What Is a Rear Main Seal? (And How to Spot It Failing)
Look out for warning signs from your rear main seal and act as soon as you notice a leak.
However, oil leaks can lead to low oil levels, which can cause more serious engine problems.
Fortunately, a bad rear main seal will only result in an oil leak - it usually doesn't have any severe or catastrophic issues outside of the leak.
The gasket is made of rubber and can easily deteriorate, leading to oil leaks. These leaks can be caused by:
The rear main seal prevents oil from getting out of the crankcase and into the transmission housing.
The crankshaft is supported by main bearings, which receive oil to lubricate the crankshaft.
The rear main seal is essentially a rubber gasket that seals the rear of the crankshaft, exiting the engine and connecting with the transmission.
Read on and find out what a rear main seal is, the signs of failure, and the replacement and repair options for worn and leaking rear main seals.
If your car suffers a rear main seal failure, you might be able to avoid a full repair - depending on the damage suffered.
Even though the seal itself is a very inexpensive part, the overall repair cost will set you back £500 to £800, on average.
This location typically requires removing the transmission to access it, which can lead to a costly repair bill.
The rear main seal is essential for keeping oil in your engine and preventing it from leaking out of the crankshaft and into the transmission.
What Is a Rear Main Seal?
A rear main seal is often called a rear crankshaft seal, as it is located on the rear side of the engine where the crankshaft connects to the transmission.
The seal prevents engine oil from leaking out of the crankshaft.
Your car puts the crankshaft under a lot of force.
It holds the pistons and is connected to the flywheel and the pulleys that power all the engine accessories.
Due to the force and stress put on the crankshaft, it has some main bearings to support it.
These main bearings hold the crankshaft in place and allow it to rotate, as well as allowing oil to pass to the crankshaft.
This keeps it lubricated and avoids friction damage.
The rear main seal sits outside the rear main bearing to hold the oil in.
The seal is typically made of silicone or rubber and wears down from friction between the seal and the crankshaft.
If an engine runs on low oil levels, it can starve the rear main seal of oil, which will cause excess friction between the seal and the crankshaft, deteriorating the seal and resulting in an oil leak.
What Happens When the Rear Main Seal Fails?
Failure of the rear main seal usually results in an oil leak from the camshaft into the transmission bell housing.
While the seal generally doesn't cause any major or catastrophic engine problems, the oil leak it causes can potentially lead to more serious issues.
If the seal is badly worn, it can cause a fast oil leak, quickly draining the reservoir.
This can lead to severe internal engine failures and expensive repairs.
More commonly, the rear main seal tends to deteriorate over time and will usually start as just a small oil leak that gets gradually larger.
While we recommend repairing a bad seal as soon as possible, it isn't necessarily an urgent repair since it results in a small oil leak.
In a best-case scenario, replacing the rear main seal requires removing the transmission.
Nevertheless, depending on the location of the oil pan and the direction your engine is mounted, you may need to remove the oil pan to be and/or the whole engine.
Therefore, replacing the seal can be a costly repair job that you might not always be able to address immediately.
Symptoms of Rear Main Seal Failure
There are three common signs of a bad or failing rear main seal:
- Oil leak where the transmission connects to the engine
- Frequent low oil light from constant oil loss
- Oil stains on your driveway
Since seal failure results in an oil leak, the most common symptoms are low oil levels and stains left behind from your vehicle.
However, oil leaks can come from various parts of an engine, like the oil filter housing, valve cover gasket, PCV, head gasket, and so on.
Diagnosing a rear main seal failure can be tricky.
The telltale sign of a rear main seal oil leak is the leak's location.
Oil leaks from a rear main seal will always come from the rear side of the engine, right where it bolts up to the transmission.
Therefore, the easiest way to detect it is to check the rear of the engine for any visible signs of a leak.
Causes of Rear Main Seal Oil Leaks
While normal wear and tear can cause issues with the rear main seal, some other potential problems can lead to a leak or failure.
