Covering your motorcycle right after riding under the rain or leaving them outside for long a time are your motorcycle's worst enemy but the front fork inner tube is one of the most susceptible parts. Basically, its hard chromium surface treatment is strong but it can damage the oil seal which lead the front fork oil to leak and will eventually need replacement. The inner tube replacement is much easier than you think in case your motorcycle is equipped with an upright front fork. Please make sure to do replace it as soon as possible if the oil on the front fork leaks due to rust.
The moisture penetrates through the invisible holes.
When disassembling the front suspension, remove the front wheel and pull off the front fork. Remove the dust seal by pressing into the upper part of the outer tube by using a flat-blade screwdriver or a wide tool as shown in the image. Among the various problems caused by aging deterioration and lack of daily maintenance, the most common problem among them is getting rust on the front fork inner tube. Moisture is the worst enemy of the inner tube. It will occur if it's stored whether outdoors or indoors in humid area or when just leaving your motorcycle as it is after a rainy ride.
The surface of the inner tube is treated with hard chromium plating, which is electrically bonded to the pipe material, and some may wonder why rust occur in a plated surface. However, the hard chromium plated film has a few µm (micrometer) to 100 µm. It is actually very thin since 1µm is equivalent to 0.001mm.
Furthermore, the plated film has countless holes and cracks that are invisible to our naked eyes. Leaving your motorcycle with moisture could cause the inner tube pipe to get rust on the part where the moisture penetrates through the invisible holes and cracks and reaches it.
You can possibly reduce the penetration of the moisture by thickening the plating film. It seems that it is not being applied by the commercial model because thick plating film can cost them a lot but hardness and wear resistance can still be secured even with thin-film.
In other words, it is really important not to leave your motorcycle with moisture and store it in high humid areas to prevent getting rust on the inner tube. Also, it may be more effective to apply grease on your motorcycle with an appropriate amount of oil instead of degreasing it using a parts cleaner. But, it is not advisable to apply oil too much on it because the dust and dirt can adhere to the parts easily. Please be careful since the dirt can damage the lip of the oil seal.
- The inner tube gets rust due to the moisture occurs when storing your motorcycle after a ride.
- The hard chromium surface has holes and cracks that are invisible in our naked eyes.
It is necessary to disassemble the suspension when replacing the inner tube.
The upper part of the ZEPHYR's fork piston has 12 corners, and the opposite 21mm bolt fits them perfectly. There is also a model with a smaller hole on the piston pipe and stop the its rotation by pressing it using a special wedge-shaped tool.
The cap bolt that secures the fork piston from the bottom of the outer tube is so alarming, so please remove the dirt from the hexagonal hole by turning it using a hexagonal wrench. Replace the copper seal washer with a new one during the restoration to prevent oil leakage.
It has a gray-colored slide bush at the lower end of the inner tube, and a brown guide bush pressed on the upper end of the outer tube. The guide bushing comes off when pulling out the inner tube since both bushes have different diameter.
The peeled chromium edges are more likely to damage the front fork oil seal lip when the front fork is expanded and contracted with the inner tube with rust. Then, the fork oil inside the suspension cannot be sealed and will start to seep out from the top of the outer tube. This is an example of an upright front fork but, oil will flow out from the bottom part of the outer tube if it is an inverted fron fork.
There is a way of repairing the rusted part by filling in the recess with a soldered metal but in most cases, this is only a temporary repair countermeasure. If you add a strong chromium-plated surface around the rusted part, the surface must be leveled in order for the soldered metal to adhere. It is much more reliable to replace the parts than to use the solder halfway if you can get the inner tube.
It is necessary to remove the front suspension from the motorcycle when replacing the rusted inner tube, which can be a huge task to do. Also, the difficulty level of the work will depend on the construction of the suspension, which should be determined earlier before start working on it.
