The atmosphere in the crankcase contains a large amount of relatively large oil droplets (200 micron) in warm air. Because of the droplets small surface area to volume ratio, the possibility of ignition by a heat source is very low. Oil Mist is formed when a moving part inside the engine fails, it then overheats and vaporizes the oil droplets, which will travel away from the hotspot and condense into much smaller droplets (5 to 10 microns).
Oil mist becomes ignitable whenever it reaches the LOWER EXPLOSION LEVEL (LEL), which is approximately 47mg/liter or 13% oil mist / air ratio. At this point any hot spot with temperature above 850oC (1562oF) can trigger an explosion.
Surfaces that can generate intensive oil mist in addition to the crankshaft bearing system include:
Pistons in cylinder liners
Crankshaft bearings such as main bearings and big-end bearings
Camshafts, their bearings and cams
Timing gear shafts and their bearings
Gear boxes with their bearings, and in some cases pumps