While some failures are natural, others are not.
The most common causes of rear main seal leaks are:
- Natural wear and tear/age
- Worn main bearings
- Low oil levels
- Clogged or bad PCV valve
- Crankshaft or transmission misalignment
Natural Wear and Tear
As the crankshaft is constantly rotating against the rear main seal, it is common for the seal to wear down over time and begin to leak.
When this happens, you will typically notice a small leak gradually getting more significant.
The other causes usually result in a bigger leak that expands more quickly.
So if you have a small and gradual leak, it is likely from normal wear and tear over time.
Preventative maintenance is the best way to prolong the life of the rear main seal.
This includes frequent oil and filter changes and topping up with engine oil.
Worn Main Bearings
The main bearings hold the crankshaft in place, allowing it to rotate in a fixed position.
Main bearings are made of metal, as is the crankshaft, and can wear down from the friction of the crankshaft.
When these bearings wear down, it can create some 'slop' in the crankshaft, allowing it to move or wiggle slightly.
This then puts excess force on the main seal, which can cause it to deteriorate quickly.
Unfortunately, bad main bearings will also lead to several performance-related issues and require a new bearing and possibly a new crankshaft.
Due to the location of the bearings, it will require the engine to be taken apart, making it a costly repair.
Low Oil Levels
When an engine runs on low oil levels, various components are starved of oil and lubrication.
This results in more friction and heat inside the engine which then takes a toll on the rubber seal causing it to wear down and leak.
Low oil levels can also cause a number of other issues, like worn main bearings, so it is essential to keep your engine oil topped up.
Clogged PCV Valve or Failure
Engines produce what is called "blow-by gasses" which occur when oil, gas, and exhaust fumes make it past the piston rings and end up in the crankcase.
The PCV valve is responsible for venting this bad gas back into the intake tract, where it is reburned.
This reduces emissions and releases the pressure it creates in the crankcase.
A clogged PCV valve will prevent the blow-by gasses from venting out of the crankcase, which causes pressure to build inside the engine.
The excess pressure puts outward forces on the rear main seal, which can cause it to slip out of place, breaking the seal it creates between the crankshaft and the transmission.
This will ultimately result in an oil leak.
Crankshaft or Transmission Misalignment
A misaligned crankshaft won't rotate in a perfect circle, putting excess stress on the rear main seal.
Additionally, the input shaft on manual transmissions or flex plate on automatic transmissions can be misaligned or damaged, putting stress on the seal and causing it to fail.
These issues are less common - unless you have recently had a transmission or crankshaft replacement resulting in a potential misalignment during the installation.
Can You Drive with a Rear Main Seal Leak?
If you think that your car's oil level is too low, then you should stop driving the vehicle.
Driving with a rear main seal leak is dangerous, and you could cause permanent engine damage if you continue to do so.
Rear Main Seal Replacement Options
The simple answer to a rear main seal leak is to replace the part.
However, as we've discussed, this can be very expensive.
If you are looking for a temporary solution until you can afford a complete fix or if your car isn't valuable enough to justify the repair costs, you have a few other impermanent options.
We would strongly recommend getting a professional opinion from a garage to diagnose the cause of the issue before attempting any of the DIY fixes below.
Use an Oil Additive
Oil system additives - such as BlueDevil's Rear Main Sealer - are added directly to your engine's oil to repair gasket and seal leaks.
The additive creates a chemical reaction that causes the rear main seal to expand, stopping the leak.
It works on any rubber seal, whether cracked, dried, or deformed.
You can use it more than once if the seal continues to wear down and leak. This is an effective short-term solution but will not permanently fix the issue.
However, for older and less valuable vehicles, it is a great £15 fix compared to the cost of replacing the seal.
Use a Thicker Engine Oil
Using thicker engine oil will slow the rate of any oil leak, as thicker oil has a harder time getting through small cracks.
Switching from something like 5w-20 to 10w-30 can drastically reduce the rate of any oil leaks.
