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Old 12-12-2005
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Luke_N Luke_N is offline
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The USD fork seal guide

Iíve noticed that some people seem to be a bit intimidated by the humble fork seal, there is really nothing difficult about it and no special tools required, this procedure should work on almost all upside down style forks around. One problem that could come up is the 17mm spanner to hold the rod while you remove the top cap, on some bikes with a spacer at the top you need a thin spanner to fit between the spacer and the spring guide. Any bolt sizes I list should be good for Japanese bikes but they are just a guide unless you have a Yamaha.

This is the diagram from the WR manual, the middle row is inside the inner tube, it will differ from bike to bike but we donít need to touch that anyway, where I can I will use the names on this diagram. Most of the manuals will tell you to remove the cartridge but there is no need for fork seals, you are supposed to check a heap of measurements but are you going to replace the fork tubes if they are out of spec? I didnít think so.





Remove the front wheel, brake caliper, fork protectors and anything else your bike might have attached to the forks. If you have a steel brake line running under the fork leg you should be able to unbolt it and tie the caliper up out of the way without unbolting the line, otherwise just leave it hanging from the hose.

Double check nothing is attached to the forks, now the easiest way to do this isÖ
1. Back of the rebound adjuster all the way.
2. Loosen upper pinch bolts (10mm).
3. Loosen the top cap slightly (19mm).
4. Loosen bottom pinch bolts (10mm) and slide the leg out, do the same for the other side.





Unscrew the top cap all the way and the outer tube will fall away from the cap, as long as you keep the leg right side up you can let the outer tube all the way down without any oil coming out.
Use a spanner (17mm) to hold the rod while you unscrew the cap (19mm) from it.
Take the whole thing over to a container, have a couple of rags ready as well. Lift the spring out, it will have oil on it so lay it on a rag. There is also a rod down the middle of the damper rod for rebound damping (picture), just take it out and put it somewhere safe.





Tip the whole thing over a bucket and while holding it upside down stroke the damper rod up and down to get the rest of the oil out. You might want to put some gloves on for this, it gets pretty messy. Leave it upside down over a rag for a while to drain, oil hides in all sorts of places.

Right so you have two pretty much dry fork legs on the bench.
Remove the dust seal with a small screwdriver and slide it out of the way.





You will now see the spring clip that holds the fork seal in. Find the gap in it and pop both ends out like the picture, take it out and pull the ends apart to take it off the inner tube.





The outer and inner tubes should now separate with some persuasion. Grab both tubes and stroke them apart with a bit of force, the seal and bushes should pop out attached to the inner tube. Use a small screwdriver to pry the slide metal apart so it slides off the inner tube.





The piston metal, plain washer and the oil seal can come off as well, pop the dust seal off for a good clean too but donít forget to put it back on. If you plan to reuse them like all sliding surfaces itís a good idea to keep all the bushes facing the same way as they came out so it goes back together exactly the same as it was. I like to give them a scuff with some 1200 wet and dry, you can replace them but I think they are good for a couple of sets of seals. If however you left the factory fork sludge in there for too long they will probably be pitted with rust and need replacing.

Examine the fork legs carefully they should be completely smooth all over, a light rub with some emery paper should remove any small scratches and imperfections. You can get away with 1200+ wet and dry with fork oil on it if you are careful.

I didnít end up taking pictures of putting the new fork seal on but I donít do anything special. Coat the whole inner tube lightly with fork oil and very carefully slide it on, the lip will want to keep hanging up on the edges so hold it on a slight angle and work it around carefully. Slide it all the way to the end of the tube out of the way, like all oil seals the writing faces to the outside (writing side towards the axle). People have suggested using everything from cling wrap to the new seal packets over the end of the forks to stop the seal hanging up. This is a pretty good idea , I particuarly like the cling wrap idea just soak it in fork oil to make it slippery. Next the plain washer then piston metal can be slid on, you should have all the pieces in the picture on the inner tube.





Pull the slide metal apart slightly and slip it back into its groove at the top of the inner tube.





Now you are ready to put the two back together, if youíre like me and doing all this alone you need something to hold the inner tube up while the leg is upside down, a piece of pipe about 250mm long will do nicely.





The outer tube fits over the pipe and sits on the bench and the pipe holds the inner tube up so you can push in the bushes and seals easily.





This is where you need to come up with a way of pushing the piston metal then the seal into place. They sell specialized tools that cost upwards of $100 and there are people who have very carefully used a screwdriver and had good results, you may even be able to pop into your local bike workshop and get them to use the proper seal driver to push them in. I have done a few sets with the PVC pipe method and had no trouble at all, just find a piece of PVC pipe usually 50mm diameter works well because most forks are 46-48mm inner tubes. Cut it in half and clean up any burrs so itís all smooth, if it doesnít fit nicely around the inner tube you can heat and bend it to any position you need.





