Rear Crankshaft Seals: Protecting Your Engine from the Back End

The crankshaft is a central moving part in any internal combustion engine, and keeping it properly sealed is critical for engine health and performance. While the front crankshaft area is frequently discussed due to its role in timing systems and accessories, the rear crank seal is just as important. Located at the junction between the engine and transmission, this seal plays a vital role in preventing oil leaks from the back end of the engine. In this article, we'll explore everything you need to know about the rear crankshaft, rear crankshaft main bearing seal, rear engine oil seal, rear main crank seal, and the process of removing crankshaft seal when maintenance is needed.

What Is the Rear Crankshaft Seal?

The rear crank seal also known as the rear main crank seal, is positioned at the rear of the engine, where the rear crankshaft exits the engine block and connects to the flywheel or flexplate, ultimately linking the engine to the transmission.

Rear Crankshaft Overview

The rear crankshaft is the part of the crankshaft that protrudes from the engine and connects to the transmission system. Since the crankshaft rotates at high speed and is surrounded by engine oil, this area must be carefully sealed to prevent leaks into the transmission bell housing.

Rear Engine Oil Seal

The rear engine oil seal ensures that oil circulating in the crankcase does not leak out through the rear of the engine. It also keeps dirt, debris, and moisture from entering the engine via the transmission opening. Most rear crank seals are made of high-temperature resistant rubber or PTFE (Teflon), designed to endure the extreme pressure and rotational speed of the crankshaft.

Failures in the rear engine oil seal can result in visible oil leaks, slipping clutches (in manual transmissions), or even damage to the torque converter in automatic transmissions.

Rear Main Seals and Bearing Seals: Keeping Oil Where It Belongs

In addition to the rear crank seal, the rear crankshaft main bearing seal plays an equally important role in engine reliability. The main bearings of the crankshaft are responsible for supporting its rotation under load. Any failure in sealing at this point can compromise the lubrication system.

Rear Crankshaft Main Bearing Seal

This component helps contain the oil within the crankcase near the rear main bearing. It typically works in conjunction with the rear crankshaft seal to form a double-layered barrier against leaks. If the rear crankshaft main bearing seal fails, oil can seep out around the crankshaft journal, potentially damaging the bearing or leading to low oil pressure.

Rear Main Crank Seal

The rear main crank seal is another term for the rear crankshaft seal and is one of the most common points of oil leakage in older engines. Because of its location—behind the engine, in front of the transmission—it is often one of the most labor-intensive seals to replace.

Symptoms of a failing rear main crank seal include:

Visible oil leak at the transmission bell housing

Low oil levels

Burning oil smell due to oil dripping on exhaust components

Clutch slippage (in manual vehicles)

Routine engine oil changes and use of high-quality engine oil can prolong the life of the rear seal and reduce wear caused by dirt and heat.

Removing and Replacing the Crankshaft Seal: What You Need to Know

Because of its challenging location, removing crankshaft seal—especially the rear one—is not a simple task. It often requires detaching the transmission or removing the engine entirely, depending on the vehicle design.

Steps for Removing Crankshaft Seal

Disconnect the battery to prevent any electrical issues.

Remove the transmission or transaxle, exposing the rear side of the engine block.

Detach the flywheel or flexplate to access the seal.

Extract the old seal using a seal puller or carefully with a flathead screwdriver. Be cautious not to damage the crankshaft surface or engine housing.

Clean the seal surface thoroughly to remove oil, grime, or old sealant.

Install the new rear crankshaft seal using a seal installation tool to ensure even pressure and proper alignment.

Reassemble the engine components in reverse order.

It's also a good practice to inspect the crankshaft surface for scoring or wear during this process. In some cases, a repair sleeve may be necessary to provide a new surface for the seal to grip.

When to Replace

You should consider replacing the rear crankshaft seal if:

You see persistent oil leaks from the rear of the engine

You're replacing the clutch or transmission

You're rebuilding the engine or replacing the crankshaft

Because of the labor involved, it’s most cost-effective to address the rear main seal during other major engine or transmission service.

The rear crank seal, or rear main crank seal, plays a crucial but often overlooked role in maintaining engine performance and oil integrity. Along with the rear crankshaft, rear crankshaft main bearing seal, and rear engine oil seal, it ensures that oil remains contained within the engine and prevents contaminants from entering the lubrication system. While removing crankshaft seal components—particularly at the rear—is complex and labor-intensive, it's an essential task for addressing oil leaks and preserving engine health.

By understanding the purpose and importance of these components, vehicle owners and technicians can better diagnose leaks, plan maintenance, and extend engine life. Don’t ignore small oil drips—what seems like a minor issue could be a failing seal that, if left unchecked, may lead to costly repairs.