How To: Rebuild your KTM LC4 640 Forks / Seals

Wasnt much info on here as far as a specific tear down for the LC$ 640 Forks, so here it is!

My forks have been leaking for, uh, a while. After my last ride I decided it was high time to do something about it. I got the forks have torn apart before I decided to do this, so I'll come back and fill in with a few pictures as I go.

This is on an '02 KTM 640 LC4E, if it makes any differencs.

***Disclaimer*** - I know nuthin about suspension... I just like to tinker. Anyone who spots my many mistakes, please chime in and I will fix the info!!!

Tools Required:

10 mm Socket
8 mm Socket
Crescent Wrench

24 mm Wrench
22 mm Socket
19 mm Socket
Pick
Flat Bladed Screwdriver

Supplies:

LOTS and LOTS of shop towels
Fork Seals
Dust Seals
Fork oil- pick yer own poison

Step 1

String the bike up. If you have a way to hoist the front of the bike off the ground your life will be a lot easier. If not, find a way to jack it up from underneath enough to get the front tire off of the ground. Take off anything thats going to be in your way- headlight, lower fork protectors, etc.




Step 2

Remove the front tire. Take the large nut off of the left side with the crescent wrench unless you have an ultimate set of tools and can use the correct size socket / wrench. Loosen the 4 pinch bolts holding the axle in and remove the axle. Roll the tire out, and be careful of the brakes!

Step 3

If you havent already, remove the lower fork protectors using the 8 mm socket (3 bolts on each protector). Loosen the 2 bolts on the top triple clamp on each side. Using your 24 mm socket, loosen the top cap on each fork. If you dont loosen the top 2 bolts on the triple clamps, this is going to be very, very hard.

Step 4

Loosen the lower 2 triple clamp bolts and slide the fork down and out of the clamps. It helps if the top and bottom bolts are very loose. If its still tight, you can VERY CAREFULLY wedge the tip of your screwdriver in the slot to open the clamps a bit more. You shouldn't need to do this though.

Ta da! The forks are off!!!

Now the fun begins: disassembling the forks.



Step 1

You should have loosened the top cap (#1) already. Unscrew it the rest of the way and be careful because if you have any pressure on the fork leg the cap will pop up. Tip it over into your drain pan and let it drain while you repeat this process on the other leg. If you can, pump the fork back and forth and let it sit over night. Otherwise, you'll just have a lot more fork oil to clean up as you dissassemble them! (remember the lots and lots of shop towels!)



Step 2

Takin out the innards. Screw the top cap back on but just leave it finger tight. This is to keep everything inside from spinning while we do the next step. Remove the rubber cap from the bottom (#82) and using the 19mm socket, loosen the lower compression cap. If its really tight, you can use your front axle as a lever by sticking it back in its hole and using it as a handle to keep the leg from spinning. If it just spins and spins, its probably out, but the o-rings are keeping the compression cap in place. Check by taking the top cap back off- the entire internal assembly should slide out with it like so:



Pic of the lower compression cap removed:



You should now have two empty metal tubes held together at the fork seals. Lets get them seals out!

Step 3

No idea what its for, but I started by taking off part #13, the "guiding ring". Next is the dust cap which pops out easily with a flat bladed screw driver.



Next we need to get the Lock Washer (#18) out. Its kinda hard to see, but its got one pointy end thats really easy to get the tip of your pick or screwdriver under to pop it out. Its the rusty stuff in the pic below:



And one with the washer out:



Now comes the fun part! to get the actual seal out, we use the fork leg like a slide hammer. The KTM booklet recommends heating the upper fork tube a bit to make this easier. Slide the fork together and then pull it apart real quick. It might take a try or two, but it will pop the seal out and the two fork tubes will come apart leaving you with this:



From left to right on the lower fork leg: Parts # 22, 15, 16, and 17- Bushing / Retainer, Bushing, Support Ring, and Fork Seal. I think the support ring in the picture is bad- it should be FLAT, not CONICAL as mine is. ???

If all you are doing is fork seals, STOP HERE and go to reassembly!!!

If you want to continue tearing it down, please continue!

