Fork oil takes serious abuse when subjected to dirt bike riding. The demand on the oil increases with agressive riding such as motocross and supercross. Because of this, the oil will lose its ability to dampen and lubricate over time. Furthermore, particulate from component wear can accumulate in the oil, thus resulting in contaminated oil. This is why it is necessary to change the oil.
This article discusses oil change and fork spring replacement in sealed chamber forks from a 2010 KXF450. These forks are typical of many models however. Showa and Kayaba forks of this type across a range of many years and models of dirt bikes are very similar, sometimes having only subtle differences. Always refer to the motorcycle owner's manual for service information such as interval, oil type, procedures, special tools etc.1
Never do this job without first thoroughly cleaning the bike. Before proceeding, record the compression and rebound adjuster positions and fork tube height. Once you have recorded the values, adjust the compression and rebound settings to fully soft (counter clockwise).
Remove the number plate and fork guards. Usually, you can remove the bars and controls as a set by removing the bar clamp retaining nuts. With the nuts removed, you can bump the underside of the hand grips (left to right, back and forth) to work the studs out of the triple clamp body. With the number plate removed, you can lay the bars across the fender. If you think a hose may kink when doing this, remove that control from the handle bar.2
Remove the axle nut using an exact fit socket or box end wrench. Do not use any kind of adjustable wrench.3
This axle is still held securely by the axle pinch bolts. Loosen these in even increments, a small amount per bolt until the clamp is no longer tight against the bolt heads. The reason for this is to prevent a single bolt from carrying all of the load exerted by the clamping of the axle. This method is a good practice and reduces the risk of stripped threads in the axle clamps, which are usually aluminum.4
Use a long punch to drive the axle out (the punch must be smaller than the axle). Gently tap and don't try to drive it out all at once. Careless hammering can expand the axle end making it difficult for it to slide through the housings. You could also deform the threads which will prevent the nut from going back on. An added benefit of a long punch is that the wheel will not drop when the axle is out.5
Remove the brake caliper. Sometimes you can carefully let it hang. Possibly, you may be able to lay it across the fender, but don't kink, twist, or over-bend the brake line. If you need to, tie it out of the way with tape or something suitable. Otherwise, remove it and the lever assembly as a set, leaving the hose connections in tact.6
Loosen the fork clamp bolts on the upper triple clamp only. Loosen them incrementally from bolt to bolt, until the clamps are no longer putting pressure on the bolt heads. As with the axle pinch bolts, this is to avoid excessive loading on a single bolt which reduces stress on the threads in the aluminum triple clamp.7
Loosen each inner fork cap while holding the outer cartridge. Then, loosen the outer cartridges. Leave them in enough that you see the o-rings still contacting the sealing surfaces. You will remove these later. Loosening these now makes removal easy when the forks are out of the bike.
It is crucial that you use the correct tools on these parts. You must use wrenches that are made specifically to fit your application. The tools I am using I fabricated myself and fit the parts exactly. The adjustable wrench you see here is being used on the tool, not the fork parts.7a
Loosen the lower triple clamp bolts on one side, but be ready for the fork to drop out. You want to prevent the fork from hitting the floor which can cause damage. You may find that your clamp is very tight though, which was the situation here. The last bike I did, the forks slid right out on their own.
If the fork does not slide down on its own, pull the fork straight down while twisting the outer tube. If it will not slide out, you will have to spread the clamp apart somehow. I use a stiff, thin putty knife to do this. Use a tool that fits well and covers as much area along the seam of the clamp as you can. You don't want to deform or gouge the edges of the clamp. Another option is to use a heat gun to expand the clamp (I prefer to avoid this). However, heat can discolor or damage coatings and melt decals and plastic, so you would need to remove what is appropriate.
