Auto-Trans Rear Seal Replacement

Curing leaks at the driveshaft

Do you ever observe oil stains on your garage floor or driveway and wonder what to do about them? You should never have an oil leak even if you own a classic car, which are traditionally leakers. If your vehicle leaks, it needs to be fixed for both durability and the environment. Leaking vehicles stain the driveway and pollute anywhere they are parked. And if the leak won’t stop, you need to find the fix.

Transmission leaks, especially automatics, are quite common yet they don’t have to be. Automatic transmission fluid and gear lube are completely different lubricants. One thick. One thin. Yet they leak with the same frustrating consistency. Seal and gasket leakage is normally caused by gasket/seal deterioration, improper installation, irregular contact surfaces, and sometimes a combination of all three. Here’s what you need to know to stop fluid leakage in its tracks.

Leaks happen because there are imperfect mating surfaces where fluid has an opportunity to escape. Proper sealing is the process of keeping fluid inside and contaminants outside. To get that result you must have perfect mating surfaces, gasket or seal integrity, and understand what it takes to get it. Federal-Mogul/National Oil Seals & Bearings tells us seals are engineered to perform three basic functions — retaining lubricants and fluids, exclude contaminants, and seal or separate dissimilar fluids or gasses. Bearings and bushings count on seals to both keep dirt out and lubrication in.

For a seal to properly do its job, it must have every advantage. It must be made of the right material for the application (heat, pressure, and shaft speed). A shaft seal consists of an outer case, inner case, and a flexible lip or lips. When it comes to seal design it gets very involved and something only engineers understand. We’re going to keep it simple to where you can install seals and gaskets with confidence and have a clean garage floor.

Types Of Seal Material

There are four basics types of flexible polymers used in seals:

  • Nitrile: Basically a rubber cocktail consisting of Butadiene and Acryonitrile. Appropriate for -40F to +250F temperatures. Resistant to petroleum, water, grease, glycol-based coolants. Does not do well with ozone, extreme dryness, and wind/weather.
  • Silicone: Can withstand extreme temperatures ranging from -80F to +400F. Is resistant to oil (but not petroleum based oil), water, ozone. Durable material.
  • Fluoroelastmer: Can withstand temperature extremes – -30F to +400F. Resistant to many different types of fluids and lubricants.
  • Polyacrylate: Temperature range -20F to +300F. Resistant to all kinds of lubricants including grease. Resists deterioration from the elements.

Seal design and materials used depends on the application. Of course the auto parts industry has made it easy for the consumer to buy seals and gaskets because it has already designated seal and gasket types for given applications.

When you buy seals and gaskets, examine them for integrity before putting your money down. There are paper gaskets and there are composition silicone/paper gaskets. Opt for the composition style gasket for better sealing. Same can be said for seals. If you have a choice between Nitrile or silicone seal material, spend the money and buy silicone. It is durable and tends to be self-lubricating. Not all auto parts stores will offer you a choice.

Other Points of Leakage

Transmissions have many potential leak points including subassembly gasket locations, manual/kickdown shaft seals, and band adjustment seals. Seals must be free of flaws and properly installed. Shafts must be smooth and free from scoring. If shafts have deep scoring, find a replacement. Otherwise, dress shaft surfaces with a super fine emery paper or crocus cloth and WD-40 as a lubricant.

Keep in mind leakage isn’t always the fault of seals and gaskets, but instead irregular mating surfaces, which must either be machined or replaced. A good machine shop can give your transmission’s contact surfaces a shave to get them perfect. Just make sure they don’t take too much off throwing tolerances off inside.


TRC Transmission (800)987-2676 – (818)882-0082 -

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