Bearings become a continual source of trouble unless personnel entrusted with operating the engine follow the recommended operation and maintenance procedures exactly.

Severe bearing failures are indicated during engine operation by a pounding noise or by the presence of smoke in the vicinity of the crankcase. Impending failures may sometimes be identified by a rise in the lubricating oil temperature or a lowering of the lubricating oil pressure. Impending bearing failure may be detected during periodic maintenance checks or during engine overhauls by inspection of the bearing shells and backs for pits, grooves, scratches, or evidence of corrosion.

The indication of an impending failure does not necessarily mean that the bearing has completed its useful life. Journal bearings may perform satisfactorily with as much as 10 percent of the load-carrying area removed by fatigue failure. Other minor casualties may be repaired so that a bearing will give additional hours of satisfactory service.

Bearings should not be rejected or discarded for minor pits or minute scratches; however, areas indicating metallic contact between the bearing surface and the journal do mean replacement is needed. Use a bearing scraping tool to smooth minute pits and raised

Figure 3-30.Using a torque wrench to tighten a main bearing.

surfaces. After working on bearings, make every effort to ensure that the bearing surfaces are clean. This also applies to the bearing back and the connecting rod journal Place a film of clean lubricating oil on the journals and the bearing surfaces before you reinstall them.