Understanding Rear Main Seal

The rear main seal keeps oil in your engine and prevents it from seeping out of the crankshaft and into the gearbox. To gain access to this location, the gearbox must normally be removed, which can result in an expensive repair bill. However, depending on the extent of the damage, you may be able to avoid a full repair if your automobile experiences a rear main seal failure.

Rear Main Seal

Well, in this article, we’ll delve dip into the rear main seal, providing you with an overview. That said, we’ll be addressing the following questions:

  • What Is a rear main seal?
  • How do you tell you to have a bad or leaking rear main seal?
  • What Happens If the Rear Main Seal Goes Bad?
  • Why does the seal leaks oil?
  • Is a Rear Main Seal Leak a Serious Issue?
  • How much does it cost to replace the rear main seal?


Rear Main Seal Overview

The rear main seal keeps oil sealed inside the engine’s rear, where the crankshaft meets the gearbox. It can be costly to repair, often costing $650 or more, because it often necessitates the removal of the gearbox and, in some cases, all or part of the engine.

Most front-wheel-drive vehicles have the engine situated transversely, so the rear of the engine is the end opposite the one with the pulleys and drive auxiliary belt. An oil leak from the main seal, also known as the rear crankshaft seal, will often drip from the location where the gearbox links to the engine, though it may appear elsewhere, such as on the oil pan.

Rear main seals can be constructed of rubber or silicone and can wear out due to age, crankshaft rotational forces, corrosion from road salt, and other environmental variables. The standard solution for a leaking main seal is to replace it, but due to the high labor costs, some car owners utilize oil additives designed to restore seals.

This can, in some situations, stop or slow leakage. Other drivers may opt for a thicker oil, such as 10W30 rather than 5W20. When a gearbox has to be removed for any reason, mechanics frequently recommend replacing the rear main seal, even if it isn’t leaking, because much of the disassembly work will already be done.

What Is a rear main seal?

A rear main seal is also known as a rear crankshaft seal since it is placed on the engine’s rear side, where the crankshaft links to the gearbox. The seal’s purpose is to keep engine oil from seeping out of the crankshaft. The crankshaft of your car is subjected to a great deal of force.

It houses the pistons and is linked to the flywheel and pulleys that power all of the engine’s components. Because of the strain and stress placed on the crankshaft, it is supported by some main bearings. These primary bearings keep the crankshaft in place and allow it to rotate while also enabling oil to travel through. This maintains it lubricated and protects it from friction damage.

To keep the oil in, the rear main seal rests outside the rear main bearing. The seal, which is usually constructed of silicone or rubber, wears out due to friction between the seal and the crankshaft. Low oil levels can starve the rear main seal of oil, causing excessive friction between the seal and the crankshaft, degrading the seal and resulting in an oil leak.

What Happens If the Rear Main Seal Goes Bad?

When the rear main seal fails, oil leaks from the camshaft into the gearbox bell housing. While the seal does not usually cause substantial or catastrophic engine difficulties, the oil leak it produces can lead to more serious problems. If the seal is badly worn, it might create a rapid oil leak, depleting the reservoir quickly. This can result in catastrophic internal engine breakdowns and costly repairs. More typically, the rear main seal deteriorates over time, beginning with a minor oil leak and progressively increasing in size.

While we recommend repairing a faulty seal as soon as possible, it isn’t always necessary because it causes a minor oil leak. In the best-case scenario, removing the gearbox is required to replace the rear main seal. However, depending on the placement of the oil pan and the orientation of your engine, you may need to remove the oil pan and/or the entire engine. As a result, replacing the seal can be an expensive repair job that you may not always be able to solve right away.

How do you tell you to have a bad or leaking rear main seal?

The following are the most common way you can tell you have a bad or leaking rear main seal:

  • The Oil Light Is On
  • There’s an Oil Spill in The Driveway
  • Underbody Oil Saturation of a Vehicle
  • The Need for Frequent Top-Ups

The Oil Light Is On

If you’re driving along and suddenly notice your vehicle’s low oil indicator illuminate, there are two things you should do. The first thing you should do is pull over to the nearest shoulder of the road and turn off your motor as soon as feasible. The second critical duty is to determine where your engine’s oil went. In this type of situation, the rear main seal of an engine is doubtful.

When an engine is warmed to operating temperature and the affected vehicle is in motion, a serious rear main seal leak can cause rapid oil loss. If oil loss surpasses a certain point set by the manufacturer, a low oil light will illuminate the dashboard. Similarly, certain vehicles will show a low oil pressure indicator or warning in the same way.

There’s an Oil Spill in The Driveway

When an engine has warmed up to its normal operating temperature, rear main seal leaks tend to worsen. As a result, a leaking rear main seal is frequently most noticeable minutes to hours after a vehicle has been parked for the day. This, in turn, causes oil to pool beneath a vehicle’s engine, leaving clear proof of such problems.

