Leaking Fork Seals? Not Anymore! Front Fork Tips, Tricks and Tools from our Tech DepartmentMay 19, 2010
Discovering a leaking fork seal is never a happy moment. Most people consider this to be a very complicated job, and you have to do a fair amount of disassembly to even get your forks off your bike before you can start to work on the fork seal. Taking your forks to the shop or a suspension tech can be costly and time consuming.
In reality, 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 supply the right tools to make the job easy, so you have one less thing to worry about.
There are many different kinds of forks out there, upside down (UD), right way up (RWU), damper rod, cartridge, and even exotic pressurized forks that work like a shock absorber. If we were to cover procedures for all of the forks out there, this article would be hundreds of pages long. So we are just going to cover some of the basics, focusing on how Motion Pro suspension tools can make this project easier and less time consuming. When doing this sort of work, it is critical that you have a shop service manual for your particular motorcycle. The disassembly and reassembly procedures are somewhat complex, and you have to do everything in the right order and keep things organized. 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 for the proper function of a very important part of your bike. In addition, we highly recommend purchasing an official factory service manual for your bike. These manuals have the most detailed information, and are the most thorough. There are many independent writers of manuals available, but in my experience, they can have omissions and incorrect or misplaced information. A factory manual may cost more, but it is a good investment, and a valuable resource.
So, you have your forks on the work bench, now what? One of the first things you have to figure out how to do is hold on to the parts you are working on. A standard vise can do the job, by clamping on the axle surface, or the brake caliper mount, but it is somewhat awkward. Motion Pro makes the Vertical Suspension Vise, and while it is probably not imperative 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 his shop. The Suspension Vise will hold fork tubes, fork sliders, shock bodies, shock shafts and just about any other round item. By solidly fixturing the component you are working on, in an accessible position, the job becomes much easier. Anyone who has worked on an MX fork leg clamped in a vise and had to reach above their heads to remove the fork cap will know what we are talking about and appreciate this handy tool.
As mentioned, going into particulars is not possible in an article of this type, so we are going to just cover the basics. The first job you have to do is loosen and remove the fork caps. If you are not using a suspension vise, loosen the fork caps even before the forks come out of the triple clamps, by loosening the upper triple clamp while the lower is still tight, and use a regular open end wrench or one of the special Motion Pro fork cap wrenches. 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. 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 you to pump 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 is a lot of variety here, so make sure to consult your service manual. Here too, Motion Pro makes several damping rod and cartridge holding tools for many types of forks to make this part easier.
Once the cartridge is free from the fork bottom, now you can start to work on the seals. There is generally a dust seal over the fork seal itself, and it 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. Before pulling the seal though, most forks have a retaining clip in the slider, and you have to remove it first. If you don’t take it out, the seal will never come out, no matter how much you struggle. With the retaining clip out, now is the time to pull the fork seal. This part may sound kind of barbaric, but this is the real way you remove the seal. Slide the fork tube inside the slider, and then quickly pull the tube outwards and the seal will pop out of the slider along with the tube. Again, this is not universal, so take a look at your service manual.
Before we talk about installing new seals, this is the time to check out your fork tubes and see if there is a rock chip 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 and started the leak. If you have a small rock chip, sometimes you can polish it out 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, because the sharp square edges of the bushing groove can cut the new seals. Motion Pro has a solution for this too. 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.
When installing the fork seal into the slider, you 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 driver, and the seal goes easily in place. Always make sure to check that the seal has been all the way seated so 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. It is recommended to replace it every time, so that you do not have a leak at the bottom of the fork.
Once the fork and the slider are reassembled, it is time to refill the fork with fork oil There are many brands and weights available, each has it’s own characteristics. Your service manual will have recommendations, and 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, more commonly, oil level in the fork. This is generally measured with the fork tube fully inside the slider, and no spring in the fork. Motion Pro makes the Fork Oil Level Gauge, 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. Piece of cake.
Next up, most cartridge style forks require bleeding, to get air out of the cartridge before final assembly. Anyone want to guess where you can get a tool for this? Yes, you in the third row. Motion Pro? Good job, you get a gold star for today’s class. The Damping Rod Bleed Tool 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 damping rod without getting your hands all oily. It also makes for a handy extension to the damping rod, so that when you install the spring, you don’t have to chase the damping rod back into the fork as it retracts.
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, and 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 service manual for this very important step.
Tighten up the caps, reinstall your forks, and you are done. Time for more riding!
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