Main Bearing Service

NOTE: Selective fit main bearings are used. If a .009" undersize and a .010" undersize bearings were used for precision fitting during production, the main bearing cap is painted Light Green on each side of affected journal. The crankshaft throw will also be stamped with a "9" on one side of undersize journal, and have a large spot of Light Green paint.

NOTE: Following procedures are with oil pan and cylinder heads removed. Main and rod bearing size is indicated by letter stamped into bearing tang or actual bearing size stamped opposite of tang. Bearing marks are as follows: A = .0005", B = .0010", and C = .0015". During production, .009" undersize bearings may have been installed for close tolerances on some journals.

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.
NOTE: DO NOT scrape gum or varnish deposits, clean inserts and caps in solvent. DO NOT file or lap caps to obtain proper bearing clearance. If main bearing caps are replaced, shimming may be needed. Laminated shims are available in various sizes.
Measure outside diameter of crankshaft main bearing journals in at least four places to determine if out-of-round or tapered. Standard journal out-of-round is .0002" (.005 mm) and must not exceed .0010" (.025 mm). Standard journal taper is .0002" (.005 mm) and must not exceed .0010" (.025 mm).
NOTE: 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 bearing clearance, 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.
DO NOT position jack under crankshaft pulley. Crankshaft post damage will result. With all bearing caps (other than one being checked) tight, check clearances using Plastigage method. DO NOT turn crankshaft with Plastigage installed.
NOTE: New bearings are available in standard size and .0010", .0020", .0090", .010" and .020" undersize. Replace both upper and lower inserts together. 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 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. Remove and replace crankshaft sprocket as necessary.
NOTE: Crankshaft sprocket removal will require Sprocket Remover (J- 5825). Sprocket installation will require Sprocket Installer (J-5590).
If journals are remachined, ensure same journal shoulder radius is reproduced. Too small a radius results in fatigue failure of crankshaft. Too large a radius results 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 in solvent and blow out oil passages with compressed air.
When main bearings are installed, ensure oil grooves on bearings are installed on same side. Oil new upper bearing and insert plain (unnotched) end between crankshaft and notched side of block. Rotate bearing into place. Ensure main bearing caps are installed with arrows pointing toward front of engine. Install main cap bolts and tighten.

NOTE: Some production engines may have rear main bearings that the distance between thrust faces is .008" wider than standard. Crankshaft will have .008" stamped on rear counterweight. Ensure proper distance between thrust faces is maintained if rear main bearings are replaced.