Again, this temporary fix can be used with a seal repair additive, as mentioned above.
Above are only temporary methods to deal with the rear main seal failure.
In the long run, we recommend booking an appointment with a local garage.
A professional garage will carry out an inspection to find the problem and will provide you with a transparent quote for a rear main seal replacement if required.
Compare garages near you instantly with BookMyGarage to find the best deals.
You can see instant prices and availability, and won't have to pay until the work is complete.
3 Reasons for Oil Seal Failure and Their Solutions Discussed
3 Reasons for oil seal Failure and Their Solutions Discussed
The oil and gas industry is one of the most challenging industries due to adverse conditions and demands involved. A few engineered components such as oil seals are required to perform in these adverse conditions, where they may be exposed to various extreme conditions. Although most of these seals are precision designed for a long-term performance, they may fail due to several reasons. This post gives a brief insight into the challenges that the seals are exposed to, and explores various reasons behind their failure.
An Overview on the Challenges Endured by Gas and Oil Seals
Mechanical seals are always designed in adherence to the conditions of a particular application where they may be used. Oil and gas seals are designed to resist the following:
- Aggressive Corrosive Chemicals
- Sour Gas (H2S)
- Decomposition
- Explosive Conditions
- Extreme Pressures and Temperatures
Analyzing the Top 3 Reasons for Failure of Oil Seals and Corrective Actions
Like any regular mechanical seal, oil seals fail due to several reasons or one of these reasons. Here, the failure reasons are also followed by their solutions.
- Chemical Attack: As said, the oil seals are exposed to several types of chemicals. The seal may exhibit a variety of signs ranging from cracking, blisters, and discoloration to a change in hardness levels. Why does this happen? The material of your seal is not compatible with the chemicals it is being exposed to. This chemical attack will affect its cross-link density, increasing or decreasing it. The seal material becomes hard and brittle when this cross-link density increases and becomes softer when the cross-link density decreases.
Solution: Selection of the right sealing material is important for ensuring the compatibility of the seal with chemicals. This chemical incompatibility leads to chemical attacks, which accelerate at high temperatures or pressures. So, check the compatibility of the elastomer seal material before choosing it for your application.
- Extrusion and Nibbling: At times, the oil seals may have a chipped or nibbled appearance. In extreme cases, the surface of these seals may peel on its own, thereby lending it a shaved appearance. What is the cause of extrusion and nibbling? Stress induced by high pressure is one of the main reasons for extrusion and nibbling. This stress widens the clearance gap between the mating edges, where the seal may get entrapped, and it leads to severe physical damage.
Solution: Many industry experts suggest using backup devices to avoid the formation of clearance gaps between the mating edges. The selection of a properly sized oil seal is also important to avoid extrusion and nibbling.
- Out-gassing: This is one of the common causes of oil seal failure, which happens due to the volatility of any of the constituents of the elastomer. These constituents may be the part of elastomer formulation, or gases may get trapped in the elastomer during the process of molding. Although the oil seals may not show any visual indications of out-gassing, at times they may shrink due to extreme out-gassing. The out-gassed molecules may cause contamination in wafer processing in several semiconductor applications, and also it can affect vacuum performance in industrial applications.
Solution: Use the materials that are rated to the temperature of application. Also, ensure that the elastomer used is free from volatile ingredients such as waxes, plasticizers, and so on. Both these will help you avoid the chances of outgassing.
The above mentioned are only the top three causes of oil seal failure; however, there can be several other reasons for the same. In addition to these solutions, it is important that you source these seals from some trusted manufacturer such as SSP Seals. The company provides customized oil seals in different specifications to its clients in the oil and gas industry.
Related Post
Oil Seals (Part 3): Handling of seals and seal failure causes and countermeasures
In Part 2, we explained how to select the right oil seal.
