Andrew Birkett's nobugs.org
(email@example.com, June 2001)
After I’d had the bike for a few months, it developed a leaking fork seal. I noticed some milky liquid on the right fork tube, but didn’t do anything about it for a while. This was a Bad Idea. The white stuff was an emulsion formed when the fork oil got passed the seal and mixed with water. As you bounce along the road, this oil gets scraped off by the dust seal and drips down your forks towards the front brake. This is a Very Bad Thing.
Changing a fork seal is quite easy, but you need a special tool to put the new seal in place (or you need a suitable bit of metal tube). Rather than spend money on a tool which I’d only use once, I took the stripped-down fork to my local shop and got them to tap the new seal into place. This cost 7UKP, whereas the tool would be 20-30UKP.
First, remove the two bolts holding the brake calliper in place. (BTW, the manual recommends against leaving the calliper dangling by the brake hose. And don’t go anywhere near the brake lever while the calliper is off the bike). Then undo the screw which holds the speedo cable in place and remove the speedo cable. Take the mudgard off too. Then loosen the axle pinch bolt and undo the axle nut. Put something under the engine to stop the bike from tipping forward when you remove the front wheel. I used a twelve-inch bit of wood wedged under one of the outcropping bits of the engine block. Don’t prop it up by the oil filter though. Now remove the axle and take out the wheel. Don’t let the speedo bit fall out of the wheel.
To remove a fork leg, loosen the two pinch bolts which hold the top of the fork leg in place and the fork leg should slip out. Before you do that, loosen the fork caps a little bit. If you don’t do this just now, you’ll have great fun trying to loosen them with the fork leg twisting in your hand.
Now to dissasemble the fork leg itself. Undo the fork cap, which will pop up once it’s undone from the force of the spring. Inside there’s a spacer tube, a big washer and the main fork spring. And a lot of fork oil. Remove all this and drain the fork oil into a suitable container. Take off the dust cover (I used a screwdriver to carefully lever this off, which probably isn’t a stunning idea) and remove the circlip thing from the top of the oil seal.
There’s an allen bolt at the bottom which needs to be removed. This is fixed with loctite, so it’s quite hard to undo. Use the ‘wrong’ end of an allen key and put a bit of metal tube over the other end give you a bit of leverage. There’s a copper coloured washer in there too which sometimes sticks to the bottom of the lower fork. Now you can tip the fork upside down and catch the piston/spring which falls out. Now here comes the tricky bit - removing the old oil seal. The oil seal is wedged into the lower fork pretty strongly, and it doesn’t come out without a fight. There’s a bushing attached to the bottom of the chrome fork tube, and usually you try to pull the fork apart and hope that this bushing whacks against the bottom of the seal hard enough to dislodge it. Usually, people recommend putting the lower leg into a vice and pulling hard on the fork tube. This didn’t work for me. Now that I’ve seen the disassembled fork, I don’t see how this could possibly work on a CB500 fork. The two bushings just get wedged together.
The succesful method is much more fun. I put the piston and the allen bolt back into the fork, extended the leg fully and filled it with water nearly up to the top. Then, I put the fork cap back on. The idea is that if you press hard on the fork tube, the water pressure will be sufficient to dislodge the seal. You’ll know when the seal has come out because you’ll get soaked by the fountain of water. Make sure the fork cap is on properly, otherwise the o-ring will pop out under the water pressure. If all goes well, the oil seal should pop out. If it comes out at an angle, push it back in a bit and try agin. Now, some people recommend using oil rather than water. That’s going to be really messy and expensive. Getting water in the forks isn’t really a problem since you have them apart and can leave to to dry out (after wiping them with WD40).
Once everything is dry, you can reassemble everything and put the new oil seal in place. Use loctite on the bottom allen bolt. The manual also says to replace the washer there too. It’s very important that the new seal gets pushed into place perfectly flat. At this point I took the leg and the new seal to my dealer since I didn’t have the special tool for the job. A bit of metal pipe around 15 inches long, 10mm thick walls and 40mm internal diameter would do (in case you happen to have one just lying around!). The shop also pointed out that the bottom of the tool has to be stepped so that it presses on the metal part of the seal and not the lips. Each fork leg takes 320cc of fork oil, which should be a depth of 150mm. Once everything is back together, bounce the forks and hopefully there shouldn’t be any telltale streaks of oil. When I bounced them, there was a lot of gurgling sounds so I opened them up again to let any trapped air out.