Fork Seal Replacement 101posted Jan 1, 2008
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Discovering a leaking fork seal is never a happy moment. Unfortunately, taking your forks to the shop or a suspension tech is costly and time consuming. Many people think that changing fork seals is a complicated job, but truth is that anyone with a decent amount of mechanical experience can change fork seals, as long as you have the right information and the right tools. Motion Pro can help you overcome both of these obstacles.
There are many different kinds of forks out there, upside down (USD), right way up (RWU), damper rod, cartridge, and even exotic pressurized forks that work like a shock absorber. Covering the procedures for all the forks out there would require hundreds of pages. So we are just going to cover some of the basics, focusing on how Motion Pro tools can make this project easier and less time consuming. It is critical that you have a factory shop service manual for your particular motorcycle when doing this sort of work. The disassembly and reassembly procedures vary from one model to the other and you have to make sure you are following the right procedures for your particular fork. Also, the service manual will provide important torque values for many components, which is an essential part of doing the job right, not only for safety, but also for the proper function of a very important part of your bike. A word of caution, improper disassembly and assembly of your forks could result in great bodily injury or death, so it is imperative that you follow all the instructions in the factory shop manual. Make sure you wear safety glasses when doing this job and follow all the safety precautions listed in your shop service manual.
So, now you have your forks on the workbench, now what? One of the first challenges is finding a way to hold on to the parts you are working on. You can try clamping the axle surface or the brake caliper mount in a standard bench vice, but that can be awkward and unless you use a jaw inserts it could damage the fork legs. The Motion Pro Soft Jaw Vertical Suspension Vise (p/n 08-0391) is the ideal tool for this job. While it is not essential for the home mechanic, anyone who does any amount of suspension work will find it is one of the most valuable time saving tools in their shop. The Motion Pro Soft Jaw Suspension Vise holds fork tubes, fork sliders, shock bodies, shock shafts and just about any other cylindrical part. Working on your shock or fork is much easier when it is clamped firmly in an accessible position. Another word of caution, do not attempt to work on your rear shock unless you have extensive experience and training. Rear shocks are highly pressurized with nitrogen and extremely dangerous to work on.
When working on your forks, the first task you have to do is loosen and remove the fork caps. Make sure you bleed off any accumulated air pressure before you do this. If you are not using a suspension vise you should loosen the fork caps before the forks come out of the triple clamps. The best way to do this is to loosen the upper triple clamp while the lower is left tight, and then use a regular open-end wrench or one of the special Motion Pro fork cap wrenches to turn the fork cap. Once the cap is loose, most cartridge style forks require you to loosen a locknut at the underside of the fork cap to remove the cap completely (see your shop manual for details on how to do this). Once the cap is removed, you can pull out the fork spring and any guides, and then turn the fork over to drain the fork oil. Most cartridge forks require pumping the damper rod to get all of the old oil out of the fork. Make sure to dispose of used fork oil just like you would engine oil, and recycle!
Once the oil is drained, you have to dismount the cartridge or damper rod from the bottom of the fork. Again, there are a lot of variations here, so make sure to consult your shop service manual for the procedure for your particular fork. Motion Pro makes several damping rod and cartridge holding tools for many types of forks to make this task easier http://motionpro.com/motorcycle/tools/suspension.
Once the cartridge is free from the fork bottom you can start to work on the seals. Usually there is a dust seal over the fork seal. The dust seal can be removed simply by prying in between the fork slider and the dust seal. A small screwdriver works fine for this, but be careful not to damage the seal or the fork slider. After sliding the dust seal up the tube, you will be able to see the actual fork seal recessed in the fork slider. Most forks have a retaining clip in the slider and you have to remove it before pulling the seal. The fork seal will never come out if you don't take this retaining clip out. With the retaining clip out, slide the fork tube inside the slider and then quickly pull the tube outwards. The seal should pop out of the slider along with the tube. There are some variations on this process and not all forks are alike, so take a close look at your shop service manual beforehand to confirm the correct procedure for your particular forks.
Before installing new seals you should check your fork tubes and see if there are any rock chips or other damage that caused the seal to fail. Sometimes seals just wear out and start to leak, but often there is something that caused damage to the seal. It may be possible to polish out small rock chips with a very fine grain whetstone, but you have to be careful not to damage the fork tube chrome. If you find something like this, it's best to head to your local bike or suspension shop to have them look at it.
With RWU forks, swapping seals is simple. Slide the old ones off the top, and slide the new ones on. With UD forks, you have to remove the fork tube bushing, and slide the old seals off over the bushing groove. However, you have to protect the new seals during installation because the sharp square edges of the bushing groove can cut the new seals. Motion Pro Fork Seal Bullets are designed for this very purpose. The Motion Pro Fork Seal Bullets are available in seven different sizes to fit just about any fork tube. The Fork Seal Bullets slide over the tube and the bushing groove and provide a nice taper to make the job of installing new seals a snap.
You will need to have a fork seal driver to seat the new seal in the slider. Motion Pro makes fork seal drivers in many sizes to fit virtually any fork. A couple of good strokes with the seal driver will push the seal in place. Always make sure to check that the seal is seated all the way and that the seal-retaining clip will seat fully in its groove. The dust seal will just push into place by hand.
The rest of the assembly is the reverse of what you did to disassemble it. Make sure to torque everything carefully. Also many forks have a soft copper seal at the lower cartridge or damping rod bolt. We recommend replacing it every time to prevent leaks.
Once the fork and the slider are reassembled you can refill the fork with fork oil. There are many brands and weights available and each has its own characteristics. Your service manual will have recommendations for your application. Using different weights will change the damping characteristics of your fork. It is best to stick with factory recommended oil, or inquire with a local suspension specialist if you want to make changes. Most manuals will have a specification for either oil quantity or oil level. Usually this is measured with the fork tube fully inside the slider and no spring in the fork (check your service manual for the correct procedure for your fork). Motion Pro makes the Fork Oil Level Gauge (p/n 08-0121), which makes this part of the job a snap. Slightly overfill the fork leg with oil, and then set the ring on the gauge to the fork oil level height specified in the manual. Insert the gauge into the fork leg, and pull on the plunger of the syringe. This will pull excess oil out of the fork leg until it is at the set height.
Next up, most cartridge style forks require bleeding to get air out of the cartridge before final assembly. The Motion Pro Damping Rod Tool (p/n 08-0337) has a number of different adapters to fit just about any cartridge rod, and the handle allows you to easily pump the air out of the cartridge without getting your hands oily. It also makes for a handy extension to the damping rod, so you don't have to chase the damping rod back into the fork as it retracts when you install the spring. Make sure to recheck the oil level height after bleeding the cartridge, as the level can drop slightly.
From here the rest of the assembly is pretty simple. Reinstall the springs and any spring guides, and then get ready to install the fork cap. One thing to look out for is some forks have a damping adjuster on the cap. In this case the adjuster has to be in a specific position and the cap has to be threaded a specific amount onto the damping rod before tightening the lock nut. This is very important, as it will affect the adjustability of the fork. Refer to your factory shop service manual for this very important step.
Tighten up the caps, reinstall your forks, and you are done. Time for more riding!