Leaking fork seals are considered by many to be a nuisance. Left untreated, they usually trigger performance degradation along with an unsightly mess. Fork seals leak as a result of a tear in the lip of the seal, which occurs when the seal slides over a fork-tube surface that is nicked, chipped or scratched. Bushing wear can also contribute to fork-seal leakage. There are a few ways of dealing with a leaking fork seal problem. One is to send the fork to a suspension specialist (like Lindemann Engineering, Pettersson Pro Suspension, Race Tech and others), where it will be repaired, revalved and respringed for certain financial considerations. Be advised that fork tuning is considered by many to be a black art and you may be subjected to some different opinions. Experience and knowledge count and should be taken into consideration when choosing a specialist. Another option is to repair the leaking fork seals yourself.
1. Repairing leaking fork seals can be difficult and messy if attempted halfheartedly. This task requires the use of unique techniques, tools and equipment. The basic goal is to replace the fork seals and bushings (if needed), do a little metal refinishing, refill with suspension fluid, and reassemble. To determine if new bushings are needed, remove the front wheel and fender and feel the play at the bottom of each leg. Most of the time the bushings will need replacement. To complete this job in one session it will be necessary to have at your immediate disposal all the necessary replacement parts, tools and equipment.
2. This project demonstrates how to rebuild a conventional damper rod fork; the newer cartridge-style forks will be covered in a later segment. Having the right parts, tools and equipment will make the job easier. Consult your service manual and obtain any required specialty tools you don't already have, plus:
- A well-stocked tool chest and basic shop support equipment (jack, stands, drain pans, etc.)
- A Motion Pro, or any other similar splitting fork-seal driver set
- Silkolene or any other seal grease
- Service manual
- A half-inch air or electric impact driver
- A fluid measuring container-like a Ratio Rite
- Fine grit (600-1200) wet/ dry sandpaper
- Fork-seal kits
- Fork fluid
3. Place your bike in a position that will allow you to work on the front wheel. Record the suspension settings and then back out the spring-preload adjuster. Remove the front fender and any other bodywork that makes you uncomfortable leaving to chance, including the fuel tank. Loosen the axle bolt, axle pinch bolts, caliper bolts, handlebar pinch bolts and the fork caps. Raise the motorcycle until the front wheel is just off the ground. Remove the calipers and suspend them out of the way. Slide out the axle and then remove the front wheel.
4. Position a drain pan under the fork. Remove the fork caps and then the drain plugs in the back of each leg, allowing the fork fluid to drain into the pan. Gently push up the sliders one at a time and remove the spacer, washer and spring from the top side of each fork tube. Note and record the components' order and the orientation of the fork spring (one side is more heavily coiled). Slide the dust seals up and pop off the fork-seal retaining clips.
5. Use a half-inch air or electric impact driver with a 6mm Allen wrench to remove the fork slider retaining bolts on the bottom of each fork slider. Pulling lightly on the slider helps to keep the inner components from spinning during removal. When the bolts come off, some fork fluid usually follows. Look for the copper sealing washers. They often stick to the bottom of the fork slider. Remove the base (a cup-shaped aluminum piece) from inside the slider.
6. After checking to make sure the bike will not tip over, use long, slide-hammer-like strokes to free up the fork seal and bushings. Internally the bushing at the bottom of the fork tube slams into and forces up the bushing at the top of the slider, which forces up a large washer and the seal above it. This action tends to damage the bushings but it's unavoidable (another reason why the bushings should be replaced). Note the order and direction of the washer, fork seal and bushings as you remove them.
7. Use fine grit wet/dry sandpaper to smooth over the fork-tube surface. Soak the sandpaper in solvent, or spray it with WD-40(r). Use your whole hand while sanding, with long even strokes. When the surface of the fork tube is smooth, rinse off the tubes with solvent or contact cleaner. Wash all of the parts (including the new ones) in solvent. New parts are coated with protective compounds, which should be washed off before they are used.
8. Slide the new dust seal (dry) all the way up the fork tube to the bottom triple clamp. Apply seal grease to both the inside (lip) and outside (slider) surface of the new fork seals. Work the seal onto the fork tube and slide it up. Lube the top bushing inside and out and then slide the big seal washer and both bushings onto the fork tube. Once the bottom bushing is in place it will keep the rest of the components from sliding off. Apply ordinary wheel-bearing grease around the inside of the aluminum base, then push it onto the bottom of the piston rod. The grease should keep it in place during reassembly.
9. Insert the fork spring and spacer into the fork tube, then thread in the fork cap about halfway down. This temporary assembly will make it easier to reassemble the slider onto the fork tube. Work the slider into place and tighten the retaining bottom bolt with the sealing (copper) washer in place. The torque value for this step is listed in the service manual and should be observed. Assemble the seal-driver set on the fork tube above the fork seal. Use it to work the fork seal into place. If you find the going tough, remove the fork cap, spacer and fork spring. Move the slider partway up the tube and, as you hold the seal driver and slider in place with one hand, use a plastic mallet to drive the fork seal into place by tapping on the bottom of the slider.
10. Insert the retaining clip into its groove and slide the dust seal into place above it. Stroke the slider up and down a few times to confirm smooth operation. The next step is to pour in the fork fluid. Consult the service manual for the fork fluid value/viscosity and pour the specified amount of fluid into a measuring cup. Put a mark on the cup to improve the accuracy for the next fork leg. Pour the fork fluid into one fork leg, being careful not to spill. Repeat the process for the other leg. Gently move the slider up and down to reconfirm smooth operation and to get a feel for the damping. At this time, if you so desire and are equipped for it, you may want to check fluid level (listed in the service manual). This can be accomplished by using an adjustable suctioning device. A tape measure is a bit cruder but just as effective. Fluid level has input on suspension performance and may be altered to meet specific suspension needs. Consult your local expert for details.
11. Carefully insert the fork spring into place followed by the spring washer and spacer. Lifting up the slider partway allows for fork-spring insertion without a splash. Replace the fork cap and torque to specification. Next, tighten and torque the handlebar pinch bolts to spec. Put the rest of the bike together, then inspect and evaluate your work. Clean up any spillage and evidence of old leaks. Adjust the suspension settings to the previous values, and the job is done.