Do all of this work with great care. DO NOT hit on the tops of the forks!8
Once the forks are out, make certain they are completely clean, paying careful attention to every nook and cranny. Clean and inspect the triple clamps and surrounding area. Insure your work area is clean as well. Any dirt that can get inside the suspension is very VERY bad.9
Now you can unthread the cartridge assemblies from the outer fork tubes and drain the oil into a drain pan as shown (leave the inner fork caps in place for now) . Let the oil drain for as long as you can (20 minutes minimum).10
After the oil has drained, temporarily screw the cartridges back in so that the o-ring of each just barely seals against its tube.11
Using soft jaws (urethane here) to hold the lower portion of the fork, I am loosening the cartridge rod retainer, which is also the rebound adjuster.12
Each adjuster will unscrew from the fork leg but will remain attached to the inner cartridge. You will feel the fork spring holding the adjuster body against the fork tube.13
Next, you'll need to compress each fork to extend the rod retainers away from the forks for removal. The best method is to place a fork upside down on a protective piece of material (such as soft wood), then push down on it enough that you can insert the holding tool as you see here. The fork is spring loaded and will apply a load against the retaining tool, holding it in place. Be careful not to pinch yourself in between the parts.14
With the tool holding the adjuster body out, you can now remove it. Hold the cartridge rod lock nut with an open end wrench while you unscrew the adjuster body.15
There is a rod that will slide out of the damper rod once the rebound adjuster body is removed. This rod is an extension from the adjuster body to the actual internal rebound valve adjustment. IMPORTANT: It may, depending on the model of fork, have a rotational position when inserted. A rod with a rotational position requirement will stick out too far if not aligned internally.16
Place the fork upside down on the floor again (on protective surface). Compress the fork and remove the holding tool while allowing the fork to extend fully in a slow controlled manner. The end of the fork will now extend past the cartridge rod.17
At this point, you can now unscrew the inner cartidges from the fork tubes. The damper cartridges and fork springs slide right out. Clean all of these parts so they clean and dry. Place them on a surface that is equally clean.18
The remaining fork caps atop of the removed cartridges are actually the compression valve/accumulator assemblies, and they are to be removed next. Unscrew them with the appropriate special wrench. When these become fully unthreaded, you should notice they will not come out just by unscrewing them. I am holding this cartridge in a vise with soft jaws.19
To get a compression valve/accumulator assembly out, you will need to compress the cartridge damper rod while wiggling and pulling up on the cap. Compressing the rod causes pressure in the accumulator chamber which aids removal by pushing the assembly outward. Be patient and wiggle it out carefully. You should only do this with your hands to prevent damaging these delicate parts.20
CAUTION: The now open cartridge is full of oil. Drain the oil by pouring out. While holding the cartridge upside down, pump the damper rod in and out until all remaining oil is expelled. As with the fork tubes drained earlier, it is beneficial to leave the cartridges drain for a minimum of 20 minutes.21
Disassembly and draining is complete now. Do a final cleaning and inspection of all parts. Inspect all o-rings, seals, and bushings for wear or damage. Buy replacements as needed. For information on inspecting and replacing fork tube seals and bushings, please refer to How to replace fork seals.22
Here is a view of cartridge rod end and retainer (adjuster body) lock nut. At this point, be certain that the lock nut is fully threaded as shown. The lock nut should be threaded to the point it can no longer turn with your fingers. It is highly important that these threads be guarded against any kind of damage or deformation.23
Fill the damper cartridges with oil. To do so, you need to know the volume of oil specified by the manufacturer. Pour in as much of the specified quantity as possible without letting oil spill out of the hole(s) in the side of the cartridge.24
Slowly pump the damper rod in and out several times. Refill oil and repeat until all specified oil is poured in and air bubbles no longer appear during pumping. Complete the process leaving the damper rod fully compressed (not extended).25
Lubricate o-rings on the compression valve assembly with fork oil. Insert the assembly into the top of the cartridge (you may need to wiggle it a bit). When it becomes difficult to insert, gently pull down on the cartridge rod (extending it some) while pressing down on the valve assembly top cap. This should aid in drawing the assembly in by causing a slight suction within the accumulator chamber.26
You will need to push down on the valve assembly top cap while threading it into the cartridge. Tighten fully with a light torque. I find it sufficient for these to be lightly tightened so long as they are fully inserted and sealed. When the forks are clamped into the triple clamps, the triple clamps tend to lock the threads. The 2010 KXF 450 cartridge top plug torque specification is 20 ft-lbs. Drain excess oil from the top holes of the completed cartridge.27
Apply fork oil to the wiper seal on the fork spring retainer located on the damping cartridge. Make sure the damper rod is now fully extended. Insert the cartridge (damper) into the fork tube along with the fork spring and any spring preload washers where applicable. Prior to insertion, you can tie one end of a length of mechanics wire to the end of the cartridge rod to guide the rod through the bottom end of the fork tube.28
To get the damper rod completely through the fork, you will need to compress the fork spring and use the retaining tool as was done during disassembly. Before installing the adjuster body onto the rod, remember to insert the adjusting extension rod, being careful that it aligns internally to its correct position. Rods that do not require aligning will likely be round. D shaped rods are position critical. Thread on the adjuster body. When the adjuster body is fully threaded, there should remain a gap between the lock nut and the adjuster. If not, the adjuster is not fully on and/or the lock nut has moved. If this happens, remove the cartridge, check and correct the lock nut and re-install. The adjuster body must be on correctly.
When the adjuster body (retainer) is correctly installed, you can now tighten the lock nut against it to the specified torque (usually around 20 ft-lbs). Finally, compress the fork to remove the retaining tool and carefully allow the fork to extend and engage the adjuster body. Screw the adjuster into the fork bottom and torque to specification.29
Apply fork oil to the top cap o-rings. Complete the oil change process by filling each fork leg with the appropriate oil volume as specified by the manufacturer and screw the top caps onto the tubes. A light torque is sufficient for the top caps because the triple clamps tend to lock the threads.> 30
Slide the forks in one at a time, from the bottom up. Rotate the outer fork tube back and forth if needed to make sliding easier. You may have to spread the clamps as previously described. Positition the top of each fork tube to the same height as before removal and position the compression adjusters and vent plugs so they can be accessed when the handlebars are mounted. Tighten the bolts evenly (incrementally, going from bolt to bolt on one clamp at a time) to the correct torque specification. If you over tighten the bolts you risk not only damaging the bolt threads, but also hampering fork action. These bolts are usually 8mm diameter and typically require about 15 foot-pounds of torque. Check your service manual for the correct value.31
Complete assembly of the bike considering the following suggestions.
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