Rear Main Seal
A heavy-duty rubber or silicone seal at the back of the crankshaft that prevents engine oil from leaking into the transmission housing.
The rear main seal is a critical engine gasket located at the very back of the engine block where the crankshaft connects to the transmission flywheel or flexplate. Its primary function is to prevent engine oil from escaping the crankcase, keeping the oil inside where it belongs to lubricate vital internal parts.
Because of its location, the rear main seal is exposed to constant heat, high rotational friction, and crankcase pressure. Over time, the rubber or silicone material of the seal will harden, shrink, or crack, leading to oil leaks. A leaking rear main seal typically drips oil from the bottom of the transmission bellhousing, which can contaminate the clutch on manual transmission vehicles or cause a messy puddle under the car.
Replacing a rear main seal is extremely labor-intensive because the transmission, clutch or torque converter, and flywheel or flexplate must be completely removed to access the seal. While the seal itself is relatively inexpensive, the labor cost makes it a major repair. It is common practice to replace this seal whenever the transmission is removed for other work, such as a clutch replacement.