A rear main seal typically fails due to normal rubber aging, prolonged exposure to high engine operating temperatures, crankcase overpressure caused by a clogged positive crankcase ventilation (PCV) valve, or scoring on the crankshaft journal. Over years of continuous thermal expansion and contraction, the synthetic rubber or polytetrafluoroethylene sealing lip hardens, cracks, and loses its elasticity, allowing engine oil to seep past the crankshaft.
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