PG-1 (green) must be used.

IMPORTANT: When using the Plastigage to measure play, be careful not to get any in the oil holes.

Measure ovality with a micrometer in two positions 90° from each other, checking clearance can be done with Plastigage as follows:
1. Clean the parts to be measured so that they are dry and free from oil and dirt. Place a strip of Plastigage along the shaft.





2. Fit the bearing cap.
Tightening torque 20 Nm + 70° (15 ft. lbs. + 70°).
The crankshaft must remain absolutely still while measurement is in progress.
3. Remove the bearing cap. The strip of Plastigage should now have adhered to either the bearing cap or the main journal.





4. Measure the width of the Plastigage strip using the scale that is printed on the package and read off the clearance. One side of the package gives mm measurement, the other side in 1000ths of an inch. Measure the strip at the widest point but do not touch it with your fingers. See Specifications regarding measurements.