Oil Seals (Part 2): How to select the right oil seal
In order for the sealing property of the oil seal you selected to really shine, attention needs to be paid to how it is handled. In the event of seal failure, it is necessary to take effective countermeasures beforehand. Therefore, this column will explain the handling of seals, seal failure causes, and their countermeasures.
1. Oil seal storage, handling, and mounting
Carelessness in seal handling may cause seal failure. For this reason, it is necessary to take care to perform appropriate storage, handling, and mounting.
1) Storage
Table 1 shows the main points of seal storage.
Table 1: The main points of seal storage
No. Main points to be aware of Remarks 1 Keep at room temperature (max. 30°C) and humidity 40 to 70% on average. See Figure 1. 2 Avoid direct sunlight and electrical devices that emit ozone. 3 Do not hang them (to prevent lip deformation and failure). 4 Store in a tightly sealed container and protect from contaminants such as dust and sand. 5 If storing for a long time, use the ones with the earliest manufacture date first.Figure 1: Storage method and conditions
2) Handling
Table 2 shows the precautions for handling oil seals.
Table 2: Precautions for handling oil seals
No. Main points to be aware of 1 When carrying, avoid excessive impact. 2 When opening the wrapping with a sharp object such as a knife, do not damage the seals. 3Do not leave seals out on a table (to prevent the adhesion of contaminants).
4 Do not hang them (to prevent lip deformation and failure). 5 Use kerosene for cleaning.N.B.
Using abrasive cleaners, solvents, or the like will negatively affect the rubber material.
3) Mounting
Table 3 shows the precautions for mounting oil seals.
Table 3: Precautions for mounting
No. Main points to be aware of Remarks 1 Before mounting, confirm that there is• no dirt,
• no foreign particles, and
• no damage
on the seals. Prevents the entry of foreign matter 2 Feed (apply) lubricant for pre-lubrication (initial lubrication).
• Use clean lubricant for the lip.
• Use clean grease between the main and minor lips (see Figure 2).
See the list below for recommended grease. Prevents heavy wear on lip 3 Recommended grease to be used (applied) between the main lip and the minor lip:
• Soft grease (small penetration)
• Small penetration change by temperature
• Wide serviceable temperature range
• Lithium base type N.B.
Avoid using the following combinations of rubber and grease, as they may cause the rubber to deteriorate or harden
Ex. 1: Silicone base grease
for silicone rubber seals
Ex. 2: Urea base grease
for fluoric rubber seals 4 When the seal is mounted in a cold area, it should be warmed up and its lip flexibility restored before mounting. 5 When the structure is such that the component is mounted on the shaft with an interference fit, reducing the lip contact surface shaft dimensions to be roughly 0.2 mm smaller than the bore diameter of the component helps prevent damage on the shaft surface (see Figure 3). Prevents abnormal contact between the lip and the shaft 6 When the seal is pressed into a housing bore, use a pressing jig and mount it perpendicularly to the shaft and avoid failures such as scuffing on the fitting surface (see Figure 4). Prevents fitting surface failure
Prevents oil seal lifting (spring back) on the rubber O.D. wall 7 If the shaft has a spline, keyway, or holes,
• use a seal-protecting jig (see Figure 5)
• If a seal-protecting jig cannot be used,
round the edges of the spline/keyway and apply sufficient grease to the area in question before carefully mounting. Prevents lip failure when the oil seal passes above the shaft 8 If mounting a long shaft,
or if mounting heavy housing,
use a guide jig to center the seal and shaft and prevent failure resulting from the part of the seal knocking against the shaft (see Figure 6). Prevents failure caused by contact with the shaft 9 If removing the oil seal, use a new oil seal and spacer so the sealing edge doesn't overlap with the trace of the previous lip (see Figure 7). Prevents abnormal contact between the lip and the shaft
Figure 2: Pre-lubrication for seals with minor lip
Figure 3: Recommended shaft profile and machine construction to avoid damaging shaft surface
Figure 4: Recommended seal press-fitting jig
Figure 5: Seal protecting jig for spline, keyway, holes on shaft
Figure 6: Guide jig for inserting of long shaft into seal bore
Figure 7: Avoid old seal lip track
For a more detailed discussion of seal handling, please see the following:
Oil seal storage, handling, and mounting
2. Seal failure causes and countermeasures
Leakage from the seal can be broadly divided into "leakage from lip" and "leakage from seal fitting area."
