Motorcycle fork seals
If you wash and polish your motorcycle only to find that the fork tubes are oily and dirty again after a few kilometres, then the problem is worn fork seals (often referred to as radial shaft seals).
Changing the fork seals on your motorcycle
In which case, a dirty fork tube is the least of your worries. Ineffective damping can seriously affect road holding, but worst of all, leaking oil may flow onto the brakes and cause them to fail. And don't forget – if you're replacing seals – always replace them on BOTH sides at the same time.
Removing the fork
Before jacking up the front end of your bike securely, loosen the sealing plugs of the fork tubes, which means undoing the clamps of the upper triple tree.
Once your machine is safely jacked up, remove the front fender, wheels and brake calipers. It's sufficient to just secure the calipers to one side with wire, as you don't need to open the hydraulic system for this job. From this point, you must not touch the brake lever. Loosen the clamps on the lower triple tree and pull the fork tubes down and out with gentle twisting motions.
Dismantling the fork
To remove the sealing plugs, use a ratchet and suitable socket, while pressing gently, as the plugs are still under slight pressure even when the fork springs are in the relaxed state. You can now remove the spacer tubes, if fitted, and the spring collars and fork springs. Be sure to make a note of the position of each component and the order in which you remove them. It's also advisable to work on only one fork assembly at a time so you don't mix up the parts. Drain the oil into a suitable drip pan. Remove the drain plug if your bike has one. You'll find that pushing the fork tube in and out a few times will drain the fork oil a bit quicker.
Your next task, which can be a little tricky, is to remove the hexagon socket screw which holds the slider and the damping rod together from underneath. To prevent the damper rod from turning together with the screw, you need to insert a suitably sized socket and two extension bars down inside the fork tube to grip the rod. If you don't have the right tools, it might help to re assemble the fork and hope that the tensioned fork spring will prevent the damping rod from spinning. Underneath the hexagon socket screw, you'll find a sealing washer that must always be replaced with a new one. And bear in mind that you should be treating the chromed fork tubes with the same caution as a basket of raw eggs! Even the tiniest of nicks in the area of the seal is enough to ruin the fork tube, as the seal would be leaking again after just a couple of miles back on the road. If you put the fork in a vice, only clamp it on the slider, e.g. on the brackets of the brake calipers. If all else fails, you're sure to find a garage that will remove the screws with an air impact wrench for a small charge.
Changing the seals
Now you need to remove the circlip that secures the seal underneath the dust cap. Use a small screwdriver to carefully prise the dust cap loose. Then slide it up the tube and remove the circlip by pressing it inwards to get it out of its groove.
Once you have clamped the slider securely, pull on the fork tube several times with a jerking action so that the guide bushing drives the seal out of the slider. You can then pull the seal off upwards. If your fork is not fitted with a guide bushing, then the seal will not come out when you pull out the fork tube, but it should be fairly easy to lever out afterwards. If you are dismantling the fork even further, e.g. to clean it and check for wear (min. fork spring length, wear in the slider, etc.), then you really must consult your bike repair manual first. Bear in mind that dismantling the fork completely often means replacing various O-rings and slide bushings. If you are at all unsure as to what you're supposed to be doing, check with a professional first to find out exactly what is required for your bike model!
Before installing the new seal, apply a little grease to the outer surface of the seal to make it easier to push into the slider, and then to the sealing lip to prevent damage when you re-assemble the fork. Ideally a professional seal driver should be used for installing the seal, but you can also use a suitable pipe or a drift punch. Use the old seal as an underlay when you start tapping. You should not need to tap very hard to drive the seal in, but whatever you do, never use any sharp object that could damage the seal. It's also a good idea to wrap some insulating tape around the fork tube just in case you slip with the hammer! You need to drive in the seal far enough so that you can press the circlip into its groove with a screwdriver.
Installing and filling the fork
Fill the fork tubes with fork oil, either on your workbench or on the bike, whichever works for you. We've decided to fill them on the bike. The key thing is to make sure you use the right viscosity and exactly equal quantities (check your repair manual and use a measuring cup, and maybe a funnel to avoid any spillage). If the fork bottoms out when you brake hard, it's no use changing to an oil with a higher viscosity! The cause of the problem is the fork springs, not the oil, which is only responsible for damping. The inherent chassis problems of some motorcycles can be eliminated with after-market fork springs from Wirth. These springs have type approval, are progressively wound and are approx. 30% harder than the original springs. In fact, if you have air-assisted forks, Wirth springs need little or no air assistance, which also extends the life of your seals.
Once you have tightened and torqued all screws and nuts as specified in your repair manual and fitted the bolts that hold the damping rods with new copper washers at the bottom of the sliders, you're ready to get back on the road. And your bike is sure to have a whole new feel.
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The Louis Technical Centre
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These tips for DIY mechanics contain general recommendations that may not apply to all vehicles or all individual components. As local conditions may vary considerably, we are unable to guarantee the correctness of information in these tips for DIY mechanics.
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