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.
This location typically requires removing the transmission to access it, which can lead to a costly repair bill.
Even though the seal itself is a very inexpensive part, the overall repair cost will set you back £500 to £800, on average.
If your car suffers a rear main seal failure, you might be able to avoid a full repair - depending on the damage suffered.
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.
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The rear main seal is essentially a rubber gasket that seals the rear of the crankshaft, exiting the engine and connecting with the transmission.
The crankshaft is supported by main bearings, which receive oil to lubricate the crankshaft.
The rear main seal prevents oil from getting out of the crankcase and into the transmission housing.
The gasket is made of rubber and can easily deteriorate, leading to oil leaks. These leaks can be caused by:
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.
However, oil leaks can lead to low oil levels, which can cause more serious engine problems.
Look out for warning signs from your rear main seal and act as soon as you notice a leak.
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.
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.
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There are three common signs of a bad or failing rear main seal:
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.
Normal wear and tear can cause issues with the rear main seal, and certain problems can even lead to a leak or failure.
A diagram highlighting the main causes of rear main seal oil leaks.While some failures are natural, others are not.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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Having a problem with the rear main seal in your vehicle can be a scary thing, mostly because many of us don’t entirely know what that seal is or does. We’ve only heard horror stories about friends who had great reliable vehicles that developed a rear main seal leak and suddenly their car was only fit for a junk yard. Here at BlueDevil we’re hoping to change that myth.
The rear main seal is a seal in your engine that is very important and usually difficult to reach. If you pop your hood and start looking around for the rear main seal, you aren’t going to have much luck seeing it. It is buried deep, usually under a lot of protective covers, wires, hoses and unfortunately, the bell housing of your transmission. Let’s first break down the name to see what we can learn about the rear main seal.
First, the rear main seal is at the rear of your engine. This can be confusing today because the rear of the engine isn’t always at the rear of your engine bay. On a rear wheel or 4 wheel drive vehicle the engine is mounted with its front (the side with all the pulleys and belts) towards the front of the car. With modern front wheel drive cars, and some all-wheel drive vehicles the engine can be mounted transversely in the vehicle. This means that the front of the engine can be facing either the passenger or driver’s side of the vehicle. So the “rear” in rear main seal refers to the seal’s location on your engine, and not necessarily relative to your vehicle. The rear main seal is always on the opposite the side of your engine as the pulleys and belts.
Second, the rear main seal is a seal for the main crank shaft bearings in your engine. Since the crank shaft has the piston attached to it, the pulleys with all your accessories connected to it and the flywheel which connects to your transmission on it, it carries all the forces and power your engine creates. Since the crank shaft has the toughest job, it gets the biggest and best bearings to make sure it can stand up to the work load. These bearings are called the main bearings in your engine and carry the majority of the stresses on your crank shaft. The rear main seal is just on the outside of the rear main bearing and it keeps the engine oil from leaking out where your crank shaft exits the engine.
The outside circumference of the rear main seal creates a seal between the engine block and sometimes the oil pan as well. The inside of the rear main seal creates a seal on the crankshaft where it exits your engine to connect to the transmission. As the crank shaft spins, it can wear down the rear main seal, especially if a low oil condition exists in the engine, or the vehicle sits for a long time between uses.
Because of its location in the engine, replacing a rear main seal includes at least removing the transmission from your vehicle, if not completely removing the engine as well. Removing either the engine or the transmission from your vehicle is a difficult and time consuming job, usually making a rear main seal replacement a very costly project. This is why you hear so many horror stories about the unbelievable costs of fixing a leaking rear main seal.
The good news is that BlueDevil has created a way to seal your rear main seal leak easily without costly maintenance and long projects. BlueDevil Rear Main Sealer is a specially formulated additive that you add directly to your engine oil. BlueDevil Rear Main Sealer is not a petroleum distillate, powder, or hazardous chemical so it will not harm your engine in any way and can stay in your engine oil until your next oil change. Once BlueDevil Rear Main Sealer is in your engine it will revitalize the old seals you have, restoring them to their original size, shape and flexibility, sealing your rear main seal leak – guaranteed!