Step 5

We need to dis-assemble the piston now and its a bit tricky. You need three hands- one to pull up on the top cap, one to pull down on the spring to uncover the nut, and one to get your wrench on the nut!

*****insert pic

Once you have your 22 mm wrench on the hydraulic stop and your 24 mm socket on the top cap, unscrew the top cap and set it aside. Remove the Spring.

The black plastic part, the washers, and the nut are all one piece and according to the diagram its called a hydraulic stop.

Step 6

If you want to take apart the innards, its pretty simple. Just unscrew the Hydraulic stop from the top of the piston (if its tight, wrap the piston in shop towels and clamp with channel locks CAREFULLY, and loosen the 22 mm nut on the hydraulic stop), then slide the piston down and out of the inner tube (#58) being careful of the bushing on the bottom of the piston. When your done, heres what you should have:













FURTHER DISASSEMBLY

Lets say you found some items of concern (like I did) and want to tear it down further.

Step 7

Lower Compression Cap Dis-assembly. Have a spot ready to lay out all the pieces in the correct order. Using your 19 mm socket on the bottom and a 17 mm wrench on the top, take off the check valve nut (#63). Lift off the spring, shim, piston, and shims. Apparently, this is where all the action takes place- defined by these shims. There are a couple of threads out there that talk about mods / different stacks but I know nothing about it, so check out the other threads!



Step 8

Dis-assembling the piston / rebound piston. You have the piston out already- its the long skinny rod with a short spring and a piston / bushing at one end of it.

FYI- the short spring is the rebound spring. There are TWO different springs- one for the SM, and one for the Adventure / Enduro. The Enduro spring is 75mm long and the SM spring is quite a bit shorter. THE ENDURO SPRING IS NO LONGER AVAILABLE FROM KTM (which, as murphy dictates, is the only broken part in my forks...).

Reassembling the forks. Time to put in the new fork seals! By now, you've cleaned the tubes outside and in really good, right? Right. Using some of our lots and lots of shop towels, I like to swab em out real good, kinda like cleaning a gun barrel.





Place the seals (see hint on seal installation down a ways) / clips / bushings onto the lower fork leg in the following order:

Guiding Ring
Dust Seal
Seal Retainer Clip
Fork Seal
Washer
Big bushing
Small bushing

Should look like this (just not with the conical washer, I replaced that...):




HINT ON SEAL INSTALLATION...

Lube them up good with whatever oil you plan on filling your forks with. I like to then use our tiny sharp (read pointy) object to help slip the inside of the seal over any obstacles to keep from tearing them up. Works good if you angle the seal and then work the pointy tool around while applying slight pressure until it pops down to where its sposed to be:



And now for the coolest little trick for installing fork seals you have EVER SEEN!!!

You need about 6 feet of electric tape per seal. Clean the oil of the lower tube and wrap a buncha tape around it. The tape will be our seal pusher, so it needs to be on the outside of the seal as shown below. We're looking for about 1/8" thick of tape:



Lube the outside of your seal. Slide the tube down so the ring of tape pushes the seal in:



Give her a shove and Walla:



Seal installed! Grab the end of the tape and yank- it just spins right off!

Install the seal retaining clip. Works best if you stick the two ends into the grove, then push the opposite side down in with a screwdriver.

Install the dust seal. Lube the outside and slide the two legs together as far as they will go. The very bottom of the fork will act as a seal pusher for the dust seal- pop it in!

Install the guiding ring.

Viola! You've got new fork seals!

Filling the forks and installing them! Getting close!

Not 100% sure on this, but heres what I ended up doing, based on this thread here: http://www.advrider.com/forums/showthread.php?t=165680

I ended up using 550cc's of oil to get my oil level up to 130 mm from the top of the leg.

With the top cap off, the spring out, and the fork collapsed, pour in your oil. I started with 500cc's, although I think the recommended starting point is 420cc based on the above referenced thread.



Pump the piston back and forth several times, then measure the level from the top of the collapsed fork to the top of the oil with the piston pulled up (A LITTLE HELP HERE!?!? The other thread says cartridge collapsed... With my piston down I could barely get 450cc's in, leaving basically NO air gap. Lifting it up got me really close to the 130mm, am I right or do I have way to much oil in my fork!?):



Heres the tool I used to do my measuring:



Set it to 130mm and locked it there. Basically just used it as a dipstick! (pic shows it OUTSIDE of the leg cause I had already installed the sping... but you get the idea).