In case of using an inverted front fork, it is often necessary to use a special tool exclusive for the model together with the universal hand tool, so it is better to get involved with it easily. Also, please note that the model with damper adjustment function on the top of the fork may not be disassembled easily even if it uses an upright fork. On the other hand, the spring will come off and the oil will drain if you remove the bolt on the top of the fork, and the inner tube can be disassembled by removing the fork piston bolted at the bottom part of the outer tube. This type of front fork is called Cerianitype or piston slide type.
- It is necessary to disassemble the suspension when replacing the inner tube.
- It is possible to disassemble the suspension using a hand tool if it is an upright fork piston slide.
It is necessary to stop the fork piston from turning during disassemble and reassemble.
The inner tube may be chane dependng on the model even if on the same modelyer. Please make sure to use a parts that correspond to your motorcycle. You can find inner tube for different motorcycle models at Webike Shopping site.
The piston sliding type, which is widely used as a standard upright fork, has the fork piston that generates the damping force held in place by a bolt under the outer tube. This piston also serves as a stopper when the inner tube is fully extended, so it must be removed when replacing the inner tube.
It turns the bolt with an Allen key while holding the caliper mountings on the outer tube in a vise but the piston in the outer tube along with the bolt may turn and become stuck. It may be helpful to restore the fork springs and upper bolts once the bolts have been serviced and tensioned when the inner tube is in a fully extended position, the piston is held in place.
In the case of the ZEPHYR 400 forks, which I'm showing you its working procedure, there is a 12 square recess at the top of the fork piston, and a bolt with a width of 21mm on two sides must be inserted to prevent rotation. If you combine the bolts and nuts and extend them with a socket tool extension bar, you can hold the fork pistons firmly in place and make it easier to attach and remove the lower bolts.
Once the fork piston is removed, the inner tube can be pulled out. At this point, give the inner tube several strokes to pull out the guide bushing because the slide bushing at the bottom end of the inner tube stops against the guide bushing at the top end of the outer tube. Since the backup ring and oil seal also come off at the same time, the oil seals have been replaced unconditionally with new ones, and the guide bushing and backup ring should be reused if there are no scratches or rust on the surface.
In addition to the manufacturer's original parts, the most popular models are sold with an external inner tube to be replaced sometimes. Whichever one you use, dimensions such as outer diameter and overall length are very important, so it is important to choose the right parts for your model.
For restoration, please insert the fork piston through the inner tube and then into the outer tube, and secure the nut at the end of the fork piston with a cap bolt over the lower end of the outer tube. If the fork piston is off-center, cut it into the oil lock piece (White Plastic part in the photo) at the tip of the piston when the inner tube is full bottom. This will change the damping force at maximum compression, so it has to be fixed in the center.
The special tool seal pusher makes the work go smoothly when hitting the brown guide bushing into the outer tube. Please prepare it in advance because it can increase friction loss and cause oil leaks if you hit in at an angle. The price is around 46 USD.
This is what happens when the guide bushing is pressed into the outer tube (the backup ring is removed). The upper and lower bushings allow the inner tube to expand and contract smoothly while the internal springs and fork oil to absorb the impact and generate damping forces.
The assembly process, which is the reverse of the disassembly procedure, involves placing the fork pistons in the center of the outer tube bottom which is important to fix it. If it is off-center, parts will interfere with each other which makes it difficult to operate, and internal parts will be worn out. It takes a few times to fully tighten the inner tube before you can main-tighten the bolt at the bottom of the fork piston. By stroking it, the center comes out and can be fixed in the correct position to avoid such problem.
It isn't nice to look an inner tube with rust spots but it is a messy trouble which affect to the other parts of the system as well such as damage the oil seal and fork oil leaks. It's one of the topic you want to tackle as you gain experience but disassembly of the front forks is a necessary part of the oil seal replacement process, so make sure to remember the steps and tips.
When setting the oil seal, it is possible to prevent the damage to the lip of the seal by covering the top of the inner tube with thin plastic or plastic wrap. Apply the seal pusher adapter evenly to the top surface of the seal and press into the outer tube, and make sure not to tilt it.
- It may necessary to stop the rotation of the fork piston when disassembling the suspension.
- Centering is effective when assembling the fork piston