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Jan 5, 2010

How To Rebuild a Dirt Bike Rear Shock, 2001 Honda CR250

The following was done on a 2001 CR250 although most bikes are very similar and the tips shown here can be applied to many bikes. The rear shock on a dirt bike is something that often gets neglected. It seems as though it will slowly leak oil and gradually looses performance in a way that can go unnoticed. It's also one of those things that unless you've done it before, can seem mysterious and more difficult than it really is. For the most part, this job can be done without any special tools or knowledge. If you leave the valving untouched, an oil change and seal replacement can be done in little over an hour. The only specialized "tool" needed is the nitrogen to charge the bladder when your all done, and that can be done at a shop for pocket change most of the time. Some tire shops even have nitrogen to fill car tires.

Start by removing the rear shock from the bike. I actually found it easier to release the preload on the spring while the shock was still attached to the bike(with the airbox removed). Release the preload all the way so the spring is free to move around. With the shock removed, release the nitrogen pressure from the bladder, then find the nut right next to the clevis end of the shock. Mount the clevis in a vise and turn the nut counter clockwise. In my case, the nut moved a bit before tightening down on the threads, then it began to unthread the shaft from the clevis.

Once the clevis is removed, remove the spring and retainer. Now get a dull chisel of some sort and pry the dust cover away from the shock body.

Now that the dust cover is free, and there is no nitrogen pressure, push down on the head seal with your fingers. It should move by just pressing on it, but might require some force. Push it down until you can see the snap ring. Get something pointy and sharp and remove the snap ring. Once the snap ring is removed, all the guts should come right out. Sometimes releasing more pressure in the bladder helps if it is sticking, and sometimes a good jerk will get it to come as well.

Now you can use the top nut to aid in removing the bottom nut that was next to the clevis. Mount it in a vise and turn. The top nut should be locked on very tightly and if removed, must be replaced with a new nut. Either way, one of the nuts needs to come off to replace the seal head, the bottom one seemed easier to me. Next, remove the bladder. This is done in much the same way that the seal head was removed. Push down to reveal the snap ring and remove the snap ring. A tool is made to thread onto the Shrader valve in order to pull the bladder out. I didn't have one, so I used a piece of rubber and some small vice grips to clamp on softly but firmly onto the Shrader valve, then worked it out. Clean everything up well enough to eat on and put the new head seal on the shaft. Make sure you get it back on the right way so you don't have to take it back apart.

Now that everything is clean, it's time to fill it back up with oil and re-assemble. I used Pro Circuit 7wt shock oil. First, pour the oil about halfway up in the reservoir. The oil will self-level in the shock body. Get a catch pan under the shock and push the bladder into the reservoir. The oil should overflow out of the top of the reservoir. Push the bladder past the circlip grove slightly. Put the circlip in and make sure it is seated all the way around. Add 35 lbs of compressed air to the bladder. This is to bleed out the air in the reservoir.

Next, with the new seal head on the shaft, put the bump stop and the nut back on the shaft behind the head seal in the order it came off, again using the top nut to get the bottom nut tight on the shaft. Add oil to the shock body until it's about 3/4 of an inch from the top. Now put the assembled shaft, with the new seal head, bump stop and nut into the shock body. Push down slowly and steadily. Watch for air bubbles in the oil as you push down. If necessary, add oil so it remains about 3/4 of an inch from the top and push the new seal head in, holding pressure on it until it stops going into the body. While pushing down, release the air pressure from the Shrader valve on the reservoir. This will allow the seal head to go in
further, just slightly past the circlip groove. Install the
circlip, fully seated. Now the spring, spring retainer, and clevis can be
installed. Again, mount the clevis in a vice and use the nut
to turn the threaded end of the shaft back into the clevis.

Now your ready to take the shock to a shop to have it
charged with nitrogen. My service manual calls for
142 psi, but somewhere between 130 and 175 is normal.
The shop I took it too did it for free, but it shouldn't cost
much more than $5 to have it done. Air can be used in a pinch,
but nitrogen will hold it's pressure better and has less
contaminants. Nitrogen is even used in some car tires nowadays
because of the benefits it has over just plain air. Put the shock
back on the bike and adjust the preload to where it was before
and your done!