Tech Tips

Replacing Fork Seals

Written by  January 31, 2005

Now that there’s snow on the ground here in the Midwest, have you been looking at your bike sitting there in the garage and can’t wait to get busy fixing things? If so, have you considered tackling those leaking fork seals?

Here in the great state of Missouri, we have yearly Motorcycle Vehicle Inspections that must be completed in order to renew our state issued sticker. Many riders dread inspections, and quite frankly, I think it is quite a hassle for the shop staff and the customer. This is also the time when we, as customers, have to open our wallets and have a shop perform repairs, or attempt it ourselves. Either way, we will most likely need to buy some parts.

The problem with leaking fork seals is that after time, this will eventually happen to all motorcycles. When they start leaking, your braking can be greatly reduced due to contamination, and the suspension travel will bottom out more easily, giving you a mushy feeling that increases over a short period of time.

Repairing your fork seals can be tricky and very messy if you attempt this without the proper tools and a service manual. If you do not have a service manual for your bike, I suggest purchasing one. There is so much valuable information in these manuals, and you may just become a pro after reading it over your morning coffee.

Before we get started, I want to point out that this article is for conventional forks only. Cartridge forks, like you find on most late model dirt bikes and sport bikes require special fork seal drivers and fluid, which we will be covered at a later time.

Here are a few items you will need to gather in order to perform this procedure:
Service manual
Motion Pro or similar quality fork seal driver
Fluid measuring container, such as Ratio Rite
3/8” or 1/2” impact wrench or electric impact driver
Fine grit sandpaper (600-1200) wet/dry
Fork oil per your service manual specifications (usually 10 or 15 weight)
Allen wrenches, either SAE or Metric (sockets preferred)
Fork-seal kits with wipers
Replacement bushing (if needed)

The first thing you must do is get the motorcycle’s front wheel in the air, either on the center stand (if equipped) or by using a jack. Always remember safety, and have the bike secured so it does not fall over and damage you, your bike, or anyone around you. Once you have the front wheel off the ground, it is time to see if you also need to replace the bushings.

Next, remove the front wheel and fender. There are two caliper mounting bolts your need to remove, or you may have a drum-type front braking system, which comes apart once you have the front wheel down.

Grab the bottom of the fork leg and lift up. If the fork is easily pushed, the fluid is most likely gone and the bushings have been compressed, and may be scarred. The old fluid normally runs down the fork leg and onto the front brake caliper or shoe. Once this happens, the brake pads or shoes must be replaced due to contamination. This is what the state inspectors are looking for when you have your motorcycle is inspected; and will normally fail inspection if your seals are leaking.

For those of you who have a windshield or fairing, take the time to remove these items so you don’t scratch the plastic. Do what makes you feel comfortable, not what your buddy tells you, unless he’s willing to pay for the repairs. Remember, shop techs are paid a flat rate and book time. You are not on that pay scale, so slow down and take your time.

With that said, it is time to get the forks down from the triple clamps. You need to loosen the upper and bottom pinch bolts on both sides. This is a great time to note the pre-load settings on the fork caps, which may or may not have adjustments setting. Once the forks are removed from the bike, set them on a rug or lean them against your workbench, so they do not fall over.

Next, you must remove the outer dust seal to see if there is fluid under this seal. Usually, a flat-bladed screwdriver works fine for this. There will be either a C-clip or a giant E-clip in the top of the fork leg that must be removed.

You must now remove the top fork cap, fork springs and washers, but keep in mind that different systems use different parts. This is why I stress that you use your service manual for reference. It’s just good insurance. When you remove the fork springs, write down how you found them so you can reassembly them is reverse order.

If you have an oil pan, you can tip the fork down to remove the remaining oil in the tube and prevent oil from leaking all over the place when you remove the bottom Allen bolt.

Once you have removed the upper components, check for damage. If any part is damaged due to scratching or galling, it must be replaced. Otherwise, you will be doing this procedure again shortly.

You will find that the fork tube collapses when the spring and top cap is removed. This is normal, because the weight of the tube falls into the fork leg and gives you a tip.

When you remove the bottom bolt from the fork leg you can grasp the top tube and the bottom leg and use it as a ram to separate the two halves. Or, for those of you who have a vise, this works as well.

Once you have separated the fork tube and the lower fork, it is time to clean and inspect all parts for damage and wear. I currently use a parts washer from Safety-Kleen Corp, which is ideal for cleaning, and is the choice of most service departments.

Now that you can see all the components, blow them dry or wipe them with shop towels to remove the solvent. Once this is done, there are a couple of tips I want to stress. First, use a little grease on the new seals, and if there are nicks or rust on the upper fork tube, remove any burrs with the finest sandpaper you can purchase; usually, between 600 and 1200 grit. I also have a trick that a lot of dirt bike fans use. You can place a plastic grocery bag or saran wrap over the tube and install the fork seal and wiper. This prevents any tearing of your new seals. Reinstall the bottom Allen bolt into the dampening rod holder and torque to your manual’s specification.

Now, you are ready for the new fork oil. Look in your manual and note the fork oil capacity. There is also a measurement that is sometimes overlooked, which is the fork oil level with the top tube collapsed and the spring removed. To check the oil level, you can simply place a ruler into the tube and it pull out to see how close the oil level is. You will see oil on the ruler, and you may need to add oil depending on that measurement.

Once the oil has been checked and adjusted, reinstall the fork springs right side up! What do I mean by right side up? Progressive fork springs are tightly wound at one end and must be facing up. In other words, conventional springs look the same the entire length of the spring. This is why I had you write down how you found the springs when you removed them, so you can reassemble them in reverse order of how they were disassembled. I just love it when manuals state that!

Now that you have the springs reinstalled correctly, install the top cap and torque to the manual specifications. You have now completed your fork seal replacement. Don’t you feel great?

Next, take the fork, reinstall it in the triple trees, and torque to manual specifications.

Reassemble the front fender and wheel, and torque accordingly.

That’s the skinny on the fork seal replacement. If you follow these instructions, and use your manual, you should be a pro by your second fork tube. Now you are one up on your neighbor, because he might have to pay for this service. Maybe he’ll even pay you?

If you need help with this procedure, please feel free to e-mail me and I can help walk you through this. On my Tough-O-Meter scale, this procedure scores around a 4 on a scale of 1-10, so I feel this is a good afternoon project for the beginner do-it-yourselfer.

Again, I want to stress safety, and always wear eye protection and make sure your bike is supported securely when the front wheel is removed.

B-safe out there!

By Dave Miller