The bearings are thin curved strips of metal that sit between the crankshaft "journals" and the connecting rods, and between the engine block and crankshaft. They usually have three layers of metal. The first layer is real soft so dirt and grit embed in it rather than going around and scratching the polished surface of the crankshaft journals. The next layers are harder to absorb impact.
The gap, or "clearance" between a bearing and the metal part riding on it is very tight; in the order of.005". Oil is forced into that gap under pressure to isolate the moving parts from each other. That oil oozes out the sides, then falls back into the oil pan or gets whipped around to coat other areas, namely the cylinder walls.
The oil pressure is maintained because it's hard to push the oil through the tight clearances. Anything that makes it easier for the oil to run out will result in lower pressure. Oil gets thinner when it gets warm. That lets it run out faster, so the volume being pumped has to increase. Most oil pumps have no problem pumping enough volume, but with low pressure, those moving metal parts can start to make contact. The soft first layer of metal begins to rub off of the bearings. You can actually see that as a significantly discolored area. The missing metal results in increased clearance so the oil can run out even easier and will be less effective at isolating the moving parts. Once that wear starts, it accelerates real fast until you hear a knocking noise from the parts banging against each other.
If this is caught early enough, sometimes new bearings can be installed and the wear stops taking place. That has to be done before the journals becomes scored or scratched. Any roughness on a journal will tear up the new bearing. For a V-8 engine, the crankshaft sits on five bearings, and there's eight connecting rod bearings. It only takes one of them to be worn to allow the oil to run out too fast and cause low pressure.
Depending on the engine design, there are usually five or six camshaft bearings too. Those get oil in such a way that there's not much pressure left. They don't have to handle the same pounding forces as the other bearings so they rarely cause a problem.
Friday, November 27th, 2015 AT 10:17 AM