Replace Fork Seals
If your classic bike is showing oil on the top of the fork sliders, it's time to replace the fork seals. It'll just get worse, and the oil could start dripping onto the caliper, causing a loss of brakes.
Motorcycle Shop vs DIY
Most dealerships charge $250-$300 to replace fork seals, which is an hour or two of their labor plus the price of the seals and fork oil. If you have a 25+ year-old bike, is not uncommon for a shop to decline the work.
Chances are if you're reading this article, you're already thinking of replacing the fork seals yourself. I replaced the fork seals on my 1976 Sportster, and I can tell you, it is time consuming. But on the other hand, it's an opportunity to learn new skills while saving a few hundred bucks.
If you decide to do it yourself, you will find it's a great time do other related work as well, like polishing the forks or adding gators. And if the steering head bearings on your classic bike haven't been serviced in years, it's probably past due. Dry old rusty bearings are a safety risk you don't need to take.
Fork Seal Replacement
Replacing fork seals requires removal of the front forks, which involves removing the front wheel, fender, brakes, and hand controls. You will need to prop up the front of the bike while you have the bottom forks removed. A motorcycle lift stand makes jobs like this much easier.
Read: Motorcycle Lift Stand Review
If your speedometer cable goes to your front wheel, it needs to be removed. For bikes with front disc brakes, the caliper will also have to come off.
The next step is to drain the old fork oil. On my Sportster, the drain plug was a small Phillips-head screw on the bottom of the slider-tubes near the axle. (You can discard the used fork oil with regular motor oil.)
Remove Lower Fork From Upper Fork
With the bike securely on my motorcycle stand, I raised the front of the bike with a scissor jack until the front wheel was free. I used extra tie-downs to ensure the bike was secure.
Remove the axle nut and push the axle out of the wheel using a mallet or hammer and a piece of wood. Remove the wheel from the front forks. Either write down a diagram on how the spacers came off, or arrange them neatly in the order removed.
Remove the large cap nuts on the top of the fork tubes. Use the correct size socket on the cap to prevent damage to the nut. The cap is under mild spring tension; use caution when removing the cap.
The slider bolt, or pinch bolt, is located at the bottom of the fork tube inside the axle saddle. Removing the fork slider lower bolt on the Sportster required a 6mm Allen socket.
Pull the fork tube out of the slider, and drain out any residual fork oil.
Fork Seal Replacement Tools
My Sportster had lock-rings over the fork seals, which required lock-ring pliers to remove. Other tools required are some sort of seal driver. These can be bought, usually as a kit, or can be fabricated from PVC pipe.
Remove Old Fork Seal From Fork
Fork seals can be pretty stubborn to remove, especially if they've been in there for 30+ years. You may get lucky and they may come out by prying them with a dull flat-tip screwdriver. If not, there are many other methods to get them out.
My old fork seals wouldn't budge, so I cut slices in them with a cut-off wheel, then carefully pried them out. If you do this, be careful you don't cut into the inner or outer surface of the slider - the aluminum is fairly soft and easy to gauge.
Another way to remove stuck fork seals is to thread a screw in them, then try to pull them out using the head of the screw as the fulcrum.
Once the seal is out, wash the lower fork out with cleaner and a rag. Closely inspect the top of the lower fork for any grooves or dings. Minor scratches can be cleaned up with a small piece of Emory paper.
Polish Aluminum Fork Legs
If you want to polish your forks, now's the time. I did these with a home-made bench sander fitted with buffing pads.
Read: How To Polish Aluminum Motorcycle Parts
Install Fork Seals
Lightly moisten the inner surface of the new seal with fresh fork oil. Carefully slip it over the top of the stanchion and slide it down to the slider.
If you have a seal driver set, drive the seal into the slider. If not, find a suitable substitute. I scrounged around the garage and found an old small-block Chevy balancer. The outer diameter fit perfectly.
Replace the oil seal lock-ring shiny side down. Ensure it has snapped correctly into the ring groove.
Remember to re-install the dust seals before putting the lower forks back on!
Slide the fork slider onto the fork tube, and fasten the original screw and washer. The factory manual called for my fork slider screw to be at 11 ft-lbs.
Reinstall the front wheel onto the fork sliders. Make sure that spacers are reinstalled in the correct locations. Replace the axle nut, fork slider cap nuts, being careful not to cross-thread the nuts.
Re-install the speedometer cable and brake caliper. Tighten the axle nut - check your shop manual for specific year and model.
Install New Fork Oil
Use 20 weight for average conditions. For extreme conditions, use either 10 weight or 30 weight accordingly.
Dry fork requires .5 to 1.0 ounces more than the wet amount due to residual oil left clinging inside the fork.
WET: Means you just drained the oil out of the plug and want to refill it.
DRY: Means you disassembled the forks and cleaned them dry inside and out.
Another way to measure is to push the forks down all the way while the caps are off, and fill it to 5" below the upper edge of the fork tubes.
Although this is a laborious task, replacing the fork seals yourself can be done in your home garage. By not paying a shop, you'll have saved up money for that motorcycle lift table you need and want.