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Disclaimer: CMG Online and/or Sonic cannot accept responsibility for any advice given. All advice given is on the assumption that the work is being carried out by a suitably qualified technician, especially when it comes to safety critical parts such as braking systems and chassis components.
We have been known and will no doubt once again make the occasional error. Please use the advice given as only a rough guide as to how to complete any job. All work carried out on a motorcycle should always be done with the aid of a suitable workshop manual.




It's an exploded diagram init? Click for bigger pic and component listing.

Many new riders, as well as some seasoned pros, rarely give their front suspension a second thought. That is until something goes wrong. Don't feel too bad, most humans don't do anything they don't absolutely have to ... until they absolutely have to!

Most modern street bikes, as well as off-road motorcycles use telescopic forks in some form or another. Some have air assist while others have adjustable dampening, but all have fork oil and a pair of seals that keep the oil inside the forks, where it belongs. As I said earlier, most of us don't give any of this a second thought until that oil that should be on the inside, ends up all over the outside.


This becomes evident in the form of a dirty oil ring that appears on the upper fork tube, in-between the slider and the lower triple clamp. This ring is the telltale sign that the fork seal has failed and that it must be replaced. If this problem is ignored, the oil that is leaking out will eventually run down the fork leg(s) and on to the brake disc(s). Note - Once the oil finds its way onto the disc it will be quickly mopped up by the brake pads. Oil contaminated pads cannot be cleaned off, the pads are junk! Oh, by the way, oil on the brake pads will also somewhat impede the stopping ability of your bike. Strange but true.


Fork seals are relatively inexpensive as far as buying parts for your machine go. They range in price from $20-$45. If you purchase the OEM parts they will probably also come with a new dust cover. The aftermarket seals won't but do come with two seals to a package. Since the dust covers just help prevent crap getting into the seals, you should be able to reuse them unless they are obviously damaged

If only one seal is leaking, you do not necessarily have to change the seal in the fork that is not leaking. However, since you've already done half of the work required to change both seals, and the other seal is most likely just about to let go, it's worth doing them both at the same time. If not, I would at least recommend you take the opportunity to change its fork oil, which should be replaced every two years whether the seals are leaking or not.


Tim removes the fork brace and looks very professional in the process.

In order to replace a fork seal, the forks must be removed from the bike and then disassembled. This sounds like a big job but it's really not that bad, unless of course your bike only has a side stand. Fortunately the '85 Honda Nighthawk 650 that we were working on has a centerstand as well.

With the bike on the stand, the calipers must be removed from the fork lowers to give enough space for the front wheel to be removed and to also isolate the front forks for their removal. I recommend tying them back out of the way, supporting the weight of the calipers and thus preventing undue stress to the brake lines. A small bungy cord is ideal for this job.

The Nighthawk is equipped with an Anti-Dive mechanism that connects the caliper of the left side to a push rod on the fork lower of the same side. The idea being that as the brakes are applied the push rod is activated by the pivoting of the caliper. This restricts the flow of the fork oil through the fork dampening system, thus slowing the rate of decent on the suspension during hard braking. Because of this system, the brake pivot mount must also be removed in order to allow enough clearance for wheel removal.

Before removing the wheel, it's also a good idea to remove the speedometer cable from the speedo drive unit. It's not absolutely necessary, but it does make the job a bit easier. The axle lock nut can now be removed and after loosening the axle pinch bolt, and the axle may then be taken out. With the front wheel out of the way, the front fender and fork brace can now be taken off. Almost there, just two pinch bolts left to loosen on the upper and lower triple clamps, enabling the left side fork to be slid easily out through the bottom of the clamps.


Removing the retaining bolt with fancy special tool.

With the fork removed from the bike we can start the task of getting the old seal out. These forks are of the air assist type so it's wise to release the air pressure before undoing any sealing bolts. There isn't much air pressure in a fork at the best of times (6-10 lbs, or 0 lbs if it's already leaking), however it is enough to cause the oil to squirt out unexpectedly, and directly on to the person removing the bolts!

The fastest way to remove the old oil is to release the retaining bolt located in the very bottom of the fork. Because the retaining bolt has to be removed anyway in order to remove the fork tube, it provides a lower and larger drain port than the standard small drain screw located on the side of the fork lower.

