Monoshock Rebuild

I am posting this thread mostly for those who have done the monoshock conversion on their KZ, but it should be applicable to those who own a GPZ, or a dirtbike, or a modern sport bike with a monoshock as well.

Recently, I decided to rebuild the rear monoshock from a 1995 GSXR 750 which is going to be the rear shock on my 1978 KZ1000. If you have ever read the owners manual for any modern bike, it always tells you to replace the oil in the front forks approximately every two years or however many miles. It usually also tells you to inspect the rear shock, but there is seldom any information on how to service it. Normally the service manual tells you to replace the shock if there is any sign of damage. It is my belief that this policy exists due to the compressed nitrogen charge and the potential danger it poses. Now it stands to reason that if the oil in the forks breaks down due to use, then the shock oil would do the same.

I'm here to tell you that it does. The particular GSXR shock in question had no dampening effect on the bike. It was a pogo stick. In a few pages we'll see why this was the case.

Now I will begin to detail the rebuild process and I will include the installation of a Race Tech Gold Valve which is totally optional.

The usual disclaimer applies. This thread is for information purposes only. Any damage or physical harm that you receive is as the result of your own actions. The shock absorber assembly contains compressed nitrogen under high pressure and a very strong compressed spring.

Also, this thread will have numerous posts and at least 50 pictures, so bear with me if I don't get it complete tonight.

Now on to the fun. Prepare to get out your multi-lingual curse word thesaurus.

Step 1) Raise the rear of the bike in such a way as to remove any load on the rear suspension. This usually means lifting the bike by the frame. Obviously, you cannot use a conventional rear stand as it places the weight on the bottom of the swingarm.

Step 2) Remove the shock absorber, and any remote mounted reservoir if present. This will vary by bike, and if you can't handle the shock removal, then you don't need to be getting into the shock itself.

Step 3) Remove the dust cap on the nitrogen reservoir. It is normally just a sheetmetal cap that is pressed into the end. You normally have to damage the cap to remove it. I pried mine out with a screwdriver by cutting a slit in the cap with an angle grinder.

Make sure to put the slit off center so as not to damage the nitrogen valve under the cap.

Now pry the cap out with a flat screwdriver.

Now you should see a schrader valve just like your tires have. This holds in the compressed nitrogen. It should be under quite high pressure. This particular shock requires 200 psi.

Step 4) Remove the cap and then using a valve core tool, SLOWLY unscrew the valve core. You just want to crack it enough to bleed off the pressure. Once the pressure is gone, then fully unscrew the core, and keep it in a safe place.

Here's what the core looks like when removed.

Now this is where my first problem was discovered. My shock had no pressure beyond normal atmospheric pressure. The nitrogen pressure in the bladder is there to keep the oil in the shock from foaming, and the bladder separates the nitrogen gas from the oil, and allows for the oil to displace when the shock compresses. My guess is that my oil would probably have had some included gas by this time which is not good for consistent dampening. Also, it would have no resistance to foaming, also not good.

Now, this is to be expected since my shock was from a 15 year old bike itself. However, just a little research on the GSXR forum lead to the discovery that more than one owner of a new GSXR had found no pressure upon the teardown of their shock. Interesting. :ohmy:

Step 5) Push down on the aluminum piston at the top of the reservoir to reveal a circlip. You must remove this circlip carefully with a small flat screwdriver, but ONLY AFTER YOU HAVE RELEASED THE NITROGEN PRESSURE! If you can't press the piston down with your hands, then you probably forgot to bleed the pressure. Return to step 4.

Step 6) Adjust your air compressor to a low pressure, lets say 40psi. Using a tire chuck, apply some air to that stem on the piston. You need to cover the end of the piston with your hand as the air pressure will force the piston out of the reservoir quite rapidly. Make sure that the piston is pointed in a safe direction away from your face. If it should slip, it will fly with great force.

Once the piston clears the circlip groove, you should be able to pull out the piston with the attached rubber bladder. Now pour out any oil into a drain pan.

Step 7) Now we move on to removal of the spring so we can get into the main shock body. First, you need to measure the position of the preload adjuster so that you can return to its original position. Now back off of the spring preload adjusters as far as they will go. This will make it easier to compress the spring. Who knows you might be one of those lucky guys who doesn't have to compress the spring if the retainers will come out. I'm not that lucky. Insert your favorite cuss word here.

It's worth mentioning that this is one of those areas where you need to take extreme caution. A compressed spring is potentially very dangerous. The correct spring for my setup is 8.5 kg/mm or 475 lbs/in. That means that it takes 475 lbs of pressure to compress the spring just one inch. If the compressor were to slip off an otherwise unrestrained spring the results would be disastrous. Do not ever get yourself in line with the compressed spring, and while it is compressed treat the thing like a bomb. Ever question what a spring can do? Google Felipe Massa sometime, a flying spring ended his F1 season last year.

Step 7) Compress that spring. :S

Step Remove the spring retainers. They are usually a the bottom collar of the spring. They vary from one manufacturer to another, but the story is usually about the same. This Showa shock has a split retainer that is retained by a continuous ring. The picture explains it better than I can. First pull out one side:

Then the other. At this point you should be able to slip off the continuous collar.