Why Do I Have a Rear Main Seal Leak?

Why Does a main seal leakYou may have a rear main seal leak for a variety of reasons.  First we will take some time talking through what rear main seals are designed to do, then we will look at the causes for a leak.  The rear main seal in your vehicle holds the engine oil in the oil pan while allowing the crank shaft to exit the engine block.  The bearings that support the crank shaft are called the main bearings in your engine, so the rear main seal is a seal just outside of the last main bearing at the rear of your engine.  The rear of the engine is the side opposite, all of the pulleys and accessories like the power steering pump or alternator.  Your rear main seal may be one or two pieces and is likely made out of a special type of rubber or silicone that is designed to handle the temperatures and chemicals in your engine oil.  Ideally the outside of the rear main seal will press tightly against the engine block and/or oil pan not allowing engine oil to leak out and the inside of the rear main seal will slide over the crank shaft keeping oil from slipping past it as the shaft spins.

Your rear main seal may develop a leak for a variety of reasons:

  1. Old Age
  2. Lack of use
  3. Outside elements
  4. Corrosion
  5. Extreme operating conditions

Anytime you have a surface rubbing on another, no matter how much lubricant you use or how smooth the surfaces are, overtime one will wear down.  In the case of your rear main seal, rubber is softer than metal so the rubber seal will wear down first causing a leak.

A lack of use is one of the silent killers of the seals and gaskets in the engine of your vehicle.  Regularly driving your vehicle will keep the oil circulating though the engine and this will help keep all the seals soft and conditioned.  Without a regular oil bath, your rear main seal can become dry and hard causing it to not seal properly around the rotating crank shaft.  Also, when your vehicle is not running, the crank shaft will sit down on the bearing surfaces they ride in.  This very small movement stretches the rear main seal opening just slightly, but if left for a long time, can permanently deform your rear main seal causing a leak around the crank shaft.

Most vehicles have inspection covers or skid plates to keep the elements like water, sand and dirt away from the outside of the rear main seal, but over time, some outside elements can make their way to the seal.  Things like sand, salt or road grime can cause premature failure of your rear main seal from the outside in.

Rear Main SealThere are many reasons you should regularly change your engine oil.  Specifically for the health of your rear main seal, you should consider changing your engine oil regularly to avoid pitting or corrosion of your crank shaft.  Old oil can become acidic over time as it absorbs water and goes through the heating and cooling cycle of a running motor.  This acidic oil sits between the rear main seal and the crank shaft lubricating it, but also corroding and possibly pitting your crank shaft.  This corrosion will create a rough surface which will quickly wear out your rear main seal, causing a leak.

Extreme operating conditions like extraordinarily high heat or foreign chemicals can also cause rear main seal failure.  High heat can cause seals to get brittle or hard and chemicals can cause reactions with the materials some seals are made from.  For example, try leaving a pair of rubber gloves in transmission fluid overnight and they will be brittle and hard in the morning.  Be careful to operate your vehicle only with proper coolant temperatures and never put abnormal chemicals in your engine oil.

Some oil leaks can easily be stopped with our Oil Stop Leak, however, if you do find yourself with a rear main leak you can fix it using BlueDevil Rear Main Sealer.  BlueDevil Rear Main Sealer will restore your worn, dried, cracked or distorted rear main seal back to its originally shape, size and flexibility sealing your leak and returning your car to reliable operation.  We’re so confident in our Rear Main Sealer that it is guaranteed to permanently seal your leak!

BlueDevil Rear Main Sealer

Regardless of the cause of your leak, try BlueDevil Rear Main Sealer available online here: Rear Main Sealer

You can also purchase any BlueDevil product at our partnering local auto parts stores like:

  • NAPA
  • Pep Boys
  • Prime Automotive Warehouse
  • CarQuest Auto Parts
  • Advance Auto Parts
  • AutoZone
  • Bennett Auto Supply
  • O’Reilly Auto Parts

Pictures Provided by: www.neown.net and nesengineering.blogspot.com

16 responses to “Why Do I Have a Rear Main Seal Leak?

