Genuine BMW Rear Main Seal, BMW B48

The rear main seal, also known as the rear crankshaft seal, is a crucial component in an engine.

Genuine BMW Part Number: 11148664904

Purpose

1. Prevents Oil Leaks: The primary function of the rear main seal is to prevent engine oil from leaking out of the rear end of the crankshaft. It seals the gap between the crankshaft and the engine block or the transmission housing, ensuring that the oil remains within the engine.

2. Maintains Oil Pressure: By preventing oil leaks, the rear main seal helps maintain the proper oil pressure within the engine. This is essential for the lubrication of internal engine components, reducing friction and wear, and ensuring smooth engine operation.

3. Protects the Clutch and Transmission: In manual transmission vehicles, the rear main seal prevents oil from leaking onto the clutch assembly. In both manual and automatic vehicles, it prevents oil from contaminating the transmission components.

Location and Function

- Location: The rear main seal is located at the rear of the engine, where the crankshaft extends out to connect with the flywheel or flexplate and, subsequently, the transmission.
- Function: The seal fits tightly around the crankshaft and within the engine housing, creating a barrier that prevents oil from seeping out and contaminants from entering. It is designed to handle the rotational motion of the crankshaft while maintaining a tight seal.

Symptoms of a Failing Rear Main Seal

- Oil Leaks: Visible oil leaks at the rear of the engine, typically around the area where the engine meets the transmission.
- Low Oil Levels: Frequent need to add oil due to unexplained oil loss.
- Oil Drips: Oil dripping from the rear of the engine, often noticed on the ground beneath the vehicle.
- Clutch Slippage: In manual transmission vehicles, oil leaking onto the clutch can cause slippage, leading to poor performance and difficulty in gear shifting.
- Burnt Oil Smell: An odor of burning oil, especially after the vehicle has been driven, indicating oil may be leaking onto hot engine or exhaust components.

Maintaining the integrity of the rear main seal is essential for the health and performance of an engine. Regular inspections, particularly during routine maintenance or when the transmission is removed for other repairs, can help identify any potential issues with the rear main seal. If a leak is detected, timely replacement is crucial to prevent further damage to the engine or transmission. Replacing the rear main seal can be a labor-intensive process since it often requires removal of the transmission or engine, but it is necessary to ensure the longevity and reliability of the vehicle.