Main bearings are of the precision insert type and do not utilize shims for adjustment. If clearances are found to be excessive, a new bearing for both the upper and lower halves, will be required. Service bearings are available in standard size, .013mm (.0005 in.) U.S. and ,026mm (.001 in.) U.S.
Selective fitting of main bearing inserts is necessary in production in order to obtain close tolerances. For this reason you may find one half of a standard insert with one half of a .001″ (.026mm) undersize insert which will decrease the clearance .0005″ (.013mm) from using a full standard bearing.
INSPECTION
In general, the lower half of the bearing (except #1 bearing) shows a greater wear and the most distress from fatigue. If upon inspection the lower half is suitable for use, it can be assumed that the upper half is also satisfactory. If the lower half shows evidence of wear or damage, both upper and lower halves should be replaced. Never replace one half without replacing the other half.
CHECKING CLEARANCE
To obtain the most accurate results with “ Plastigage” (or its equivalent) a wax-like plastic material which will compress evenly between the bearing and journal surfaces without damaging either surface, certain precautions should be observed.
If the engine is out of the vehicle and upside down, the crankshaft will rest on the upper bearings and the total clearance can be measured between the lower bearing and journal. If the engine is to remain in the vehicle, the crankshaft must be supported upward to remove the clearance from the upper bearing. The total clearance can then be measured between the lower bearing and journal.
To assure the proper seating of the crankshaft, all bearing cap bolts should be at their specified torque. In addition, preparatory to checking fit of bearings, the surface of the crankshaft journal and bearing should be wiped clean of oil.
1. With the oil pan and oil pump removed, and starting with the rear main bearing, remove bearing cap and wipe oil from journal and bearing cap.
2. Place a piece of gaging plastic the full width of the bearing (parallel to the crankshaft) on the journal. Do not rotate the crankshaft while the gaging plastic is between the bearing and journal.
3. Install the bearing cap and evenly torque the retaining bolts to specifications. Bearing cap MUST be torqued to specifications in order to assure proper reading. Variations in torque affect the compression of the plastic gage.
4. Remove bearing cap. The flattened gaging plastic will be found adhering to either the bearing shell or journal.
5. On the edge of gaging plastic envelope there is a graduated scale which is correlated in thousandths of an inch. Without removing the gaging plastic, measure its compressed width (at the widest point) with the graduations on the gaging plastic envelope. Normally main bearing journals wear evenly and are not out-ofround.
However, if a bearing is being fitted to an out-of-round (.001″ max.), be sure to fit to the maximum diameter of the journal. If the bearing is fitted to the minimum diameter and the journal is out-of-round .001″, interference between the bearing and journal will result in rapid bearing failure. If the flattened gaging plastic tapers toward the middle or ends, there is a difference in clearance indicating taper, low spot or other irregularity of the bearing or journal. Be sure to measure the journal with a micrometer if the flattened gaging plastic indicates more than .001″ difference.
6. If the bearing clearance is within specifications #1, 2, 3, 4 (.0018″-.0032″) and #5 (.0022″-.0037″), the bearing insert is satisfactory. If the clearance is not within specifications, replace the insert. Always replace both upper and lower inserts as a unit.
7. A standard, .013mm (.0005″), or ,026mm (.001″) undersize bearing may produce the proper clearance. If not, the crankshaft may be ground up to .010″. But bearings in that size range are not available at this writing.
8. Proceed to the next bearing. After all bearings have been checked rotate the crankshaft to see that there is no excessive drag. When checking #1 main bearing, loosen accessory drive belts so as to prevent tapered reading with plastic gage.
9. Measure crankshaft end play (.05-.18mm) by forcing the crankshaft to the extreme front position. Measure at the front end of the rear main bearing with a feeler gage.
10. Install a new rear main bearing oil seal in the cylinder block and main bearing cap.
REPLACEMENT
Main bearings may be replaced with or without removing the crankshaft.
