Your 1979 Yamaha XS1100 Special relies on a hydraulic front fork to reduce the shock generated by an uneven road. Oil in the fork legs slows motion of the front fork as it passes over a bump, reducing the shock to a minimum. Seals inside the lower fork legs keep contaminants out and prevent the oil from seeping out. A broken seal will create an oil leak, reducing the efficiency of the forks on the road. Replacing the fork seals is a big job, but it can be completed by most XS1100 owners armed with the right tools.
Tools Used: 10, 12, 22 mm socket, Socket wrench, Phillips screwdriver, Bungee cord, Soft-jawed vise, Rubber-jawed pliers, 6 mm Allen wrench, Container, Flat-head screwdriver, 1 qt. SAE 10 motor oil
Change Fork Seals
Park the motorcycle on its kickstand. Remove the cotter pin from the axle nut on the right side of the front wheel with pliers. Loosen the axle nut with a 22 mm socket and a socket wrench. Loosen the pinch bolt on the bottom of the right fork leg and the brake caliper's mounting bolts on the left lower fork leg, using a 12 mm socket. Remove the speedometer cable's set screw from the speedometer drive on the left side of the front wheel, using a Phillips screwdriver. Wrap the speedometer cable around the motorcycle's handlebars.
Lift the motorcycle onto its center stand, then place a jack under the motor to raise the front wheel off of the ground. Pull the axle out from the left side of the fork legs and lower the front wheel to the ground.
Unscrew the fender's mounting bolts with a 10 mm socket and pull the fender away from the front fork. Unscrew the front brake caliper's mounting bolts from the right fork leg, using a 12 mm socket. Hang the caliper from the handlebars with a bungee cord.
Loosen the right fork's pinch bolts on the upper and lower steering clamps, using a 12 mm socket. Slide the fork out from the bottom of the triple clamp. Place the upper fork tube in a soft-jawed vise.
Press the fork's top cap valve stem to release the air pressure within the fork. Unscrew the top cap from the fork tube with a rubber-jawed pliers. Pull the cap's washer and the fork spring out from the top of the fork tube. Remove the fork from the vise and pour the fork oil out from the top of the fork tube into a container. Pump the fork tube repeatedly to drain the fork oil completely.
Place the lower fork leg in a soft-jawed vise. Unscrew the bolt from the bottom of the lower fork leg with a 6 mm Allen wrench. Pull the rubber dust boot off of the top of the lower fork leg, then slide it up and off of the fork tube. Pry the circlip off of the top of the lower fork leg with a flat-head screwdriver. Grasp the upper fork tube with both hands and pull it out of the lower fork leg. Pry the fork seal out from the top of the lower fork leg with a flat-head screwdriver, then discard the seal.
Clean the lower fork leg and the fork tube with parts cleaning spray. Coat the top of the lower fork leg and the new fork seal in fresh SAE 10 motor oil. Drive the new fork seal into place with a seal driver. Slide the fork tube into the lower fork leg. Insert and tighten the bolt on the bottom of the fork leg to secure the fork tube, using a 6 mm Allen wrench. Insert the circlip into place over the fork seal at the top of the lower fork leg. Slide the rubber dust cover onto the top of the lower fork leg.
Remove the fork leg from the vise. Pour 7.61 ounces of SAE 10 motor oil into the top of the fork tube. Pump the fork tube several times to remove any air bubbles trapped within the fork's oil. Slide the fork spring into the fork tube, followed by the cap washer and top cap. Screw the top cap into place.
Reinstall the fork onto the motorcycle, following the reverse order of removal. Repeat the steps to change the remaining fork's seal, then reinstall the fork, front wheel and brake caliper.
Tips & Warnings
The fork's spring can pop the top cap off with enough force to injure you. Point the top of the fork away from yourself and others while unscrewing its top cap.
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