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How to Replace a Crankshaft Rear Main Seal Without the Hassle

a Crankshaft Rear Main Seal

a Crankshaft Rear Main Seal

Your car’s engine block and the crankshaft output flange has in-between them. And this is what you call the crankshaft rear main seal, which is designed to keep the engine oil from leaking. This rubber seal is placed in a steel plate and is mounted to the rear of the engine block.

The seal is subjected to the usual tear and wear, heat, and engine vibration causing it to fail over time. When this happens, the engine oil will leak and will create trouble on your engine when not replaced on time. You can take your car to a service center for a quick fix or you may do it yourself to save you a couple of a hundred dollars or more in parts and labor (some auto shops charged their clients for this repair between $650 to $800).

In this article, we will teach you the step by step instructions on how to replace a crankshaft rear main seal, so you can do it yourself if you want to.

Prepare these

  • Safety goggles.
  • Safety glasses.
  • Floor jack.
  • Wrench/ socket sets.
  • Screwdriver set
  • Sealant
  • Flat steel (2 x 6 x 1/8 inches in size)
  • Wooden hammer
  • Clean rags
  • New rear main crankshaft seal

Here’s what you should do

Wear your safety goggles and glasses before you start doing this activity. Begin with lifting your car on four jack stands by using a floor jack. Lift the car as high as possible to give you more space while working on your car and make sure no other objects are blocking your way, even the smallest ones.

1. Removing the battery’s connection

Remove your battery’s negative cable to avoid any electrical shock or short circuit while working on your car.

2. Removing the exhaust system

There is a need for your engine to be lowered down a little bit to make way for the removal of the transmission. Doing this requires the partial or complete removal of the exhaust system.

3. Removing the CV axles

Remove the CV axle to give way for the removal of the transmission or allow a clearance for its removal. This is required whether if you have automatic or manual transmissions, or front or rear wheel drive.

4. Removal of the transmission

The difference between an automatic transmission and a manual one is that the former has two cooler lines that go about into the radiator which the later do not have. These should be removed using an open line wrench while the torque converter should be pulled out from the flex plate so you can remove the transmission.

5. Dislodging the flex plate or the flywheel

Most newer cars that have standard transmission system will be having a flywheel or clutch which needs to be separated in order for the rear main seal to be replaced. For automatic transmission cars, a flex plate has to be removed which is a lot easier because only a few bolts need to be loosened in order for it to be moved. The bolts are just at the rear of the crankshaft. Use the wrench that would fit, and loosen these bolts.

6. Detaching of the rear main seal housing bolts

It is now easier to reach your main goal, that is, the replacement of the main rear seal. After you have removed the obstacles to your aim, it is too easy now to do the job efficiently. The rear main seal is pressed around its housing that is bolted to the back of the engine block.

The seal sits into a cylindrical metal piece which is the crankshaft, the part of the engine that rotates when the car is running. In turn, the crankshaft is supported by a small bearing at the center. This bearing appears on most manual transmission vehicles.

Before removing the seal, there are other parts that have to be removed. Follow these steps before you can reach the seal housing and remove it:

  • Some car models have a thin metal plate placed in-between the engine and the transmission. Remove this using a screwdriver with a flat head.
  • Next are the pan bolts with sizes ranging from 10 to 13mm, depending on car make. Using a socket wrench, remove these bolts carefully and set them aside in a safe container.
  • The rear main seal housing is enclosed to the back of the engine block and mounted by a series of bolts. You can remove these using again your socket wrench by turning them to the right. When you dislodge the bolts, put them aside as you will use these bolts when reassembling.
  • It is now time to remove the seal’s housing. Using again your screwdriver, gently pry loose the housing from the engine block. With your one hand holding the screwdriver while prying around the edges of the housing, try to sway the housing with your other hand to quicken the removal. Be careful not to drop this housing as this may crack when dropped hard.
  • When you loose pry the housing, you can now clean it using a clean rag, then set aside after cleaning for re-installation.

7. Removal of the rear main crankshaft seal

To keep the seal from moving during operation, it is mounted first with a plastic retainer. You need to remove first this retainer to have an access to the seal. After removing the retainer, you can dislodge the main seal from the crankshaft by using your hands.

8. See to it that your new seal match

Compare the old and new seals if they are matched. There may be a slight difference in appearance, but the outer and inner diameters should be the same. Check visually the new seal, it must have no damage, a cut or anything that could start a leak.

9. Installing a new seal

If the seal is damage-free, you are now close to your aim. Please take note of the way you should install the replacement seal. The manner you will install the new seal will only point to two things: either it will leak or it will work. The seal’s lip (the indented side) must be placed towards the engine and over the crankshaft. Fit the seal snugly and push it gently into the bore of the housing.

10. Fitting the seal

To be able to assure that the new seal doesn’t leak, put a flat steel above the newly placed seal and strike it gently and evenly with a wooden hammer to set it into place. Do this continuously around the seal until you are sure that it fits tightly. Apply a thin layer of sealant around the housing when you are sure the seal is in place evenly.

11. Re-installing all in reverse order

Prepare all that you have previously removed, such as the block and crankshaft before re-installing all the parts. The gasket surfaces should be free from dirt and oil so everything fits snugly and without a leak. When all are prepared well, start reassembling in reverse order while assuring all bolts are tightened to the correct torque.

Here are some professional tips

This is an authentic crankshaft rear main seal, the Genuine GM 12639250 Crankshaft Oil Seal Housing, Rear. This product from General Motors is an ideal replacement for your aging parts. Easily available from Amazon.


We hope you enjoy this tutorial. You can really replace a crankshaft rear main seal easily without the hitch when you put your heart and mind into it. Not just because you can save a lot from expensive labor cost from most dealerships, but also know more about your car when something unusual happens to it.

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