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Summary
  • Also known as the rear crankshaft seal, the rear main seal is a gasket that prevents engine oil from leaking as it flows from the engine block to the oil pan.
  • Some rear main seal replacement tips include wearing protective gear, disconnecting the battery before removing the transmission, and matching the new rear main seal to the old one.
  • A rear main seal can cost anywhere from $10 to $80, but most mechanics will charge you around $600 to $900 to replace this part.

Replacing your crankshaft’s rear main seal isn’t the easiest task. It usually involves removing your vehicle’s transmission and exhaust system, so it’ll take a lot of time and patience.

However, it’s a necessary repair that you can’t just ignore. Unless you want to risk damaging your engine, you should replace a faulty rear main seal as soon as possible.

, Rear Main Seal Replacement Tips

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Older engines had a two-piece seal that could be replaced by removing the oil pan and the rear main bearing cap, but that was a big job, too. Some really old vehicles had a rope seal that could be replaced by removing the pan and rear main cap, and those seals are very difficult to replace.

What Is a Rear Main Seal?

Also known as the rear crankshaft seal, the rear main seal is one of the most important seals in your engine. It’s a gasket that prevents engine oil from leaking as it flows from the engine block to the oil pan.

See also  Crankshaft Seal Problems: What Is It, Symptoms, and What To Do

What Causes a Rear Main Seal To Leak?

Rear main seals are typically made of rubber or silicone, so they tend to wear out and may need to be replaced after some time.

Constant exposure to heat, road salt, and engine vibration can cause the rubber to become hard and brittle, allowing oil to leak from the engine.

When the crankshaft rotates, the seal grinds against the seal hub. This rotational force can also cause the rubber seal to wear faster.

, Rear Main Seal Replacement Tips

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: That being said, most vehicles never need a rear main seal replacement. Sometimes the rear main seal is blamed for oil that is coming from somewhere else, so make sure before you decide to go after the rear main seal.

How Much Does a Rear Main Seal Replacement Cost?

A rear main seal can cost anywhere from $20 to $80, but hiring a mechanic to replace it for you will be much more expensive. A rear main seal replacement can be time-consuming and labor-intensive, which is why most mechanics will charge you around $600 to $900 for it.

How To Replace the Rear Main Seal

Labor costs can be expensive, which is why plenty of gearheads prefer replacing their rear main seals by themselves. It’s not the easiest DIY repair out there, but there are a few ways you can make things easier. Here are a few tips to consider:

Preparing Your Vehicle

Before anything else, you’ll have to raise your vehicle off the floor. Giving yourself enough room to work under your car will make things easier, so be sure to adjust your jack stands accordingly.

Wearing Protective Gear

You can never be too careful when it comes to DIY repairs. Wearing protective gear such as gloves and glasses can keep your skin and eyes safe from oil, dirt, and other debris.

Disconnecting the Battery

To access the rear main seal, you typically have to remove a few components first. Disconnect your car battery’s negative cable before you begin. This should help prevent short circuits, especially if you have to remove the transmission.

See also  Crankshaft Seal Problems: What Is It, Symptoms, and What To Do

Removing the Exhaust System (if necessary)

Depending on your vehicle, you might have to first remove parts of the exhaust system before you can get to the rear main seal. In some vehicles, the exhaust system blocks the transmission, which you usually have to remove to access the rear crankshaft seal.

If you aren’t 100% sure about removing the exhaust system, consider asking a trusted mechanic for advice.

Removing the Driveshaft

Aside from the exhaust system, you might have to remove the driveshaft too. You’ll need a wrench set, sockets, a torque wrench, and a pry bar to properly remove it. Consider supporting the transmission with a sturdy support beam or a transmission jack to make things easier.

Removing an Automatic Transmission

An automatic transmission has two cooler lines connected to the radiator, which you’ll have to disconnect to safely remove the transmission.

You’ll also need to remove the crossmember and any wires connected to the transmission, and you’ll need to access the torque converter bolts. Usually this means removing the starter, which, on most vehicles, is bolted to the transmission. Never turn the engine backwards while accessing the torque converter bolts or you could throw the engine out of time.

Make sure you turn the engine the way it turns when running if you’re using a breaker bar on the crankshaft bolt to turn the engine.

You need a line wrench or the right special tool to disconnect the cooler lines. After you get the transmission out of the way, you’ll need to remove the flex plate (“flywheel” as some call it) and the big plate that is sandwiched between the engine and transmission. Make sure you put this back, too.

Removing a Manual Transmission

Keep in mind that the process of removing a manual transmission will differ from an automatic one. Before you can remove the flywheel you’ll need to remove the clutch and pressure plate.

Removing the Rear Main Seal Housing

Again, on some vehicles, you’ll find a thin metal plate (usually aluminum) between the engine and transmission that is cut to exactly match the shape of the mating surface, and it will also have a hole for the starter to pass through it. If your car has this, use a large screwdriver or pry bar to remove the engine plate. This can help you access the rear main seal housing.

See also  Crankshaft Seal Problems: What Is It, Symptoms, and What To Do
image of an unbolted rear main seal
Note that you won’t always find the seal in a housing, so be prepared for this. The seal in the photo is in a housing that has already been unbolted from the engine. Notice while you’re here whether any of the expansion plugs on the back of the block are leaking (see top right of photo) and if they are, now is the time to replace them. | Image Source: Richard McCuistian

Removing the Rear Main Seal

Removing the housing usually causes the rear main seal to come off as well. In some cases, however, the seal can get stuck on the crankshaft. Be careful not to force the seal off if this happens to you. Instead, try gently prying it off with your hand, so you can avoid damaging the crankshaft.

Matching the New Rear Main Seal

Before installing your new seal, you might want to compare it with the old one first. They don’t have to be exactly the same, but it’s best if they have the same inner and outer diameters.

Cleaning the Seal Housing

Clean the seal housing well before you install the new seal. You can use a gasket scraper and carburetor cleaner to remove the old debris, oil, and grime from it.

Installing the New Rear Main Seal

When installing your new rear main seal, you have to be careful not to install it at an angle. Doing so can deform the seal, which can lead to engine oil leaks. Most seals will come with an installation tool, so be sure to follow its instructions carefully.

Preparing the Block and Crankshaft

To mount the seal housing properly, you’ll have to first clean the crankshaft and gasket surface. You can use a shop towel and carburetor cleaner for this, but be careful not to let any cleaner or dirt get into the oil pan. Consider placing a clean towel over the oil pan for extra protection.

Mounting the Seal Housing

When mounting the seal housing, you might find it hard to secure the lip of the seal to the crankshaft. Don’t use any metal tools to force the lip over the crankshaft unless you want to damage the seal.

Where Is the Rear Main Seal Located?

On most vehicles, the rear main seal’s location can be found on the rear side of the engine, where the crankshaft connects to the transmission. Because of its location, it can be hard to get to without first removing other parts.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : Transmission , DIY Tagged With :
Garage Essentials
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