I describe this process for informational purposes only; if you choose to try it, any untoward result is on you.
The procedure requires essentially no mechanical aptitude. The only step that requires any significant force is step 6. If you find yourself having to apply more than gentle force at any other stage, something’s wrong. Be mindful not to use tools or methods that will score the shaft or damage the oil seal.
Tools and supplies:
- soapy water and a cloth or soft brush
- shop towels
- compressed air source (optional)
- snap ring pliers
- plastic hammer (optional)
- used oil receptacle
- replacement oil
- thick monofilament line or very small zip tie
- syringe with blunt needle (optional)
- hard but cushioned work surface
- 3/8" drive socket with an outside diameter just smaller than the inside diameter of the damper tube (optional)
1. Remove the damper from the bike and ensure it is squeaky clean - use soapy water and compressed air if necessary to get any crap out of the ends;
2. Using your snap ring pliers, remove snap ring on the side opposite the eye and set it aside. It has no orientation (i.e. it doesn't matter which side faces in and which out);
3. Remove the spacer below the snap ring and set it aside. It also has no orientation;
4. Slide the shaft such that the eye is as close to the damper body as it will go. You will feel a positive stop;
5. Remove the small metal cap from the shaft end opposite the eye. It is just a friction fit; mine came off by hand. Don't grip the shaft with pliers or anything else that might score it; that will eventually ruin your seal;
6. Holding the damper body vertically, strike the eye on some rigid but cushioned surface, just hard enough to pop out the greenish oil seal on the side opposite the eye. I used a rubber mat on my workbench. You can also strike it with a plastic hammer like the one ParkTool makes for working on mountain bikes, or a dead blow mallet, but if it is horizontal when the seal comes out, you're going to have oil everywhere;
7. Gently slide the oil seal off the shaft and set it aside, noting its orientation - the inner and outer surfaces are different;
8. Drain the old oil into the used oil receptacle (if this is the factory oil, it's about as viscous as sixth-beer pee). Pump the piston several times to ensure complete drainage;
9. Add your chosen oil. I used Maxima 20wt fork oil with a blunt-needle syringe. You will need to work the piston up and down several times (very slowly if you don't want to end up wearing fork oil) to get out all the air and ensure a complete fill. Be patient and do this until you see no more air bubbles emerging. Ensure that the damper is filled right to the very top, using a shop towel to dab off any remaining bubbles;
10. With the damper now filled and secured in the vertical position, slide the seal back onto the shaft, observing the correct orientation. This is the most important part: as you put the seal onto the shaft, place a piece of thick monofilament line, or the tapered end of a very small zip tie, between the shaft and the center hole of the seal. This will create a tiny gap between the seal and the shaft that will allow oil to escape as you press the seal into place, and will ensure that you have nothing but oil left inside the damper. If you don't do this, you will not be able to replace the seal unless you remove some oil, which requires trial and error and creates the virtual certainty of trapping air inside. Whatever you use, you only need to create a tiny gap between the seal and the shaft. Don't use anything big or sharp enough to wreck your seal;
11. Press the seal into the damper body, just deep enough so that the snap ring will fit back in its detent with the spacer inserted. Go very slowly. Oil should ooze through the gap created by the monofilament as you do this. You should see no bubbles. A suitably-sized 3/8" drive socket can help - slide it over the shaft and use it to put even pressure on the seal so it isn’t tempted to go in cockeyed;
12. With the oil seal now in place, gently pull out the monofilament, restoring an oil-tight seal against the shaft. Ensure that whatever object you used to create the gap comes out intact. If any part of it breaks off and remains inside the damper, it could create a leak, or worse, it could in theory plug the tiny hole in the piston and interfere with your steering, which, if it happens while you are riding, could really mess up your day. It is probably a very small risk, but even a small risk of something with such potentially dire consequences is worth avoiding;
12. Clean and insert the spacer;
13. Clean and install the snap ring;
14. Replace the cap on the end of the shaft. If it seems loose, you can tap it with a hammer to seat it, or even add a bit of non-permanent LocTite. I'm not even sure why it is there; it seems to serve no functional purpose;
15. Wipe the unit down - especially the end where you removed and re-seated the oil seal - so that any leaks will easily be visible. Cycle the piston a few times (it's harder than you think it will be) and check for leaks; and,
16. Reinstall your damper, test ride, and check once again for leaks afterwards.
That's it.
