Oil running down your motorcycle fork legs is a very common problem. There are many reasons why your fork seals may be leaking.
Motorcycle fork oil seal leaks are often caused by dirt trapped between the seal and the inner tubes. The fork seals may also be worn or the inner fork tubes may be scored, scuffed or rusted causing oil to leak past the seals. Too much fork oil or two much air pressure may result in leaky fork oil seals.
Fixing the oil leak on your motorcycle forks will depend on the cause of the leaky fork seals. Let’s take a detailed look at the reason (and resulting solution) to your fork seal leaks.
#1 – Dirt between the seal and inner tube
The most common reason why motorcycle fork seals start leaking is trapped dirt and debris between the inner fork tube and the oil seals. If the fork dust seals are cracked or torn from old age, years of sunshine or dropping the bike, dirt can get into the oil seals.
Over time, the up-and-down movement of the forks will cause dust and dirt getting in between the sealing surface of the seals and the inner tubes. Without a perfectly tight mating surface, oil will seep past the seal and down the fork legs.
FIX: Very often, dirty oil seals can be cleaned without taking the forks apart. Get yourself a cheap Seal Mate from Amazon like the one in the image below. This little plastic tool won’t scuff your inner fork tubes and can be used to pick dirt out from between the oil seal and the fork tube.
All you need to do is pry the dust seal loose and slide it down (or up) the fork leg. Insert the Seal Mate between the oil seal and inner tube, and work your way around the tube. Any trapped dirt will be remove and if you are lucky, your seals will seal properly again. Obviously, if the seals are damaged this won’t work, but it is worth a try.
More often than not, trapped dirt is the cause of a leaky seal. And spending a few bucks on a Seal Mate could save you hours of trouble replacing forks seals (a messy business requiring specialized tools).
#2 – Worn oil seals
Oil seals wear, especially if you ride in dirty or wet environments. Even more so if you never clean the seals as explained in #1 above. If your oil seals are worn, they won’t seal properly anymore causing your fork oil to leak out.
Fork seals are generally not expensive but it is a messy job to replace them. You may also need specialized tools to dissasemble your forks and seat the new seals.
FIX: If your fork seals need replacing, you need to get ready to tear apart the forks completely. If you are replacing the fork oil, you might as well replace the seals too.
Motorcycle forks vary a lot depending on the bike, so it is not possible to discuss each type of fork. The job, however, will entail most of the following steps.
- Remove the front wheel and then remove the forks from the triple clamp
- Drain the fork oil from the drain plug at the bottom
- Remove the top cap of the fork (be careful of the spring tension)
- Release the fork spring retained and remove the spring and damper rod
- Pry loose the dust seal
- Remove the oil seal retaining clip (very important!)
- Use momentum to slide the fork inner tube out from the outer tube
- Keep all the collars, washers and clips in the right order
- Install all the parts (including the new seal) in the right order
- Hammer in the new fork seal using a fork seal driver like this
- Replace the rest of the parts
- Fill the forks with the correct amount of fork oil
- Fit the forks to the motorcycle
While this is a fairly simple job, it may require special tools to remove the top cap. You might also struggle to get the new oil seal into its recess without a proper seal driver. I have struggled for hours trying to get fork seals to seat properly.
CAUTION: Be very careful not to scratch or ding the inner fork tubes and keep it squeeky clean. Any dirt in the seal or scratches on the inner tubes will result in more oil leaks in future.
#3 – Scoring on the inner tubes
For the oil seals to be effective, it is important that there is a smooth mating surface between the seals and the inner fork tubes. Any imperfections like deep scratches, rust patches or dents may damage the seals causing them to leak. Even if the seals are not damaged, the damaged area may cause oil to leak past the seal.
FIX: Depending on the severity of the damage to the inner fork tubes, the surface could be cleaned up in order for the seals to do their job. Surface rust can be removed with 2 000 grit sandpaper and WD 40. Work the lubracted sand paper around the fork tube (not up and down along the lenght – to prevent gouging marks that will leak oil) until the rust is gone.
If the damage is severe and it is not possible to remove with sand paper, the fork legs can be re-chromed professionally. It may, however, be cheaper to just get a second-hand set in good condition. The last (nad most expensive) resort is to buy next forks.
Prevent damage to your inner fork tubes by fitting plastic fork pretectors if yours is missing.
#4 – Too much oil in the forks
Motorcycle forks are filled with fork oil in order to assist with dampening and rebound. It is important to fill up the fork legs with the manufacturer’s required amount of specific weight fork oil.
It is okay to slightly over/under fill the forks, depending on the dampening effect required, but too much oil may result in too much pressure which could force the fork seals to dislodge and leak.
FIX: If you suspect too much oil in the forks, drain some oil from the drain plug on the axle lug and drive the seal back into its recess with a seal driver. If you are not sure about the amount of oil in the forks, it is best to take the forks apart and start from scratch.
On some forks it is possible to get all the old oil out without removing the forks form the triple clamp. I’ve done this on my old KLR 650 when I replaced the fork springs. Just drain the oil, open the top and refill with the correct amount of oil.
Some manufacturers require a specific volume (in cc’s) while others requires a specific oil level measured in inched from the top of the fork tube. Consult your owner’s manual.
#5 – Too much air pressure in the forks
Some motorcycle forks have air valves (similar to a tire valve) in the top of the forks. In most cases, these valves are purely to vent the air pressure built up in the forks to atmospheric pressure. That means, you remove the valve cap, depress the little stem inside the valve to release any residual pressure, and replace the cap. Nothing more!
If you pump up the forks with air, you risk building up too much pressure inside the forks which may pop out an oil seal and cause a massive leak. In fact, a buddy of mine uses an air compressor like this to remove oil seals when he wants to replace them.
Some bikes, like my old XR 650 L, has air valves in the forks that were meant to be pumped up to increase the stiffness of the suspension. It is important to not exceed the manufacturer’s recommended pressure to prevent too much pressure and leaky seals as a result.
FIX: If you suspect too much air pressure in the forks, release the pressure via the air valve in the top of the fork legs.
Conclusion
Leaky fork seals are very common and can be very frustrating. If you don’t attend to oil leaks, you could cause oil to leak onto your front brakes, resulting in a dangerous riding condition. Lubed up brake discs and pads don’t stop!
No oil in the forks will also affect the dampening ability of the front suspension. You’ll be riding on pogo sticks which would negatively affect the ability to control your bike. The suspension needs to be in good working condition to keep the front end on the ground and the grip at a maximum level.
If you see oil on your forks, find the problem and fix it.
Happy riding!
Francois