Types of wear of main and connecting rod bearings
A. Scratches due to dirt embedded in the coating material; B. Insufficient lubrication, local wiping of the bearing coating; C. Incorrect installation of bearings, shiny (polished) areas D. Tapering of the necks, wiping of the bearing coating from the entire surface along the length of the bearing; E. The formation of a shoulder on the edge of the bearing; F. Fatigue wear, craters or pockets
Although the main and connecting rod bearings must be replaced during engine overhauls, used bearings should be stored at their installation site for a thorough inspection, since wear marks on these parts allow you to restore the general condition of the engine. Bearing shells are subdivided into size groups according to their thickness, belonging to each size group is indicated by a color marking printed on the bushing.
Causes of failure of crankshaft bearings are insufficient lubrication, the presence of dirt and foreign particles, engine overload and corrosion. Regardless of the cause that caused the destruction of the bearings, it should be eliminated before the final assembly of the engine in order to avoid repeated damage to these parts.
When checking the condition of the bearings, they must be removed from the cylinder block, crankshaft main bearing caps, connecting rod caps and connecting rods. Lay the liners on a clean surface in the same order they were installed on the engine. This will allow you to install the appropriate crankshaft bearing, in which the bearing has undergone increased wear. When checking, it is not allowed to touch the working surface of the liners with your fingers, so as not to cause additional damage to them.
Dirt and foreign particles enter the engine for various reasons - during the assembly process, through filters or the crankcase ventilation system. Contaminants can also get into the oil and from there into the bearings. Contaminants often contain metal particles from the machining of engine parts, or due to wear during normal engine operation. Sometimes there are residues of abrasive materials in the dirt after lapping and grinding work, especially with insufficient flushing and cleaning of the engine after they are completed. Regardless of the cause of origin, foreign contaminants are usually embedded in the soft alloy of the bearing and are easily visible. Large particles will not be able to penetrate the bearing and lead to scratches and scuffing on the surfaces of both the bearing and the crankshaft journals. The best way to prevent this wear is to thoroughly flush the parts and maintain an impeccable cleanliness when reassembling the engine. To prevent premature wear, it is also recommended to change the oil and filters more often.
Insufficient engine lubrication ("oil starvation") may be due to a number of interrelated reasons. These include engine overheating (causes oil to thin), overload (cause oil to be squeezed out of the bearing surface) and pressure drop due to leakage (ejection) oils (due to increased clearances in the bearings, wear of the oil pump, high crankshaft speed). The reason for accelerated wear of the crankshaft bearings is also the blockage of the oil channels, usually due to misalignment of the lubrication holes in the crankshaft bearings with the holes in the cylinder block and in the connecting rods, which causes insufficient lubrication and failure of the bearings. When insufficient lubrication is the cause of failure, the bearing coating material is chafed or extruded from the steel base. The temperature rises to such a level that, due to overheating, the steel base of the bearing is chipped, which acquires a bluish tint.
The durability of crankshaft bearings is also highly dependent on driving skills. Bearings experience increased stress when driving at wide open throttle, or at low speeds in high gear (in heavy engine modes). Under increased loads, the oil film is squeezed out. Bearings experience bending deformations, which form microcracks (fatigue wear). As a rule, the bearing coating is detached from the steel base in the form of small particles.
Short trips can cause corrosive wear of bearings, because due to insufficient warm-up, condensed moisture and corrosive gases do not have time to escape. These components accumulate in the oil, causing the formation of acids and insoluble deposits. As oil reaches the bearings, acid corrosion occurs, causing the bearings to deteriorate and fail.
Incorrect installation of bearings during assembly also leads to premature wear. If the clearance in the bearings is less than normal, then their lubrication is difficult, which leads to wear as well as due to "oil starvation". Dirt or small particles that have fallen under the bearing shell lead to local lifting of the shell and to increased wear in this part.
When assembling, do not touch the bearing surfaces with your fingers, otherwise the surface may be scratched or dirt particles may be introduced onto the bearing.
As already noted, during the overhaul of the engine, the liners of the main and connecting rod bearings must be replaced. Neglecting this rule and saving on repairs lead to extremely undesirable consequences.