The Rear Crankshaft Seal: Function, Failure Signs, and Replacement Guide

The rear crank seal is one of the most critical engine components when it comes to maintaining oil control and engine longevity. Commonly referred to by several names such as the rear crankshaft main bearing seal, rear engine oil seal, or rear main crank seal, this part plays a central role in sealing the back end of the crankshaft. In this guide, we’ll explore what it does, how to recognize failure, and what’s involved in removing the crankshaft seal for repair or replacement.

Understanding the Rear Crankshaft Seal

The rear crankshaft exits the back of the engine and connects to the transmission. To prevent oil from leaking out where the crankshaft meets the flywheel or flexplate, a seal is installed — known as the rear crank seal or rear main crank seal.

Purpose of the Seal

The rear engine oil seal performs several key functions:

Prevents engine oil from leaking at the junction between the crankshaft and transmission

Protects internal engine components by maintaining proper oil pressure

Keeps the clutch or torque converter area free from oil contamination

The rear crankshaft main bearing seal is usually made of durable rubber or polymer and is often part of a larger housing or cover that bolts onto the rear of the engine block.

Location

Sits at the back of the engine where the crankshaft exits

Hidden behind the flywheel (manual transmission) or flexplate (automatic)

Often requires transmission removal for access

Symptoms of a Failing Rear Main Crank Seal

A leaking rear main crank seal doesn’t just create a mess; it can seriously impact engine performance and health. Catching the early signs of failure can help avoid more costly repairs.

Common Warning Signs

Oil leak between engine and transmission

Oil on the underside of the bell housing

Clutch slippage due to oil-soaked clutch (manual vehicles)

Unexplained oil loss with no visible external leaks

Burning oil smell from oil dripping on exhaust parts

These symptoms typically indicate that the rear crankshaft seal is worn, cracked, or has lost its ability to seal due to age, pressure, or improper installation.

Causes of Seal Failure

Age-related wear or heat damage

Excess crankcase pressure from a faulty PCV system

Misalignment of the crankshaft

Engine oil contamination or degraded seal material

Overfilled oil causing excess pressure around the seal

Removing and Replacing the Crankshaft Seal

Removing the crankshaft seal—particularly the rear one—is not a simple job and often requires significant disassembly. However, knowing the process helps vehicle owners make informed decisions.

Step-by-Step Overview

Disconnect the battery

Remove the transmission or transaxle to access the flywheel or flexplate

Remove the flywheel or flexplate

Unbolt and remove the seal housing (if applicable)

Use a seal puller or screwdriver to carefully extract the old seal

Clean the sealing surface and inspect the crankshaft for wear

Install the new rear crank seal evenly to avoid deformation

Reassemble components in reverse order

Depending on the engine, the rear crankshaft main bearing seal may come integrated into a housing or be available as a standalone part.

Cost of Replacement

Rear crankshaft seal (part only): $20–$60

Labor cost: $400–$900+ (due to transmission removal)

Total cost: $500–$1,000 or more, depending on vehicle type

Mechanics often recommend replacing the rear crank seal during clutch jobs or transmission replacement since the labor overlaps significantly.

Tips for Success

Always replace the seal with OEM or high-quality aftermarket parts

Check for wear on the crankshaft surface

Replace the PCV valve to avoid repeated seal failure

Consider replacing the rear engine oil seal whenever the transmission is removed, even if no leak is visible yet

The rear crank seal, or rear main crank seal, may be hidden from sight, but it plays a central role in protecting your engine from oil loss and damage. Whether you’re seeing early signs of failure or planning a major engine service, understanding the location, function, and process for removing the crankshaft seal can save time, money, and potential engine wear. While replacement can be labor-intensive, it’s a necessary step in preserving engine health and reliability.