Unfortunately, this is the point that can defeat many a home mechanic. Since the retaining bolt is screwed into the damper rod of the upper fork tube, you may find that you're continually turning the bolt without it ever coming loose. What you're doing is spinning the internal damper rod (front fork cylinder in the picture). You can try and get a friend to compress the fork while you're doing this (thereby hopefully holding the damper rod still under spring pressure), or you can go and buy the special tool that is inserted into the top of the fork and jams the damper rod in place while the bolt is removed. Failing that, if you have air tools, an air gun with the right attachment can spin the bolt faster than the rod and thereby release it. Just be very careful not to over do it and round off the bolt's head - uh oh!

Yanking on the tube will eventually release the slider and seal from the lower.

With the retaining bolt removed, along with its copper washer (don't loose it in the oil!) the old fork oil can be drained into a pan. Depressing the fork as you hold it over the pan can speed up this process, but make sure you're wearing old clothes, as this inevitably ends up being messy. With the old oil drained, place the fork into a vice ready for the seal removal. Note - be very careful which part of the fork you clamp the vice onto. It's hollow, with some parts being a smooth surface to prevent seal damage. Clamping in the wrong place can either distort the fork or damage a surface. I find that either the brake caliper mounting surface or the lowest point where the axle resides are good points of clamping. Also, use some soft metal inserts and/or rags in the vice to prevent marking the fork lower.

With the fork firmly clamped into place, carefully pry the dust cover up and then slide it off the tube. The seal retaining clip is now visible and requires the use of circlip pliers in order to remove it from the fork lower. Now, at long last, the seal can be removed. With the fork still held firmly in the vice, the fork tube should be pulled rapidly away from the lower until it butts against the seal. This process is repeated until the tube, along with the seal, is released. The seal can now be simply slid up and off the tube.


Sonic replaces the circlip. Well done sir.

When installing the new seal make sure that it is well oiled before attempting to slide it over the top of the fork tube (to prevent damage to the seal's lip). The tube, with the new seal in place, can now be slid back into the fork lower. A fork seal driver should be used to seat the seal into place. The home mechanic can save some $$$ by cutting a length of black plastic pipe (make sure it's the right internal and external diameters) to size, which will also do the job nicely.

With the seal back in place, the circlip can be reinstalled along with the lower retaining bolt (use some thread lock), and don't forget the copper washer. All that's left to do is to replace the circlip and dust seal and then add the fork oil. When doing any job on your motorcycle I will expect you to have a service manual handy. The manual will provide you with many tips and other important information, in this case how much fork oil to put into each fork.

This Honda is a special case. Due to the Anti Dive mechanism, the left fork requires more fork oil than the right. The service manual points this out and gives the precise measurements in cc's. I've found the ideal measuring device for fork oil is an old baby bottle. Most baby bottles are graduated in both ounces, ml. or cc's, and so it's perfect for this sort of thing. Plus it only costs $4.00 at the pharmacy.

Refilling the tube with oil. Hey, that's not a baby bottle. Scam!

Now before the new fork oil is put in, you'll have to select an appropriate grade. There are three different grades available. Again the manual will specify what is best suited for the bike in question. As a rule of thumb 5W (yellow in colour) is the thinnest and so is designed for a soft ride, such as dirt bikes. 10W (blue) is for an intermediate ride, like a cruiser, and 15W (red) gives a firmer ride for the sport bike crowd.

The quickest way to get the fork oil into the fork is through the top. This requires that the big bolt that holds the big fork spring in place, and under some compression, must be removed. Hint, big spring - do not take a haphazard approach to removing this bolt, like keeping your face right over the thing as you're unscrewing it. Pay attention and keep pressure on the wrench to prevent the whole works: wrench, bolt, and spring, from flying out and into your face. It will definitely leave a painful mark, most likely on your forehead!

That's a fork seal that is.

With the fork oil in and the fork leg back together, we repeated the procedure on the other fork. The rejuvenated forks were now installed back onto the bike and the front end reassembled. Thread lock was applied to the bolts that secure the forks, brakes, and axle pinch bolts. Also, any time the brakes are removed it's important to remember to pump the lever a couple of times, to take up any space that may have developed between the pads and the discs during the brake's removal. You don't want to be doing this on the maiden voyage, just as a car pulls out in front of you!

There, all done. And the job only took about an hour and forty-five minutes.

Cost? About $30.00, seals and oil included.

Thanks for reading, Sonic.