So now you have a seal driver you can slide the piston metal into the outer tube and put the plain washer down on top of it, tap it down lightly working your way around slowly. It will take a few turns around before it seats in fully, the photo shows it down all the way.





Next bit is the seal itself, same process as the bush, just slowly work around it tapping down a bit at a time. A good tip here is to cut the outer lip off your old fork seal and use it on top of the new one to stop any damage. It is seated fully when you can see the groove for the spring clip. Put the spring clip in the same way you took it out, the body of the clip first then clip the ends in.





It should clip into the groove easily if it hangs up at all you havenít pushed the seal down far enough.





There are a few different ideas about what to do under the dust seal, I think it is Keg that likes to cut up a ring of sponge, soak it in oil and put that under it. Sounds reasonable but I donít go in for that fancy stuff a bit of grease and Iím happy. The dust seal is a good way to size up the driver for the oil seal, in the photo the pipe is up against the inner lip of the dust seal, gives you about 2mm clearance from the inner tube.





So youíre done with the seals now you need to get some oil back in the legs and set the height. You should end up putting about 500-600mls in each leg but you need to bleed the air out before it will fit. Fill until the height is over the inner tube. It was pointed out that allowing oil down into the middle of the damper rod can be a bad thing. If it is full when you screw the top cap on it the oil expanding due to the heat can cause the rebound damping to fade.





Stroke the outer tube up and down a few times keeping the oil height above the inner tube. Now stroke the damper rod until it is hard to pull, it should be hard to pull all the way to the end of the stroke, you will need to keep topping up oil as you do this (keep in mind it doesn't need to be totally full at the end). To save wrecking your fingers trying to hang on to the threads of the damper rod you can screw the top cap back on loosely and grab that instead.

Too make absolutely sure there is no air left hiding anywhere you should leave each leg upright for a few hours to get an accurate reading on the oil height. Place the leg upright with the outer tube all the way down, the dust seal should be hard up against the axle holder at the bottom, the top of the outer tube is about 10mm higher than the top of the inner tube. Just so there is no confusion the measurement actually refers to the air gap above the oil, it is a measurement from the top of the outer tube to the top of the oil in the leg with the damper rod fully compressed.

Introducing another in the series of BSSTís (back shed service tools) and quite a trick one if I do say so myself.





The theory is fairly straightforward. Over fill the leg slightly and put the ruler down beside the damper rod, and line up height you want on the ruler with the top of the outer tube, then you just pull up on the syringe and suck oil out until only air comes up the tube. In the picture I am setting it to 120mm.





Donít forget to put the rebound rod back in.





Pull the damper rod to the top, it will fall slowly so quickly put the spring over the top and you should be able to grab the threads above the spring. Make sure you have the bump stops and any spacers that should be there then screw the top cap back on the damper rod, the threaded part of the rod actually bottoms out in the top cap and the nut tightens up against it. Tighten them up the same way you took it off then you can lift the outer tube up and thread the cap into it.

Now the torque settings here are pretty important so always use a quality torque wrench and tighten to the specifications in your manual. Make sure all threads are clean and dry

Put the leg back into the triple clamps and nip the bottom pinch bolts up enough so you can torque the top cap, donít torque up the pinch bolts yet. Put the wheel, caliper, fork protectors etc. back on and take the bike off the stand, make sure the front end is lined up and you can access your air bleed screws, torque the pinch bolts. Bounce the bike a few times and tighten up the axle pinch bolts, don't forget to set your rebound clickers back to where they were.

Enjoy the feeling of satisfaction because oil is staying inside the forks and you know they are going to feel heaps better with the new oil.


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Old 12-12-2005
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and have a clean well layed out workshop helps lot when doing for the first time make sure you have enough room to lay out your parts and tools so you dont loose anything.


Great post Luke
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Old 12-12-2005
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Great thread Luke well done
thanks for taking the time to document it
and share it.
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Old 12-12-2005
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great work luke!
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Old 12-12-2005
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Awesome work there Luke!
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Old 12-12-2005
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Brilliant post there Luke
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Old 12-12-2005
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Top post champ.
Now can you do a step by step guide to find a womens G spot!

.....then again, who really cares!
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Old 12-12-2005
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Great post Luke, appreciate all the work you've done to help those of us that are somewhat lacking when it comes to this type of maintenance.

well done!!
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Old 13-12-2005
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Awesome post, answered a few questions I had, good work.

One suspension guru told me once to wipe down the outer fork leg and inside of the triple clamps with metho or similar to clean all oil and crap before fitting forks back into the clamps.
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Old 15-12-2005
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Damn.. now I know how to do it properly.. which means I might aswell throw out the trusty 35mm Film..

great post..
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