Why Is My MTB Fork Leaking Oil? (3 Causes and Easy Fixes!)

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Mountain bike fork suspension leaking oil

If you’re a mountain biking hobbyist, chances are you’ve had your fair share of leaking forks. As a cyclist, I understand that these issues can take away from the fun of the sport.

The extreme nature of mountain biking does come with considerable impacts and vibrations. So, it’s only natural to expect some maintenance issues as you’re racking up that mileage.

With that in mind, we must understand the reasons behind a leaking fork to prevent it from happening. Learning how to deal with this leak issue can be beneficial in the long run.

So if right now you’re asking yourself, “why is my MTB fork leaking oil?” you’ve come to the right place! Stick with me, and let’s discuss the causes and fixes of a leaking MTB fork.

3 Causes of a Leaking MTB Fork

There are several causes of a leaking fork. Dirt and debris can accumulate inside, which can damage the seal and cause leaking. Too much compression on the forks (caused by a hard landing) can also force the oil out.

Seeing your mountain bike fork leaking is enough to sink every cyclist’s heart. So, here are the three common causes of a leaking MTB fork:

1.   Wear and Tear

Mountain bikes take on considerable beating caused by the trails and terrain we used them on. The forks endure a lot of impacts and shocks as you pedal through rough roads.

With that in mind, it’s natural to experience leaking fork oil over time. In fact, experts recommend that you perform regular servicing of your bike’s suspension system for this reason.

The age of your mountain bike can also play a role in this issue. If you’re using old forks and bike frames, they’re more likely to cause these types of problems.

2.   Dirt and Debris

Mountain bike suspension fork covered in mud

Mountain biking involves a lot of dirt, mud, and water. These elements can accumulate in the fork tubes and damage the seals, which causes leaks.

Though you can prevent this issue with frequent cleaning, there are instances where the debris deeply penetrates the tube, making it impossible to remove from the outside.

As the dirt sits uncleaned inside the fork tubes, it scratches the seal keeping the oil inside. Over time, the scratches can turn into tears and spots where the oil can seep through.

3.   Inner Tube Damages

Aside from dirt and natural wear, damage to the fork tube itself can cause leaking oil as well. If you observe dents, scratches, and bents, it’s better to address them before the issue amplifies.

These deformities on the fork can be factory defects, so you must check the bike for these before purchasing. It’s an issue I experienced once when I was replacing my MTB fork when I accidentally bought one with a small dent.

While riding through a particularly rough trail, I noticed oil spurts every time the fork compressed. When I brought it to a mechanic, we discovered a small dent in the tube, which grew bigger with the impact and vibration.

How to Fix a Leaking MTB Fork

If the damage is minimal and the leaking doesn’t appear to be severe, you can try fixing your MTB with the following methods:

1.   Cleaning the Seal

Applying lube to a clean fork seal

Cleaning the fork seal is a straightforward process. You won’t need any specialized equipment to get the best results.

Here’s how to clean an MTB fork:

  1. Start by thoroughly cleaning the exterior of the fork.
  2. If the dirt is too much, you can use water and a rag to get the large chunks of grime off.
  3. After you polish the exterior, you can begin cleaning the fork seal by carefully prying the dust seal.
  4. Once the dust seal is open, use a soft, clean cloth to remove the dirt underneath, careful not to damage the seal.
  5. After removing the built-in dirt, re-place the dust seal, and you’re good to go!

After cleaning, you can use liquid fork lubricants to improve its performance. Some brands of fork lubricants will also pull the dust out of your fork dust wipers, so try to use some as often as you can.

2.   Using Seal Doctor

Another way to remedy a leaking MTB fork is to use a seal doctor. It’s a device for cleaning out dust particles on the lip of the fork seal causing the leakage.

Here’s how to use a seal doctor for your MTB fork:

  1. Always start by cleaning the exterior of your fork before working on the seals.
  2. After cleaning the outside, gently pry the dust seal using non-sharp tools to avoid damaging anything.
  3. Once the dust seal is off, snap the seal doctor on the fork leg near the leaky seal.
  4. Then, insert the cleaning hook of the device inside the leaky seal and rotate it around the fork leg.
  5. After rotating, the hook should remove every dust and debris stuck on the seal causing the oil leaks.

Using a seal doctor is the simplest way to fix a leaking fork seal without disassembling it. You can use it anytime and anywhere with a few simple steps!

3.   Replacing the Fork Seal

In the case of severe leakage, replacing the fork seal might be your best option. These kinds of issues often cause damage to the seal you won’t be able to fix by cleaning.

However, replacing the fork seal is something that I don’t recommend doing on your own unless you have prior experience. It also requires special tools and methods.

That said, it’s best to visit your local mountain bike dealer or mechanic. You should also avoid riding your mountain bike to avoid further damage to the bike’s fork.

How to Identify a Leaking Fork

Leaking forks can cause several issues on your bicycle. You’ll immediately notice the reduced shock absorption, which can be very uncomfortable while riding through rocky paths.

This issue can be hard to spot in its initial stages, so it’s crucial to check your forks frequently, especially if you’re into extreme mountain biking.

A tip that experts recommend in identifying a leaking fork is to use a clean cloth. Wipe the fork tubes with the fabric and take your bike for a short ride around your home.

If you notice oil rings or grime around the fork after your ride, there’s a high chance that your seals are broken and need maintenance or replacement.

How to Prevent MTB Fork Oil Leaking

Proper lubrication is the best advice I can offer to maintain your MTB forks. It reduces the effects of the constant shock and vibration that are usually the culprits for fork issues.

Regularly cleaning your MTB fork is also a surefire way to maintain it. It prevents dirt and grime from accumulating and penetrating your fork’s oil seal.

If you find constant cleaning a hassle, you can invest in mudguards specifically for MTB forks. They protect the fork legs from the debris thrown by the wheel as you ride through dirt trails.

Check your fork legs after every ride to ensure no dirt or debris is stuck near the dust seal. Make this a habit, along with careful riding practices, and you’ll save yourself from the hassle of leaking fork oil the next time!

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Paul Tuthill
Growing up in Scotland, Paul developed a love for the outdoors and a desire for adventure from an early age.