The 1983 R80ST “project bike” came with both front fork legs, although a previous owner had painted the fork sliders blue. One leg is missing the yellow reflector and the black fork seal cover.
I had the fork sliders vapor blasted to freshen up the forks and remove the paint.
I use a pair of snap ring pliers to remove the snap ring that secures the damper rod inside the fork tube.
I use a seal puller to remove the fork seal inside the fork slider.
I use a heat gun to heat the fork slider around the fork seal so the aluminum will expand making seal removal and installation easier.
I use a 36 mm socket to help seal the fork seal in the top of the fork slider.
I use a draw rod to compress the bottom spring on the damper rod so I can install bottom cap and pull it inside the fork tube to expose the groove in the bottom of the fork tube and install the snap ring into the groove. The draw rod I use is a piece of all thread that is about 26 inches long. I use a large fender washer on the top end of the draw rod so it fits on the top end of the fork tube. I use a smaller washer on the bottom end of the draw rod, about 23 mm diameter. This washer has to be small enough that the ears of the snap ring can compress without resting on top of the washer. There are two nuts that I use to tighten the draw bar to compress the end cover of the damper rod assembly until the groove inside the fork tube is exposed.
I use a fork rebuild kit from Tom Cutter at Rubber Chicken Racing Garage and obtain the rest of the parts from Euro MotoElectrics who is donating parts in support of my goal to auction this bike with all proceeds going to support the Motorcycle Relief Project. I will install some of the parts in Tom’s fork rebuild kit and the new fork springs when I install the forks in the top plate and triple clamp.
In addition to rebuilding the forks, I replace the large Allen bolt that secures the fork slider to the damper rod (part# 07 11 9 919 778) as they got mangled a bit when I removed them. I replace the fork top cap bolts (part# 31 42 1 241 661) as they are in bad shape. I also replace the yellow side reflectors (part# 63 14 1 244 529) as one was missing and the other was cracked.
|Rubber Chicken Racing Garage Fork Rebuild Kit
07 11 9 963 073 Gasket Ring, Aluminum (2)
31 42 1 232 665 Gasket Ring Fork Seal (2)
07 11 9 963 151 Gasket Ring, Copper (2)
07 11 9 963 010 Gasket Ring (2)
07 11 9 963 645 Lock Ring (2)
31 42 1 237 213 Felt (2)
31 42 1 238 909 Cap (2)
31 42 1 237 205 Cup (2)
|31 42 1 237 232||COMPRESSION SPRING, Fork Spring||2|
|31 42 1 241 665||COMPRESSION SPRING (from 11/82), Damper Rod||4|
|07 11 9 919 778||FILLISTER-HEAD SCREW – M12X30, Fork Slider To Damper Rod||2|
|63 14 1 244 529||SIDE REFLECTOR, YELLOW, FRONT||2|
|31 42 1 241 661||GUIDE SUPPORT – M30X1,5X11, Fork Tube Top Nut||2|
|31 42 1 241 649||GUIDE RING||2|
|31 42 1 242 645||VALVE WASHER||2|
This video summarizes how I rebuild the front forks.
VIDEO: 1983 BMW R80ST Rebuild Front Forks
Remove Fork Slider
The fork slider is attached to the fork damper rod with a large Allen bolt at the bottom of the fork slider. I typically use an electric impact driver to loosen the Allen bolt with the fork springs inside the fork tube. The pressure of the springs usually provides enough friction to prevent the fork damper rod from turning when I remove the Allen bolt.
But on this set of forks, the Allen bolt would not come off. So I remove the forks as shown here.
I use a chisel and a long socket extension so I can push the sharp edge of the chisel into the top of the damper rod while I use an Allen socket and breaker bar to loosen the Allen bolt from the bottom of the fork slider.
I mark the fork tubes with “L” for Left and “R” for Right. The fork sliders are sided, so I don’t need to mark them. I like to put the fork slider on the same fork tube it was mated with when I assemble the front end.
Vapor Blast Fork Sliders
I had the fork sliders vapor blasted to remove the blue paint and restore the patina of the fork sliders. Only one tube had the yellow plastic reflector which I removed. I will replace both yellow reflectors. Here are before and after pictures of the fork sliders.
Remove And Disassemble Fork Damper Rod
The damper rods inside the fork tube are secured with a snap ring. I use a pair of snap ring pliers to remove it. Then I remove the damper rod from inside the fork tube.
The damper rods have a non-stock spring on the end. This was an after market “enhancement” that was designed to prevent the forks from bottoming out.
After removing the snap ring, I pull the fork damper out of the inside of the fork tube.