As a result, the unexpected presence of an oil stain within the driveway is cause for alarm and should be thoroughly investigated to establish its source. It is also probable that the oil pooling will intensify over time.

Underbody Oil Saturation of a Vehicle

Because a leaky rear main seal causes oil to accumulate within an engine/transmission’s bell housing, blowback can cause considerable oil saturation of a vehicle’s underbody. As a vehicle is driven, leaked oil is swept backward, coating everything behind the rear of the engine.

This emphasizes the need of doing regular vehicle inspections. In the vast majority of cases, simply looking beneath your car will reveal such tell-tale indicators of rear main seal failure. This allows you to arrange for repair before the causing leak worsens.

The Need for Frequent Top-Ups

Rear main seal leaks are notorious for leaking a large amount of oil in a short period of time. In severe circumstances, a leaking rear main seal might leak up to a quart of oil in a week. This high oil loss will demand frequent top-offs to prevent an engine’s oil from becoming dangerously low.

A worn rear main seal could be to blame if a weekly inspection of our vehicle’s engine reveals unexpected oil loss, or if you suddenly find yourself pouring more oil into your engine than usual. As a result, more time should be spent determining the cause of this increased oil use.

Why does the seal leaks oil?

While natural wear and tear might cause difficulties with the rear main seal, there are a few other potential issues that can cause a leak or failure. Some failures are natural, while others are not. The following are the most typical reasons for rear main seal leaks:

  • Worn main bearings.
  • Misalignment of the crankshaft or gearbox.
  • PCV valve clogged or damaged.
  • Low oil levels.
  • Natural aging and wear and tear.

Worn main bearings

The crankshaft is held in place by the main bearings, allowing it to revolve in a fixed position. The main bearings, like the crankshaft, are comprised of metal and can wear out due to friction. When these bearings wear down, some slop’ is created in the crankshaft, causing it to move or wobble slightly. This puts undue strain on the main seal, causing it to degrade quickly.

Unfortunately, poor main bearings will also cause a number of performance concerns, necessitating the replacement of the bearing as well as the crankshaft. Because of the location of the bearings, the engine will have to be disassembled, which will be an expensive repair.

Misalignment of the crankshaft or gearbox

A misaligned crankshaft will not rotate in a perfect circle, putting extra strain on the rear main seal. Furthermore, in manual transmissions, the input shaft or flex plate can be misplaced or broken, putting stress on the seal and causing it to break. These problems are less common unless you have had a gearbox or crankshaft repair, which could have resulted in misalignment during the installation.

PCV valve clogged or damaged

Engines emit “blow-by gases,” which occur when oil, gas, and exhaust fumes pass through the piston rings and end up in the crankcase. The PCV valve is in charge of directing this noxious gas back into the intake tract, where it is reburned. This decreases pollutants and relieves pressure in the crankcase. A clogged PCV valve prevents blow-by gases from escaping out of the crankcase, resulting in increased pressure inside the engine. Excess pressure exerts outward forces on the rear main seal, causing it to fall out of place and destroy the seal it produces between the crankshaft and the gearbox. This will eventually lead to an oil leak.

Low oil levels

When an engine runs on low oil levels, it deprives numerous components of oil and lubrication. This causes increased friction and heat inside the engine, which wears out the rubber seal and causes it to leak. Low oil levels can also lead to a variety of other problems, such as damaged main bearings. As a result, it is critical to maintain your engine oil topped up.

Natural aging and wear and tear

Because the crankshaft repeatedly rotates against the rear main seal, it is usual for the seal to wear down and leak over time. When this happens, you will usually notice a little leak that eventually becomes larger. Other causes usually result in a larger leak that spreads faster. So, if you have a little, slow leak, it is most likely the result of regular wear and tear over time. The greatest way to extend the life of the rear main seal is through preventative maintenance. This covers routine oil and filter replacements as well as engine oil replenishment.

Is a Rear Main Seal Leak a Serious Issue?

A rear main seal leak is often considered to be rather dangerous in nature, owing to the fact that such leaks only develop over time. In severe circumstances, a rear main seal might leak to the point where it is difficult to keep up with the consequent oil loss. Simply, adding enough oil to maintain a proper lubricant supply becomes difficult.

However, this is a considerably better scenario than what will likely happen if an engine runs out of oil due to a rear main seal leak. Any significant loss of lubricating oil increases the chance of bearing deterioration, which can lead to engine failure. Running an engine low on oil can and will result in premature internal wear. In any case, a rear main seal leak should be rectified as soon as possible.

While correcting a leak of this kind might be time-consuming, it is far less bothersome than having to replace an engine due to the long-term implications of chronic oil loss. If you are not capable of performing such repairs yourself (and most people are not), schedule an appointment with a reputable service center as soon as feasible.

