Another Track day
11 March, 2017
Summer first track-day
9 May, 2017

Single chamber fork service - KX125

It’s a short topic about servicing (oil and seals replacement) for single chamber forks, on a KX125.

There is another topic about dual-chamber forks service, YZ250, here: LINK

Preparation step – need to have something as below:

  • 2 x Wrench 17mm (for fork nuts)
  • 1 x Wrench 21mm (for wheel nut)
  • Screwdriver
  • Socket 12, 14mm (for triple clamps)
  • Socket 10
  • Fork oil
  • 2 x dust seals, 2 x oil seals
  • Seal driver (47mm)
  • Rags

Alright, show time.

First you need to remove the wheel and the brake caliper off the front forks.

Second, you should use the 17mm wrench to loosen the top cap of the forks while the forks still in place with the triple clams. This will make your life easier than loosing the top cap with the fork between your legs 🙂

Loosen the fork cap

After loosen the cap, you may proceed to get both the forks off the bike.

Open the fork cap, until you see internal parts like this:

On top, you can see a lock-nut inside the spring, same size with the cap nut:

You can use your fingers to push the spring down a bit, then remove the cap as below photo:

Remove whatever you can remove from the fork tube, i.e. the cap, spring, spring guide, and the re-bouncing rod:

Pour all the oil out:

You should be patient to make sure you remove as much as possible the old oil inside the chamber. One hand hold the tube, the other hand pull and push the rod slowly, slowly, up and down… I think I spent approx 20 mins for each tube 🙂

Got the screwdriver to driver the dirt seal out of the tube. Hold your screwdriver firmly to avoid scratching the inner tube:

Next would be the seal retaining clip – slowly, firmly and tenderly:

Now we will get the inner and outer tubes separated by pulling them to each side:

No need to pull too strong, even less power than when you bang the table in office 🙂 they would come right off:

Remember the order of all bushing and ring:

Use clean rags to clean all the parts, you can even use some WD-40 to support cleaning.

After all clean, you may start to put everything back together, all the seals, retaining clip and bushing… Someone may buy seal driver bullet to let the seal comes in easily. I just use some thin nylon bag to make this things in so easy. You may spray some WD-40 to the nylon surface to drive the seals down easier:

You will need this Seal driver to ensure both seals in place without damage them:

After all seals and ring in place, it’s time to fill the fork oil. I see quite many people use fork oil level measurement tool to determine right amount of oil in the chamber. I think the best way is just refer to the bike manual, how much is recommended amount of oil to be filled in, then you make sure you fill exact the same amount with recommended in the manual and for both side of the forks:

Refer to the above recommendation, I just need to make sure I put 650 mL of oil into each chamber. Simple as such.

For this single chamber fork type, you have to spend some more time, patiently, calmly, to make sure no air bubble inside the oil by pull-push the rod several, several times. You can easily hear the sound and vibration of air bubble.

When no bubble sound found and the rod is very hard to push down – it is good. Final check oil level according to the manual that “the oil level is higher than two holes and lower (near) the top of inner tube” – you can see it by your own eyes.

Put the spring in place. Put the ring and tighten the fork cap. Put everything back on the bike.

You have done the single chamber forks services 🙂

Some tips:

  • Do not take this nut off the rod, it will work as a knob for you to hold to pull-push the rod two thousand times 🙂

  • Don’t worry about the rod as someone may say it sinks into the oil and you need a rod holder or something similar. People have a lot of tools to retrieve the rod from inside of the spring. I don’t need that – make sure you have no air bubble in the oil, then the rod will be very difficult to sink.
  • In case your triple clamp is too tight, use a small wrench to make your life easier 🙂 :

Thanks for watching.


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