Main Bearings
- Main bearings are selective fit by manufacturer during production. A standard size bearing half may be used in combination with a 0.001" undersize bearing half to obtain correct clearance. This combination will decrease clearance 0.0005".
- If correct clearance could not be obtained during production, a crankshaft with 0.009" undersize main bearing journals is fitted. A 0.009" and 0.010" bearing half combination may be used to obtain correct clearance.
- Engines fitted with reground crankshafts may be identified by a "9" stamped on counterweight and a large spot of light green paint next to affected bearing. Also main bearing cap will be painted light green for that bearing.
- Main bearings may be removed and replaced with crankshaft still installed. Mark or identify main bearing caps to cylinder block before removing caps. With the exception of rear main bearing, main bearings are removed from cylinder block by inserting a bearing removal and installing tool in oil hole in crankshaft and rotating crankshaft clockwise. To remove rear main bearing, drive bearing partly out with a drift punch and hammer. Using a pair of pliers with taped jaws, grab bearing thrust surface and slinger on crankshaft and rotate crankshaft to remove bearing.
- Crankshaft clearance, taper or out-of-round conditions can be checked using the Plastigage method. If clearance exceeds specifications, a 0.001" or 0.002" undersize bearing may be installed to obtain correct clearance. Both bearing halves must be replaced.
- If correct clearance can not be obtained or if journal is tapered or is out-of-round more than 0.001", crankshaft must be removed and ground for undersized bearings. Bearings are available in standard 0.001", 0.002", 0.009", 0.010" and 0.020" undersize.
Courtesy of GENERAL MOTORS COMPANY
- To install bearings, ensure crankshaft journal and bearing surface in cap and block are clean. Lubricate journal and install bearing in cap. If bearings were removed with crankshaft still installed, use bearing removal and installation tool inserted in crankshaft oil hole to install upper bearing half. To install rear upper main bearing, place in position and use plier procedure, as previously described, to install bearing. Install main caps noting identification marks and evenly tighten main bearing bolts.