Can You Drive with a Leaking Rear Main Seal?

Resolving a rear main seal leak can be a tedious and time-consuming task.  Many times, mechanics’ bids for repairing a rear main seal are so excessive that it appears to be a better option to drive the vehicle and simply add oil on a regular basis to keep your car operating. Here are some reasons why that is a bad idea. While driving, the leaking oil will spread throughout the bottom of your vehicle.

This includes your vehicle’s suspension components, exhaust pipe, and any hoses or cables.  Spreading oil on your hot exhaust pipe is, above all, a fire hazard! It can also cause unpleasant odors and plumes of smoke to emanate from your engine. Even if you don’t plan on keeping your car for long, letting a rear main seal leak go is a bad idea.

How much does it cost to replace the rear main seal?

There is no sugarcoating the truth that a rear main seal leak will be expensive to replace. Even more frustrating is the fact that most rear main seals are reasonably priced. However, the labor expenses associated with such repairs are frequently significant. These labor expenditures are, of course, easily justified. Replacing an engine’s rear main seal is a time-consuming and labor-intensive task.

This type of repair necessitates the removal of a vehicle’s gearbox, as access to an engine’s rear main seal is not otherwise feasible. Furthermore, in the case of four-wheel drive cars, the removal of the transfer case is frequently required.

The cost of replacing the rear main seal varies depending on the vehicle and is based on the manufacturer’s quoted flag time for such repairs. Pricing for rear main seal replacement is also affected by a vehicle’s drive layout. The cost of replacing the rear main seal normally ranges between $650 and $1,800, with the great majority of such repairs falling between the $800-$1,200 range.

Related Article

Watch the video below to learn more


Is a rear main seal leak serious?

Yes, a rear main seal leak is considered serious. It can lead to significant oil loss and potential engine damage if not addressed.

What are the symptoms of a rear main seal leak?

Symptoms of a rear main seal leak may include:

  • Engine oil leaking at a faster rate
  • Drips of oil from the bottom of the bell housing at the front of the transmission
  • Oil puddles on the garage floor or driveway
  • Low oil levels and frequent oil loss
  • Oil stains on the ground after parking the car
  • The engine oil light coming on

What causes the rear main leak?

Rear main seal leaks can be caused by various factors, including:

  • Old age and wear of the seal
  • Lack of use of the vehicle
  • Exposure to outside elements
  • Corrosion
  • Extreme operating conditions

Can a rear main seal cause an oil leak?

Yes, a rear main seal leak can cause an oil leak. It is one of the common causes of oil leaks in vehicles.

Can a rear main seal affect transmission?

While a rear main seal leak primarily affects the engine oil, it can indirectly affect the transmission if the leaking oil reaches the transmission components. However, the direct impact on the transmission is minimal.

What can a bad rear main seal cause?

A bad rear main seal can cause several issues, including:

  • Oil loss and potential engine damage
  • Reduced engine performance
  • Increased oil consumption
  • Environmental pollution due to oil leaks

Will the rear main seal leak while sitting?

Yes, a rear main seal can leak even when the vehicle is sitting. The leak may not be as noticeable when the engine is not running, but oil can still drip from the seal.

Will brake fluid stop a rear main seal leak?

No, brake fluid will not stop a rear main seal leak. Brake fluid is not designed to seal engine oil leaks and is not a suitable substitute for addressing a rear main seal leak. It’s important to have the seal properly repaired or replaced to prevent further issues.

Can a rear main seal be replaced?

Yes, a rear main seal can be replaced. However, it is a labor-intensive repair that often requires removing the transmission or engine to access the seal. It is recommended to have the replacement done by a skilled mechanic or at a reputable automotive service center.

What causes the main oil seal to leak?

The main oil seal, including the rear main seal, can develop leaks due to various factors, such as:

  • Wear and tear over time
  • Exposure to high temperatures and harsh operating conditions
  • Improper installation of the seal during previous repairs
  • Seal material degradation or damage
  • Excessive crankshaft movement or misalignment

Is the rear main seal the same as the crankshaft seal?

Yes, the rear main seal is also known as the crankshaft seal. It is located at the back of the engine and seals the interface between the crankshaft (which connects to the pistons) and the engine block. Its purpose is to prevent oil from leaking out of the engine.


The easiest solution to a rear main seal leak is to replace the component. However, as previously stated, this can be highly costly. If you need a temporary cure until you can afford a full repair, or if your car isn’t worth enough to justify the repair costs, you have a few additional options, such as utilizing an oil additive or thicker engine oil. That is all for this article. Hope you learn a lot from the reading. If you do, kindly share it with others. Thanks for reading; see you around!

Write A Comment