My Dirt Bike Fork Seals are Leaking!

dirt bike maintenance forks

One of the least looked after parts on a dirt bike are the forks.  As a result, it is very common to find that your dirt bike has leaking forks, especially if you are buying a used dirt bike.  Unless the original owner meticulously cared for his or her bike, the chances that they have ever even replaced the oil in their forks is extremely low.

Let’s take a look at some of the common questions, causes, and fixes for a dirt bike with leaking forks.

What happens when fork seals leak?

The majority of dirt bike forks are going to be oil filled forks.  This means that the compression and rebound action of the fork is controlled by the amount of oil that slides through the valving.  This article is not about how dirt bike forks work, but rather what happens when they leak.  

When a dirt bike seal starts to leak, it allows air into the oil reservoir, and allows the oil to slide past the seals into the open.  When there is less oil in the oil reservoir, the valving action is affected, and the compression and rebound is affected.  

If you think about how shocks work on a car, a blown shock on a car results in the suspension springs not being dampened and the car bounces down the road.  The same applies for dirt bike forks with badly leaking seals.  Instead of the forks dampening the suspension action and controlling the wheel bounce, they will let it occur.

How do you tell if your fork seals are blown?

The easiest way to tell if your fork seals are blown is to check the exposed part of the fork tube.  Is the bottom part of your fork wet with oil?  This happens when a dirt bike fork has blown fork seals.  

If your bike is old and crusty, it might be hard to tell what is oil and what is mud.  So, take a clean lint free cloth, and wipe down the lower part of the forks.  Once they are good and clean, go for a quick ride and recheck.  Generally if your forks aren’t leaking oil then the seals are in okay shape.

What causes seals to leak?

Fork seals should be looked at as a consumable part of your dirt bike.  Their effective useful life is generally somewhere between 40 and 80 hours, after which they should be replaced.

Here are some common causes of leaking fork seals:

  • They have outlived their usefulness, are old, and have been through a lot.
  • The lower fork tube (the shiny metal part that is exposed to the elements) might have some defect, dent, or chip in it.  This would allow dirt and sand to both enter the fork oil reservoir past the seals, and allow fork oil to leak out.
  • Dirt has found its way past the dust wiper and is sitting next to the seal.  Eventually this will work its way into the seal causing it to fail.

Can you ride a dirt bike with leaking seals?

Sure, but don’t let this go for too long.  The issue with riding a dirt bike with leaking for seals is that eventually, you will damage your forks.  Forks are pretty expensive to replace, but seals are cheap to replace.  So if you notice you are leaking fork oil in the middle of your ride, go ahead and finish your ride then replace the seals when you get back home.

In the most extreme case where all of the oil in your forks has leaked out, there will be nothing to control the rebound and compression of your forks which can cause a dangerous riding condition by making the dirt bike extremely difficult to control.

How to fix leaking fork seals?

Replacing your fork seals is going to seem pretty daunting the first time you do it.  My advice is to do it yourself, but make sure that you have the correct tools to help you succeed.  This means you will need a fork seal driver (make sure that the size is the same as your inner fork tube), a fork seal bullet (a rubber or plastic part to put over the top of your lower fork tube when you are putting the new seal on), a fork seal kit, and some fork oil.

Each bike is different, but here is a rough guide of the steps involved.  For your specific bike, it will help to watch some YouTube videos to see how people do it.

  1. Open the top of the fork and drain the oil.
  2. Remove the fork top cap from the dampening rod.
  3. Remove the fork spring.
  4. Use a small screw driver to pull the dust wiper down.
  5. Use a small screw driver to remove the seal retaining clip.
  6. Pull the upper fork tube off of the bottom fork tube.  This can sometimes take a little force.
  7. Remove all the bushings, rings, seals, and wipers and lay the down in the order that you take them off.
  8. Put the fork seal bullet on the top of the lower fork tube.
  9. Put the dust wiper on, retaining clip, new fork seal, ring, and bushings on in that order.
  10. Slide the dust wiper and retaining clip all the way down on the lower fork tube
  11. Put the upper fork tube on (after you remove the seal bullet)
  12. Use your seal driver to drive the fork seal into the upper fork tube.
  13. Clip the retaining ring into the upper fork tube.
  14. Push the dust wiper into the upper fork tube.
  15. Replace the spring and cap and fill with oil to your manufacturers specs.

Seems like a lot.  I said it was daunting.  In reality, this whole process takes about 20 minutes per fork if you have done it a couple of times.  

How much does it cost to replace fork seals?

If you do it yourself, replacing both fork seals is around 40 dollars with the kit and oil.  If you take it to a shop, depending on their labor rates, expect to pay between 100 and 150 dollars.

Wrap Up

I hope I haven’t scared you away from replacing your fork seals with this article.  It is actually not too bad of a process after you have done it a couple of times.  Remember to watch a few YouTube videos before you start, and make sure that you have the correct tools (no skipping out with PVC pipe fork drivers and sandwich bag seal bullets), and you will be fine.