Figures 4 and 5 show the types of failure and the appearances of "leakage from lip" and "leakage from seal fitting area" respectively.
Table 4: Failure (leakage from lip)
No. Failure Appearance 1 Damage on sealing edge 2 Lip turned backward 3 Missing spring 4 Lip hardened 5 Lip softening 6 Heavy wear on shaft
Reduction of tightening interference due to rubber hardening
Table 5: Failure (leakage from seal fitting area)
Tables 6 and 7 show the major failure types, their causes, and countermeasures against them.
Table 6: Seal failure causes and countermeasures (leakage from lip)
No. Failure Causes Countermeasures 1Damage on sealing edge
1) Sharp edge or burrs on shaft chamfer
2) Shaft spline or keyway
3) Entry of foreign matter
4) Poor handling
• Remove burrs and polish
• Use shaft-protecting jig (see Figure 5)
• Clean surrounding components
• Improve manner of handling
Lip hardened
1) Temperature exceeded seal service temperature range
2) Poor lubrication
3) Excessive inside pressure happened
• Change rubber material to high-temperature-proof rubber
• Improve lubricating method and lubricant supply volume
• Apply high-pressure-proof seal or breather (vent)
Heavy wear on shaft
1) Entry of foreign matter
2) Chemical wear due to high temperature or excessive-pressure additives
3) Poor lubrication
4) Stick slip
• Attach prevention device for entry of foreign matter
• Take countermeasure to prevent high temperature and change lubricants
• Improve lubrication on lip including pre-lubricating (improve quantity of lubricant or lubricating method)
Heavy wear on lip
Excess heat generation due to
1) Poor lubrication
2) Running under conditions beyond specifications
a) Excess peripheral speed
b) Excessive inside pressure
• Improve lubrication
(change machine structure)
• Examine cause of heat source
• Change rubber to heat-proof rubber
• Apply high-pressure-proof seal or breather (vent)
Blister on lip
Expansion of agglomeration of high-temperature oil entering into the sliding surface
a) Deterioration of lubrication
b) Mirror finish on shaft surface
c) Higher peripheral speed
d) Higher lip radial load
• Improve lip lubrication
• Correct shaft surface finish
• Reduce lip radial load of oil seal
Table 7: Seal failure causes and countermeasures (leakage from seal fitting area)
1) Burrs on housing bore
2) Damage or blowholes on housing bore
• Remove burrs and chips
• Repair housing bore to eliminate damage and blowholes
1) Larger housing bore
2) Smaller oil seal O.D.
3) Improper oil seal press-fit position
4) Deformation of housing
• Use appropriate housing bore diameter
• Correct the oil seal press-fit position
• Improve housing rigidity
For details of failure causes and countermeasures, please see the following:
Seal failure causes and countermeasures
To identify the causes of seal failure and take proper measures, it is critical to observe the seal lip closely and evaluate the failure in all respects, such as shaft surface roughness, contaminants, and lubrication.
3. Conclusion
In this month's column, "Handling of seals and seal failure causes and countermeasures," we conveyed the following points:
1) Carelessness in the storage, handling, and mounting of oil seals may cause seal failure. For this reason, you must take sufficient care in storage, handling, and mounting. A jig should also be used in mounting oil seals.
2) Leakage from the seal can be broadly divided into "leakage from lip" and "leakage from seal fitting area," and various types of failure are categorized according to their appearance.
To identify the causes of seal failure and take proper countermeasures, it is critical to observe the seal lip closely and evaluate the failure in all respects, such as shaft surface roughness, contaminants, and lubrication.
