What Is a Rear Main Seal?by Richard Rowe
Rear main seals come in many shapes and sizes, but all are responsible for sealing the back of an engine's crankshaft. These vital seals must keep oil inside the engine while coping with the crankshaft's rotation and heat expansion.
Rear main seals are located at the back of the crankshaft, at the rear of the engine block. Many of these seals can only be installed or replaced by removing the rear crankshaft main cap.
Many older engines used "rope" seals that resembled a piece of cord. These rope seals did a decent job of keeping oil pressure in the engine but would eventually become saturated and start to leak.
These plastic seals are an improvement over rope seals but are still prone to leakage. These seals are often used because they do not require removal of the crankshaft to replace.
One-piece seals are the best at sealing but require a specially machined crankshaft and cannot be replaced without pulling the crank.
Older seals may leak due to heat shrinkage, damage and pitting, defects in the crankshaft, improper installation or clogging of the Positive Crankcase Ventilation system (PCV).
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