Rebuilding Forks

             This isn’t intended to be a complete guide to rebuilding a set of  forks. There are always variables being introduced with every new model year. This is simply a synopsis of  general things to look for when rebuilding a set of forks. If you do not possess the tools and the knowledge to rebuild your forks, we strongly recommend that you send them to a qualified local shop, or a suspension specialist for rebuilding.

            While getting this 2002 WR250F Yamaha prepped for the 2005/2006 Hillbilly Grand Prix Series, we had to rebuild the forks due to a leaking left fork seal.

Questions ??

             Why rebuild both if only one is leaking? Judgment call mixed with some economics and time management. It is generally not a lot more work to rebuild both, than it is to do one, once you have the wheel off anyway. Not to mention that the other fork has been up and down exactly the same number of times as the one that is leaking, or that it has been through the same amount of water, dust, dirt, same amount of steel/aluminum particles worn loose from inside, etc... Inevitably, if you don’t rebuild both, the other one will fail during the next ride and you will have more things scheduled and not enough time to replace it before the next ride...

             How does water get inside the forks? A friend of mine who always rode KTM’s had told me that water will get inside the forks after a muddy race. I have never spent a lot of time around KTM’s and figured that it was just a “KTM thing”. I rebuilt my forks (1989 Yamaha YZ125) and cleaned them thoroughly prior to the 1991 running of the “Hangover Grand Prix” in Hagerstown Maryland (held every January 1st). The mud ruts were so deep on that day that in places, it was possible to get off your bike and it would stay upright in the rut, not falling over. For curiosity (I find that to be a good way to challenge theories and learn), I dismantled the YZ forks (upside down design, but not sure that matters) after that race. They had a fair amount of water in them. Since I had caught it before the seal failure, I didn’t replace the seals or bushings again during that service. Could you imagine how much wear this would have caused if I had continued to ride it that way for another year?

Tips With This Bike

             I crack the caps loose before removing the fork from the bike. They can be hard to get loose off the bike, depending on how tight the last guy installed them. They don’t need to be much more than snug. I don’t drain the forks prior to removal, as they will always have enough oil in them to make a mess, no matter how much time you spend draining them. I clamp them with the fork cap higher than the axle holder and remove the base valve from the bottom. The base valve can be eased out by hand, once it is unscrewed. With an oil pan underneath, the base valve can be held partly out of the fork while oil drains. A noticeable amount of water came from these forks.

This washer is supposed to be flat. Sometimes they get bent when the fork is jerked apart. Sometimes this happens when the fork was disassembled (just part of the disassembly), sometimes it was reassembled  this way by the last person who did it and didn’t realize it. If the fork is reassembled with a bent washer, the fork seal in that leg will fail in a short time. The Pivot Works kit that we are installing has these washers, so everything is good. If your kit doesn’t, these can be hammered back flat on a vice anvil, but if you beat on them too hard, they get larger and will not fit inside the fork.

            The very bottom of the fork leg has a slight recess inside, outside the base valve hole. This is for debris to sit in and hopefully, stop circulating inside the fork. This should be cleaned and it is hard to get at. Carburetor cleaner works well as it cuts through the grime and evaporates. Don’t get it on the new seals. Carburetor cleaner is hard on rubber. Use the carburetor cleaner carefully. It is extremely flammable. Follow usage instructions on the container.

            Fork bushings should be inspected (if not replacing them) for wear. If Teflon coating is worn away at any point, they absolutely must be replaced if you expect the seals to last.

Bushings in this condition (photo) would normally not be replaced, but since the seal kit came with them, no need to take any chance.

            All parts were washed in solvent and placed on a clean paper shop towel and left to dry.

With parts clean, the axle holder is placed back in the vice (same as disassembly). A sandwich bag (Ziploc area cut off) is placed over the end of the tube so that first the wiper, then the seal can be worked onto the tube and past the bushing groove. Without the use of the bag, the seal and wiper lips will get cut when sliding them past the groove and they will not last long. Remove the bag and install the washer, then the larger diameter bushing, then the smaller diameter bushing is placed into it’s groove.

The tube is slid over the first bushing, then the seal driver tool is used to press the second bushing into the fork tube and solidly seat it.

The same driver is used to drive the seal into the tube and seat it. Then the clip ring is installed, then the wiper is installed.

