Replacing main bearings is a critical part of any engine rebuild. They are usually replaced along with other internal parts while the engine is out and completely disassembled. This allows for a complete inspection of the crankshaft journals around which the bearings sit, as well as the opportunity to check the alignment of the crankshaft and the engine block. However, it is possible on some engines, to replace them with the engine still in the vehicle. Keep in mind that the job can be difficult and care must be taken to keep all parts free of contamination.
Raise the vehicle and secure it with jack stands. Remember to never work under a vehicle supported only by a jack. Drain the engine oil into a drain pan and remove the oil pan itself with a ratchet and socket. Then remove the spark plugs to relieve engine compression. If the head has been removed for any reason, there is no need to remove the spark plugs.
Loosen the bearing cap nuts or bolts and remove the bearing cap and the lower half of the bearing. Tap the cap lightly with a plastic hammer on one side, then the other if it sticks. Be very careful not to damage the cap and mark it in some way to identify which journal it came off of. Then loosen all other bearing cap nuts or bolts a half turn.
Insert the bearing removal tool into the journal's oil hole and rotate the crankshaft, causing the bearing shell to rotate within the journal and allowing access to the upper bearing half. Be sure to rotate the crankshaft in the proper direction to raise the bearing's lock out of the notch in the cylinder block. If it is rotated in the wrong direction, the lock will jam itself further into the notch. If the bearing is dowelled in place, the crankshaft will have to be removed.
Examine the bearings before removing them for any obvious, visible causes of wear and make any necessary corrections. Use a crankshaft journal micrometer to measure the diameter of the journal, while rotating the crankshaft to check for any taper or out-of-round condition. If the journal is worn beyond specifications, the crankshaft will have to be removed to recondition it, which would then require new undersized main bearings.
Examine the surfaces of both the journal and the bearing to ensure that they are free of dirt, which could cause premature bearing failure. Then coat the journal surface with engine oil before installing the bearing.
Install the new bearing half by making use of the tool again and by turning the crankshaft in the opposite direction from that which it was turned to remove it. It should slip in with reasonable ease. If there is resistance, examine the surface into which it fits and note that at times it does not get started squarely. If it is forced, it could become distorted. Also do not file the edges of the bearing halves, even if they stick slightly above the surface. When the bolts or nuts are tightened on the cap, they will crush down to a proper fit.
Place the other half of the bearing in the cap and install the cap. Then tighten the bolts to achieve the proper crush and check the clearance with plastigage. Loosen the bolts slightly, by one-half turn as before, then proceed to the next bearing. When removing or installing the upper half of the rear bearing, hold the oil seal in place within the block to prevent it from coming out of position and take care to not nick the seal.
Torque all bearing caps to the specifications of the particular engine once the last bearing has been replaced. Then reinstall the oil pan with a new gasket, tightening the bolts also to specifications. Refill the engine with oil, then start the vehicle and check for leaks and unusual noises.
Always wear safety glasses when performing automotive repairs. Be sure to use cloth work gloves and rags when handling hot parts.