Leaking Fork

My right fork tube started leaking after a poorly landed jump yesterday. Looks to be leaking between inner and outer tube. Any sugestions? Also I'm wondering if I need new springs I've got the stock springs in (99 YZ400) and weight 240lbs. My bike bounes back into the air if I flat land a jump. I've looked at the cost for repair tools (Fork Catridge holding tool/Fork seal driver)and looks to be about $140? Would I be better off taking it to a shop? Thanks for any suggestions. :)


Sounds like fork seals leaking. Not to difficult to replace, or you can send them to a reputable sus. company. I would prolly say you need heavier springs. On www.gp-racing.com they have a listing of what recommended spring rates are for given rider weight. I know there are other sites that list this information as well. Good Luck!

You don't need ant special tools to replace the seals or the springs. I did not disassemble the complete fork, just the inner tube when replacing my seals and springs. On the other hand, every time I have put stiffer springs in a bike it has needed revalving to suit me. If you can afford it take it to someone like MX Tech or some race tech dealer and have them revalved and rebuilt. The change is amazing.

Any possibility you could give me a breif overview of the process.

After I paid 60bucks to have it done the last couple of times, I decided to have my neighbor give me a hand and save a few bucks.

I could not believe how easy it is.

Do not ruin or throw away your old seals. You can use them as a seal driver by cutting out about a little less than 1/4 ow/hack saw.

That will save a ton of $$ right there.

  1. remove front wheel, brake caliper and fork gaurds
  2. loosen top triple clamp pinch bolts
  3. loosen top cap on fork tube
  4. loosen lower triple clamp pinch bolts
  5. remove forks from triple clamps
  6. completely loosen top cap and slide outer tube down exposing spring
  7. hold cap up and pull spring down exposing the hex jam nut at the top below the cap, put a wrench on the nut
  8. hold the jam nut and remove the cap
  9. remove the spring, and associated washers
  10. pour the oil out into a bucket
  11. remove the dust seal by prying with a small screwdriver
  12. under the dust seal is a small metal clip holding the oil seal in place, remove the clip by prying it out with the small screwdriver
  13. compress the inner tube into the outer, and with one quick motion jerk it outward. this will drive the oil seal from the outer tube (it may take a few hits)
  14. this is as far as I took mine apart. I then pumped all the oil out, and pumped denatured alcahol through everything to clean it. I also decided to replace the teflon slide metals as they got banged up when pulling the tubes apart. I let them drain for a day, and pumped fresh oil through and drained again.
  15. now put the new seals on the inner tube. I used a piece of a rubber glove (air filter cleaning gloves) over the end of the tube to protect the new seals. then the slide metals go on.
  16. now i put the inner tube in the outer and smakced down and let the dust seal drive the oil seal as far as it could.
  17. i cut a 1-1/2' pvc coupling in half and used a rubber mallet to hit the coupling half and drive the oil seal in the rest of the way, replace the metal clip and dust seal.
  18. now fill the oil from the cap end with the springs out and the tubes and piston compressed. then pump everything to get the air out, you will be able to feel the difference.
  19. let the air bubble up for an hour then set the final level (95mm on mine). The level is the air gap on the top with the fork compressed, spring removed and piston compressed.
  20. reinstall the spring and all washers in the order removed, tighten caps to pistons, then caps to tubes
  21. make sure to use a torque wrench on the triple clamp pinch bolts, put on the brake and tire and ride.

I think that is all the steps i did, if you can buy a Yamaha service manual for this bike, it is great.

[ February 25, 2002: Message edited by: YZ400Court ]


Go to motomans site and check out his cartridge holder and seal driver. I built mine like his and they worked great. Really cheap and easy to do. The only advise I can give is to use a plastic bag (1 layer) to get the new seal onto the fork leg. Makes it a lot easier.

motomans site is http://mx393.tripod.com/

Thanks for all the help, I'm going to give it a shot.

Dont take this in a bad way. I use condoms to put the new seals on. Preferably new ones! LOL!

They work great.

Are you sure the seal is bad and doesn't just have some dirt in it? You may save some time by running a business card or film negative under it to try to see if any dirty comes out (so I've read this works, never tried it myself). If there is dirt then this may stop the leak.

A .002 feeler gauge works good also. I read somewhere that KTM recommended that to people. If that is the case, you may need to clean the dust seals more frequently in the future. I would also try to avoid that area with the pressure washer.

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