Main bearings are sleeve type oil film bearings that support spinning crankshafts in internal combustion engines, such as diesel and most gasoline engines. They consist of two steel half shells that present a thin layer of soft metal called "Babbitt" to the highly polished main bearing journal. Babbitt keeps the oil film in place with the shaft journal, virtually precluding bearing wear. Replacing main bearings is the heart of a rebuild requiring engine removal. Take photos at key stages to assist in the complex re-assembly.
Prepare to remove the engine from the vehicle. Drain the fluids, disconnect the battery, then the electrical connections, hoses and controls. Remove the transmission or drop the drive train out the bottom of front-drive vehicles.
Remove the engine from the vehicle for rear drive or remove from the drive train cradle for the front drive.
Put the engine in the engine stand or on a strong bench. Remove the water pump, timing cover and the timing belt or chain.
Turn the engine upside down to remove the oil pan or crankcase. Take close-up pictures of the bottom of the engine. Remove the oil pickup and the oil pump.
Remove all the connecting rod bearing caps, carefully noting their exact position and orientation of each and lining them up in a strong box in that exact orientation. Now remove all the main bearing caps, maintaining the same orientation in a second box.
Lift the crankshaft out of the engine block. Remove the bearing shells from the main bearing caps and the journals in the block. Inspect the crankshaft and note any abnormalities on the journals.
Send the crankshaft out for reconditioning. An industry adage says that if the engine bearings need replacement, the crankshaft should be reconditioned. Clean all the bolts, including the threads and coat with reassembly lube. The shaft will come back from the shop with undersized bearing shells to match the size of the reconditioned bearing journals.
Clean and dry all the bearing backing surfaces. Clean your hands thoroughly. Systematically, replace the new bearing shells, making sure their side tangs fit into their indentations. Put a coating of clean engine oil on each journal.
Replace crankshaft into block, being careful not to allow connecting rod bolts to scratch the polished journals. Put each main cap and connecting rod cap back in place. If in doubt about orientation, look at the close-up photos. Torque all the bearing cap bolts to specifications, including the tightening order.
Clean off all the old gaskets. Blow out the engine block with compressed air.
Reassemble the engine with new seals and gaskets.
Reassemble the car in reverse order of dis-assembly and refill the fluids. Start the engine and verify oil pressure within several seconds, or stop it immediately and recheck.
Let the engine run. Once the oil pressure is established, check for leaks, both under the hood and underneath. Test drive the car.