I disassemble the damper rod so I can replace some of the parts that are subject to wear and tear. To remove the parts on the damper rod, I remove the foot at the bottom of the rod. It is a press fit and can be hard to remove. I use a Philips head screw driver in my vice and put it through the hole near the foot. Then I use vice grips on the shank of the foot and twist to get it off.
Then I remove the parts and put them in order on the work bench. I remove the aftermarket spring.
I remove the bottom spring housing which has a flat bottom on one side and a cup on the other side the bottom spring fits into. Then I remove the top spring housing that has two cups in it; the deep one fits against the other side of the bottom spring and the shallow one holds the white plastic washer. The plate with holes fits up against the shallow cup side of the top spring housing.
Then I remove the top spring. Finally I remove the black plastic sealing ring from the groove in the top cylinder on the damper rod.
I lay the damper rod parts out in order on the workbench.
After I remove the parts, I clean up and polish the damper rod with some “0000” steel wool.
Check If Fork Tubes Are Straight
I use a large glass plate and some feeler gauges to check if the fork tubes are bent. The specifications indicate that the maximum bend allowed in the fork tubes is 0.10 mm (0,004 inches).
It is typical for the front forks to be bent somewhat. To reduce stiction of the front fork, I orient the fork tubes so the bend on both tubes point to the front of the bike. That way the fork sliders don’t bind. To check for bending I use a 0.05 mm (0.002 inch) feeler gauge to see if I can get it to slide between the fork tube and the glass plate. I check the straightness at 90 degree intervals.
I visually inspect the tubes and usually I find a shiny area toward the bottom of the fork tube. This is typically the face of the fork tube that was facing to the front. As the forks bend when hitting bumps and such, the tube bends a bit and rubs on the fork slider polishing a bit of the bottom of the fork tube.
I start with the shiny area pointing up as I check for how straight the fork tubes are. The 0.05 mm feeler slide between the tube and the glass plate and I mark how long that section is and then draw a line so I can be sure the line faces front when I install the fork tube in the triple clamp. I found both forks had slight bends towards the top of the fork tubes and one had a slight bend toward the bottom of the fork tube.
Rebuild And Install Damper Rod Into Fork Tube
To rebuild the damper rod, I replace the black plastic seal at the top of the damper rod, the two damper rod springs, the white plastic washer and the snap ring that secures the damper rod inside the fork tube. The new parts are shown on the top row of the picture below.
I start by installing the black plastic seal strip into the groove at the top of the damper rod.
Then I install the new bottom spring, the baffle plate with the holes and the new white plastic washer, followed by the top can with the shallow hole next to the white plastic washer.
Then I install the new bottom spring and the bottom spring cap.
After I get the damper rod assembled, I install it into the fork tube. I’m careful to be sure that the black plastic seal at the top of the damper rod doesn’t get caught on the ridge inside the fork tube. Then I guide the top spring, perforated metal plate and white washer, the top spring cap, and the bottom spring into the fork tube. The bottom spring extends out of the fork tube as the bottom spring needs to be compressed and the bottom spring cap installed and held in place with the snap ring.
After I install the draw rod inside the fork tube and through the damper rod, I push the damper rod back inside the fork tube so it is out of the way. Then I orient small washer on the bottom of the damper rod a bit off center to allow the snap ring ears to clear the edge of the washer.
After I get the snap ring installed, I put the after market spring on the damper rod to prevent the fork from bottoming and the foot back on the fork damper rod. I put the large Allen bolt and new copper washer temporarily on the end of the damper rod to retain the foot on the end of the damper rod until I install the forks in the triple clamp.
Replace Fork Seals
The fork sliders have a fork oil seal in the top of the slider. I replace them by mounting the fork tube in the rubber jaws of my vice. I heat the fork slider around the seal to sizzle hot and then pry it out with a seal puller being careful not to scratch the sealing surface inside the top of the fork tube.
Then I reheat the top of the fork slider and press the new fork seal evenly into the top of the fork slider. It almost goes all the way in. I use a 36 mm socket and a plastic hammer to tap the seal all the way into the top of the fork slider until it butts up against the shoulder inside the fork slider.
Install Yellow Reflectors
The yellow reflectors have an adhesive strip on the back. I clean the slot the reflector fits in with some brake cleaner to be sure the surface is clean so the adhesive will stick tightly to the aluminum. The reflectors are marked “TOP” so I orient them correctly in the slot on the side of the fork slider. I peal off the protective paper from the adhesive strip, center the reflector in the slot and press firmly to get it to stick to the fork slider.
Here are the rebuilt, vapor blasted fork sliders. They look much better than what I started with.
After I install the top plate and steering stem I will install the forks and align them. I will publish a separate document showing how I do that work.