Once you have the proper oil level, install the spring. Kinda tricky, read Meats thread as well, but heres what worked for me. Pull the piston all the way up. While holding it up from the bottom, lower the spring as far as it will go (till it hits your fingers). Next part you have to be quick- Get your fingers out of the way and drop the spring. The piston will fall slowly, so reach through the spring near the top and grab the top of the piston. If you're quick, it will only be about an inch below the top of the spring. Use a pair of needle nose pliers to pull the piston back up to the top, then install the top cap.

Lift the top leg up and screw in the top cap. Dont worry about tightening it yet, we'll do that when we have it clamped in the bike.

Repeat for the other leg!

Pics added!

Suffered a minor setback... Found some broken bits in the other leg. Good news is that means i'll be going a little more "in depth" for my how too. Bad news is the spring thats broken is no longer available from KTM, so I may be awhile sourcing one which means the assebmly part may take a while for pics.

" I think the support ring in the picture is bad- it should be FLAT, not CONICAL as mine is. ???"

Yes, it should be flat. Sometimes the ring gets bent while slide hammering the forks apart. Looks like you could also use some new bushings.

Make sure you don't over tighten the compression stack nut (#63) while re-assembling, it'll snap off like a twig. Clean it real good, then blue loctite it and let dry overnight if you have the time. My WP manual calls for 5NM for that nut, but I tighten it just a tad over hand tight.

When you get ready to install the new bushings, get the first bushing in the fork, get the second bushing and support washer ready to go and use a seal driver. The seal driver pushes against the support ring, which in turn drives the second bushing in. After the bushings are seated, you use the seal driver on the seal itself.

Here's a couple of good vids- not WP, but the technique's the same..
Pt1 http://www.youtube.com/watch?v=Y43k1qFVGW4&feature=related

Pt2 http://www.youtube.com/watch?v=C9ICr-DOmdk&feature=related

Scotch tape works good for protecting the new seal during installation..

Stay tuned for more posts... Finally got my parts ordered so I'll finally be able to put everything back together and RIDE!

how much oil goes back in???? great info dude, thanks!

How to service (some) LC4 forks

But I didn't remove my fork seals, so I'll add a link to your how-to in my thread.

I finished the section on how I did it, using info from Meats and another thread. STILL NOT SURE IF I DID IT RIGHT! Anyone want to let me know if it sounds right? I will be posting several links to different threads in the original post, Meats included. Still not done with this, but we're getting there! My bike is back together and operable now anyways!

The oil level should be measured with the top of the piston level with the top of the fork tube, not fully extended.....

So what am I doing wrong? If the fork leg is collapsed, with the piston down, I would be lucky to get 300cc's in there if I set it to 130mm from the top. Its obviously not enough because I can hear it sucking air and gurgling when I pull the piston up too.

THIS link from my thread might help.

OK.

I'm at the "slide hammer the living shit out of your fork legs" part of this and I'm having a real hard time slide hammering the living shit out of these.

Circlip is out. Check.
Heat applied to outer leg. (not too much!) Check.

Too me it just seems like an awful lot of force. . .

Any comments?

BTW, Thanks to DorkPunch, Creeper, Meat, etc. for all the fork threads!

It only took about 3 hits (yanking the fork legs apart)... And I wasnt busting a gut or anything. Are you sure you have the bottom cap out? All the innards are out?

I'm exactly where you're describing the washer (circlip) is out and before the seal is removed. I'll give it a whack.

dorkpunch--
Well, I got it apart. Those seals are remarkably snug for being the leaking SOB's that they are!

Did you ever decide if you had put the crown on that washer (support ring) or if it comes that way?

IIRC, its supposed to be flat and I ended up replacing it. Guess I oughta finish this thread, eh? :huh

I would like to add that I just did my forks and I had fit in ~570ccm of oil with 120cm from the top of the collapsed outer tube and piston and NO spring inside. (thats what it took for the gurgling of air to disappear when piston is moved through its range as well)

Hopefully I did them right.