Originally Posted by SlowLST2

The bearing does not provide the crankcase seal. No bearing is sealed to begin with. They are shields only and only intended to keep debris from getting in. Many engines in the past never came with any seals or shields to at all!

The crankcase is sealed through an oil film between the crankshaft and crankcase. If the bearing is loose, the crankshaft can wobble. This wobble can/will wear the crankcase if it’s a tight tolerance engine. This whole process will upset the crankcase seal causing erratic behavior. If an otherwise predictable engine starts running wonky all of the sudden - check the rear bearing. Usually if a bearing is going South, glow plugs will pop more often too.