Now the dampener (cartridge) is installed and the base valve is screwed in and tightened. The fork is set upright on the floor and filled to the top with oil. The dampening rod is stroked fully up and down until the oil level drops. Oil should be added as the level drops while bleeding, in order to keep the dampener rod totally submerged at all times. Continue to stroke the dampener fully up, then fully down until there is resistance all the way out. At first, there will be none, then there will be some as the rod starts upward, but then the rod releases and moves with no resistance. This is the process of bleeding the air from the dampener. Until the rod has resistance all the way out, the oil level cannot be adjusted, as there is  still air space inside the damper. If the rod never gains resistance, the dampener is faulty and will have to be replaced. When the air space has been bled through, the oil level can be adjusted. On some inverted fork designs, oil will come shooting from the top as the bleeding process is performed. Be  further warned that some of these designs also have pieces in the top that will come out while bleeding if the cap is removed. The cap can be threaded by hand onto the dampening rod to allow bleeding without the parts blowing out. On this bike, the recommended oil level is 5.5”. Using the fork oil level tool, this measurement is made and the level is adjusted.  

Now the rod can be extended fully, install the spring, cup, bumper, and the fork cap. Tighten the cap onto thedamper rod, then the cap can be installed in the tube. Wipe down the outside of the tube, clean the inside of the triple clamps (remove oil, paint, etc..) and install the tube. Torque the clamp, tighten the cap, make sure that the forks are at the same distance through the top triple clamp on each side, and you are done.     

Important Parts - What They Do

             Dust Wiper - This is the first line of defense for the fork seal. The wipers duty is to wipe the chrome leg clean of mud, dust, and water so that the seal only travels on clean smooth chrome.

             Seal - Obviously, this is what keeps the oil inside the fork. Some of these are designed with a wiper lip made on top to further help wipe the leg clean before the two internal oil sealing lips make contact. Seals generally fail due to damage from debris that made it past the dust wiper and the seal wiper (if equipped). It is common to see narrow cuts in a failed seal lip. These can be from metal debris getting passed on the way out, or from dirt getting passed on the way in. Prying the wipers out after every ride and cleaning the area on top of the seal and under the wiper, will prolong seal life. It will also impress you with what gets in there!

             Bushings - These are what stabilizes the fork leg to the tube. Many older designs do not use these, but most of the dirt bike designs since approx. 1980 do. These bushings are generally Teflon coated. The manufacturers recommend that these bushings always be replaced every time that the fork seals are replaced. If you don’t and they are worn, the slider can move on the tube more than intended and the seal will fail due to the lips not being able to flex enough to keep up with the front/back movement. Generally, if the Teflon is not worn away or peeling off, these can be reused. With the Pivot Works seal/bushing kits, the cost is more attractive to simply replace all the wear parts and be done with it.

             Chrome Tube -  Critical that there be no dings or major scratches in this. If there are, you can attempt to dress them with a fine file, then cross hatch sand them with 600 grit. Any rough place on this tube will either nick a seal and wiper, or an indent will make it difficult for seals and wipers to adhere to the tube and seal and do their task.

            Upper Tube Inverted Forks - Care must be taken not to over tighten the pinch bolts. The lower leg piston bushing has to travel past the lower triple clamp. If these bolts are over tightened, it will cause binding due to deforming the tube. This WR250F torques only to 14 ft lbs. This can be easily passed with a wrench.

              Axle Pinch Bolts - Do not over torque these either. You can break a lower leg. Then you get to spend a lot of money. Before tightening them at all, install the wheel and axle. Tighten the axle nut to proper torque spec. Pump the front end up and down several times. This works the forks to settle in their natural position on the axle. This will insure that they do not bind as they are compressed. Now you can properly torque the pinch bolts.        

            Fork Oil - Amazingly enough, there are absolutely zero standards for the manufacture of fork oil. A company can place Iced Tea in the bottle, sell it for fork oil, and there is no method for the consumer to use to determine that it is not of sufficient quality. Quality is totally up to your dealers or suspension specialists experience, or your confidence in the company who makes the oil. We use Silkolene products as first line in everything. Some other oil manufacturers have some oils that I like, but rarely everything in their product line. Silkolene is different. They offer very good quality oils in every category.

             Cleanliness - Crucial to a good result. I have seen shocks dynoed on a suspension dyno that had a very small piece of dust in a valve stack. Huge difference in the function of the suspension. One revalver that I know has a “clean room”. They do dirt bike shocks and are an Ohlins race car suspension distributor. They use gloves to assemble in a room that has filters on all the heating and cooling vents. All suspension units are dynoed. Results are recorded. When that particular shock is rebuilt again, it is dynoed prior to disassembly, again after assembly, then compared with the original dyno run from the first time that the shock was rebuilt to insure consistency. We don’t go to these extremes, but we do strive to get every piece of debris out by thoroughly cleaning everything and taking little chance of debris being left inside. If these guys take it that serious, you should give it some thought as well.