Main Bearings

NOTE: Install main bearing cap with arrows pointing toward front of engine.

Inspect each bearing for peeling, melting, seizure or improper contact. Replace defective bearings. If copper-lead bearing base is visible but is not showing in more than 20% of total area, bearing is not excessively worn.
Measure outside diameter of crankshaft main bearing journals in at least 4 places to determine out-of-round or taper. Journal out-of-round must not exceed .0002" (.005 mm). Journal taper must not exceed .0002" (.005 mm).
NOTE: Always replace bearings in pairs. Never use a new bearing in combination with a used bearing. Observe location of high spots on main bearings. If high spots are not in line, crankshaft may be bent and should be checked.
To check main bearings, use Plastigage. Shim main bearings adjacent to bearing being checked. Alternate method is to position jack under counterweight adjoining bearing being checked so weight of crankshaft will not provide an incorrect reading. Do not turn crankshaft with Plastigage installed.
When checking No. 1 bearing, loosen accessory drive belts to prevent tapered reading. Do not position jack under crankshaft pulley. Crankshaft damage will result. With all bearing caps (other than one being checked) tight, check clearances.
NOTE: If undersize bearings are used on more than one journal, position in cylinder block rather than bearing cap.
If standard and undersize bearing combination do not bring bearing clearance within specified limits, crankshaft will have to be refinished and undersized bearings installed. If journal will not clean up to maximum undersize bearing, replace crankshaft.
If journals are remachined, ensure same journal shoulder radius is reproduced. Too small a radius will result in fatigue failure of crankshaft. Too large a radius will result in bearing failure due to radial ride of bearing. When journals are refinished, chamfer oil holes and polish journals with No. 320 grit polishing cloth and engine oil.
After chamfer and polish operations, clean crankshaft thoroughly in solvent, and blow out oil passages with compressed air. Oil new upper bearing and insert plain (unnotched) end between crankshaft and notched side of block. Rotate bearing counterclockwise (as viewed from front of engine) into place.
Oil new lower bearing and install in bearing cap. Install main bearing cap so that arrow faces front of engine. Install new rear main bearing seal in rear main bearing cap and cylinder block. Seal rear main bearing split-line surface with GM Sealing Compound (No. 1052357).
Tighten all main bearing caps, (except No. 4), to 70 ft. lbs. (95 N.m). Tighten No. 4 bearing cap to 11 ft. lbs. (15 N.m). Tap end of crankshaft forward and back with lead or plastic hammer, thus lining up rear main bearing and crankshaft thrust surfaces. Retighten all main bearing caps to 70 ft. lbs. (95 N.m) in 2 steps.