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've replaced fork seals on several types of bikes, and some are easier than others. It can be a time consuming task, but on the other hand, it's an opportunity to learn new skills and save a few hundred bucks.
This article covers replacing fork seals on a 1976 Sportster. These are 35mm forks commonly found on 1970s and 1980s Harley-Davidson FX models and Sportsters.
Fork Seal Replacement
For this bike, removal of the front forks was required, 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, but you can use a floor jack under the frame.
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 the 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
The 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.
These old fork seals wouldn't budge, so I carefully cut slices in them with a cut-off wheel, and then 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 into 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. Inspect the uppers for nicks, burrs, dings, and pitting in the chrome. Also check the lower fork for any nicks or dings. Minor scratches can be cleaned up with a small piece of Emory paper.
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 fork seals yourself can be done in your home garage. You'll find it's a good time do other related work, 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.
Polish Aluminum Fork Legs
I polished these Sportster forks with a home-made bench sander fitted with a buffing pad.