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A friend of mine was brave enough to take his apart and feel out the process. I've taken and tried to perfect it along with instructions and photos.
Let me start by saying this damper is built just like a rear shock. In the picture below the item marked number 1 acts like the nitrogen bladder on a shock, only instead of a rubber bladder there is a piston and spring. The item marked number 2 is the shaft and has the seal head and valve piston and shims. Item number 3 is the adjust knob -- which we will not remove during the service of the damper.
Start by removing the damper and clean it really good. This will prevent any dirt from contaminating the work area and internals of the damper.
Next, place the damper in a bench vise with the bottom facing up. Then remove the two circlips. I don't recommend using anything but circlip pliers for this job. The area is tight on space and you don't want to risk weakening the clips or scratching an surfaces. NOTE: When you slip the circlip off of the shaft, do it as close to the eyelet and bearing as possible. You don't want to scratch the shaft surface where the seals ride.
Next we need to remove the plastic cap on the spring side of the damper. The cap has two flat sides. Use a pair of needle nose vise grips set with just barely enough gripping force as to not damage the plastic cap. Then very slowly pull the cap out giving care to the rubber o-ring wrapped around the cap. You can actuate the shaft of the valve piston to help push the cap out as well.
Someone else made the discovery that the hole in the center of the plastic cap is threaded. You can thread the correct size bolt into the hole and use this to pull the cap out instead of pliers.
Under the cap will be the spring. Remove the spring.
Under the spring will be the piston, which looks a lot like an engine piston. It has a recessed hole that is threaded. Find a spare bolt with the same thread pattern and thread it in a few turns. Then lift out the piston slowly, giving care to the rubber o-ring wrapped around the piston.
Here is what the piston looks like after you have removed it.
Now slowly pull out the shaft along with the seal head and valve piston. You can wiggle this back and forth a bit in the process. Just be careful for the rubber o-ring again.
Here are the parts you should have laid out now.
An amazing discovery here is there isn't much oil in this damper from the factory, and the quality of it doesn't seem to be very good either. In the syringe (is that how you spell that) below you can see there is about 14 cc or ml of oil in the damper. I have 28 hours on my bike and look at the oil. When I'm done with the rebuild, I'm guessing there is about 20 cc or more oil in the damper.
A friend picked this up for me at the dollar store. It was in the kitchen area with the pots and pans and other kitchen tools. Marinate Injector. It really makes doing this job easy because you can control how much oil you pour in to the cavities.
Ok, now make sure everything is clean. Spray some brake cleaner inside the damper cavities and blow it out with compressed air. Then place the damper body back in the bench vise and fill the spring side cavity slowly with the oil of your choice until it is filled to the top with no more oil transfer into the valve piston cavity.
I'm using Maxima Performance 5W Fork Oil. Just for reference you could use whatever weight you want. The thicker, the heavier the action will be on the damper. My friend is actually testing out Rotella in his at the moment, but has no ride time with it. It is much stiffer than stock with the Rotella, but I wouldn't say extreme. With the 5W Fork Oil, mine is now noticable at the stiffest setting on the adjuster.
Next we will insert the spring piston back into the damper body. The goal during this process is to have no air trapped below the spring piston. Very very slowly insert the piston at an angle that allows any air under to float to the top before the o-ring begins to seal. Only insert the piston a short distance. We don't want it to drop too far. Then remove the bolt.
Now, air can be compressed, but oil cannot. So, insert a shop rag and soak up any oil on top of the piston. The area above the piston where the spring is located is the only place we want air in the system. Without air here, the damper will hydraulic and you will get no performance from the damper.
Now insert the spring and push it in until the top of the spring is about flush with the top of the cavity.
Now place the cap on top of the spring. I found that pressing it in can be kind of tough with big fingers, so I used a phillips head screw driver along with my free hand to tempt it into place. Do you best to only push it in far enough to be able to install the circlip. We want to keep as much oil as we can under the spring piston without any pressure on the spring.
After you install the circlip, the oil level will look something like this without over flowing the top. At this point, there isn't any force on the plastic cap or circlip. It will pop fully into position during the last steps of the installation.
http://www.roostzone...SD/P4030559.JPG
Slide the seal head as far as possible from the valve piston. You do this so that when you insert it into the oil, you can raise and lower the valve piston to bleed out any trapped air in the valve piston and shims.
http://www.roostzone...SD/P4030560.JPG
Raise and lower the valve piston in the damper body deep enough that the seal head doesn't touch the oil and only high enough that the o-ring on the valve piston maintains a seal on the damper body. If you accidently raise it higher than the seal, it is OK as long as you don't raise it higher than the oil level. Do this until you see no more air bubbles coming to the surface.
Keep the oil level at the top of the damper body during the bleed process. Top it off again after you see no more air bubbles.
http://www.roostzone...SD/P4030561.JPG
Now raise the valve piston as high as you can without breaking o-ring seal on the body. Then, with the oil level still topped off, lower the seal head very slowly so that about half of it is still exposed. You should see the trapped air from under the seal head bleed out through the oil.
http://www.roostzone...SD/P4030562.JPG
Press the seal head with your fingers as far as you can into the damper body. It will not go in far enough at first to get the circlip in. This is OK. While pushing down on the seal head, actuate the shaft down and up. Repeating the process several times. You should see more air and some oil bleed past the seal head in the process until you eventually have the seal head almost recessed enough to install the circlip.
Again, the goal is to get all of the air out of the system, except for where the spring is located.
When you think you are close to getting enough space to install the circlip, place it on top of the seal head. Remember to slip the circlip over the shaft as close to the eyelet and bearing as possible. You don't want to scratch the shaft surface where the seals ride.
Connect you circlip pliers to the circlip with one hand. Then use a shop towel under the thumb of your other hand and push on the seal head and circlip until it snaps into place.
http://www.roostzone...SD/P4030563.JPG
Now remove the damper from the bench vise and check that the shaft freely travels through the entire range of movement. You also want to listen for any trapped air. If you hear air or the travel is restricted, you need to start the rebuild process over.
I hope you find this how to article useful. I'm not expert, so I'm willing to hear from anyone who has perfected the process better than I've outlined above. The goal is to make this a doable project for anyone.
This rebuild can be completed in about 15 minutes from start to finish with the right tools.
ben
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