Cylinder Block: Main Bearings

NOTE: Upper and lower main bearing halves are not interchangeable. Upper half is drilled and grooved for entry of oil.

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 four places to determine out-of-round or taper. Journal out-of-round must not exceed .0006" (.015 mm). Journal taper must not exceed .0004" (.010 mm) per inch. Standard main bearing journal runout is .002" (.05 mm) and service limit is .005" (.13 mm).
NOTE: Always replace bearings in pairs. Never combine new and used bearings. 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, shim adjacent main bearings to bearing being checked. Alternate method is to position jack under counterweight adjoining bearing being checked so weight of crankshaft will not compress Plastigage and provide incorrect reading.
When checking No. 1 bearing, loosen accessory drive belts to prevent tapered reading on Plastigage. DO NOT position jack under crankshaft pulley. Crankshaft post damage will result. With all bearing caps tight (other than one being checked), check clearances using Plastigage method.
NOTE: If undersize bearings are used on more than one journal, position them in cylinder block rather than on bearing cap. Do not turn crankshaft with Plastigage installed.
If standard and undersize bearing combinations 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. Insert Main Bearing Remover/Installer (6331) into hole in crankshaft and rotate bearing (opposite of engine rotation) into place.
Oil new lower bearing and install in bearing cap. Install main bearing cap so that arrow faces front of engine. Tighten all main bearing caps to specifications in two steps. When changing rear main bearing, rear main seal must also be replaced. Refer to, in this article, REAR MAIN BEARING OIL SEAL