  1. this is some good stuff, had a small leak that I had been chasing for awhile, first oil sandwich gaskets and O-rings to oil pan, but I said its maybe the rear seal, But still had a small leak after all the work was done ,, I have used, bars stop leak and other stuff in past but only temporary,,, guy at orileys and my neighbor said pay the extra and try bluedevil it its guaranteed,, bought it put it in and I drove it for one day,, and now the engine bottom is bone dry of leaks , this is a great product,, highly recommend it , thanks BLUE DEVIL REAR MAIN SEALER…

    1. Ronnie-

      Once the product seals, it would create a permanent seal. Please contact our technical support line at 888-863-0426 so that we can get a better understanding of the vehicle’s condition and be able to make any appropriate recommendations.

      Thank you!


    1. Daniel-

      Roughly how many miles have you driven since BlueDevil was added? It usually takes between 100 – 200 miles of driving before you will start to see full results.

      Feel free to contact our tech support team at 888-863-0426 with any further questions.

      Thank you!


    2. Daniel-

      Thank you for asking about your Chevy Silverado. Roughly how many miles have you driven since adding the product? Please contact our technical support line at 888-863-0426 so that we can get a little more information on the vehicle and assist you further.

      Thank you!


  2. Old 1975 232 Jeep I6- my mechanic has replaced the rear main seal and although there is much improvement, I still have a minor leak. He showed me photos – the engine block is worn and as a result even the new seal won’t totally stop the leak. Will Blue Devil Rear Main help?

    1. Ronald-

      Thank you for asking about your 232 Jeep 16. The BlueDevil Rear Main Sealer works by revitalizing and reconditioning the rubber seals. It would not be filling in a gap or plugging a hole. Being that the seal is new, unfortunately, it does not seem the product would be able to help in this case.

      Thank you!


    1. Joe-

      It is possible that excessive engine heat could cause a rear main seal to leak. However, the seal is made of rubber so it usually starts to leak due to time as regular wear and tear will eventually start to deteriorate the gasket.

      Thank you!


  3. i have a morris 1000 traveller 1971 model,recently overhauled the engine new piston rings new crankshaft and all new shell bearings and many more new parts etc after running for a few thousand miles an oil leak developed at the rear crankshaft end this oil leak only seems to happen once the engine has stopped and parked.now this engine design does not have a rear crankshaft oil seal,it has a reverse scroll on the inner part of the crankshaft which is designed to throw any oil back into the sump and not leak out through the crankshaft but it does.i have checked the crankcase /sump vacuum and that appears to be okay.i have used lucas oil stop leak and it improved the leak for a short time, i was about to try your product which i believe to be similar so i am not to sure if it will work any better as most of these products only seem to work where there are seals or gaskets involved, my only other option is to fit an after market oil seal, or new engine, the morris minor owners club said the crankcase and crankshaft are compromised (what ever that means) so live with the oil leak.i would like a second opinion, please advise, thankyou. ps the car runs fine apart from this annoying oil leak messing up the my limestone chippings drive and wherever it gets parked.

    1. Alan-

      Thank you for asking about your Morris Traveller. Based on your description, we recommend using the BlueDevil Oil Stop Leak. The Oil Stop Leak only needs to be used one time. Once the product seals, it would be a permanent seal. Feel free to contact our technical support line at 888-863-0426 with any other questions.

      Thank you!


  4. Can Blue Devil be used to fix leak on Main Bearing Oil Seal for Chev Utility 1.8 sport petrol. How long will it last or will it need to be done more than once

    1. Hannes-

      Yes, BlueDevil is safe and compatible to use for the leak you’ve described. You should only need to add the product once. Once the product seals, it would be a permanent seal.

      Thank you!


    1. Linda-

      BlueDevil Rear Main Sealer does not harden or thicken at any point. It is 100% safe and compatible to use in your Lexus. There is no risk associated in adding this product to an engine.

      Thank you!


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