KTM Fork Rebuild & Valving

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KTM Fork Rebuild & Valving

Post by 4Strokes » Thu May 26, 2016 12:54 pm

KTM Fork Rebuild and Valving by Jon Delameter
Lots of KTM owners seem to have experienced problematic fork seals. My dealer assured me the seals themselves don't fail, but merely get buggered up. Nonetheless, I found myself descending at a far higher rate than desired when my left fork seal yakked 5w onto my brake caliper. Of course the fork seals were two months back-ordered at the time, so I decided to "bounce it" like I do my computer when it gets ornery and see if that would appease it. It did.

The cheesy little WP book that comes with a KTM is about as helpful as a Popsicle stick for changing tires. I'm going to assume you can get the fork legs off all by yourself and I'll pick up from there.

Make sure your workbench is clean, unlike mine that has oil, steel filings and a window fan to circulate it all. Getting the legs apart ... err, disassembled, is pretty easy. A big bench vice is nearly imperative for this job, as are squares of plywood and 2x4 for safely clamping the axle clamp. Need I say that the ONLY part of your fork to be clamped in a vice is the axle clamp?
  1. Unscrew the caps (loosen them BEFORE you take them out of the trip-clamps) and pour out the oil.
  2. Remove the caps from the damper shaft, the fun part here is compressing the spring down far enough to get an open-end wrench on the damper shaft locknut.
  3. Remove the cartridge retainer bolt and compression valve assembly (be prepared for lot's more oil here) from the axle clamp, then CAREFULLY separate the stanchion from the slider.
    fork1.jpg (34.02 KiB) Viewed 2598 times
  4. Set everything up to drain for a while, like long enough that you've got the other side disassembled this far too.
  5. Pump the cartridge lots of times to purge out all the oil, then let it drain some more. Patience and repetition is the ONLY way to get most of the oil out, do not use solvents.
    Top bushing, spacers and seal
    fork2.jpg (19.98 KiB) Viewed 2598 times
  6. Very carefully remove the top bushing, spacers, and seal and stack them on a clean rag in the order they came off.
    My fork seal had about a pound of dirt in between the leading and trailing edges, and packed up above the seal around that coil spring-thing. That would explain the escaped lubricants. I cleaned all this up thoroughly and reassembled it with lots of grease in the middle.
  7. If you muck up everything else on this job, don't blow the part about putting the seal back on the slider. Wrap the top of the slider and the sharp edges of the bushing recess with something like cellophane, wax paper, or masking tape before you slip the seal over those sharp edges. Reassemble the bushings and spacers (don't forget the scraper goes on first) and re-insert into the stanchion.
    PVC tool made for reseating the seal
    fork3.jpg (10.69 KiB) Viewed 2598 times
  8. To re-seat the seal I use some big PVC cut in half (as pictured to the left). I then put a couple big hose clamps around the PVC halves and gently drive the seal in. I'm sure someone has a better technique or some high-dollar tool to do this better. Make sure you seat that seal real gentle-like, and that it's square so you don't get more practice at this procedure than necessary.
  9. Next, drop in the cartridge and thread the compression valve assembly in, be patient on this one.
  10. My favorite part again is wrestling the oily, wriggly spring down while stabbing frantically at the damper rod with a box wrench before it compresses. This part can be tricky and it's important you do it right. The concern here is the rebound damper rod. After getting this wrong so many times on this bike, the Showas on my Honda RS250, GSX-R and CBR-RR's before that, I've figured out to turn the rebound screw all the way in, turn the damper rod locknut all the way down, then thread the cap on by hand until it bottoms out on the damper rod. THEN turn the locknut up to it. The point is to make sure you get all 30-32 clicks on your rebound damper. If you don't bottom out the clicker first you could lock the fork cap down too far on the damper rod and only have like 3 or 7 clicks of rebound and it's not right.
  11. Last, before you tighten the caps to the legs, oil. The manual has its recommendations on oil quantity and height, and how to measure each. I won't even go there. I can recommend at the very least, to start with a lower quantity or height as it's easy to add oil in increments for bottom-out prevention. Removing oil is not so easy unless you got a Mity-Vac or at least a turkey baster. The manual does have a required minimum oil height and recommended height ranges, I went the middle of these settings and called it an evening.
KTM RFS Fork Valving
I pulled shims out of my compression stack. The high speed damping was too severe and backing off the clicker only made the front too pogo-ish, then harsher on big impacts. I pulled out the 2nd and 3rd biggest shim from the compression stack, very easy to do without even draining the fork oil! Then put the clicker back to 20 out with 375ccs of Motorex 5w and the front end feels great. Plenty of folks have good things to say about aftermarket shim stacks, Race Tech valves, etc. But I'm too much of a fiddler not to try and change things on my own first. The other big influence on front end feel is the rear compression setting. I tried upping the rear compression damping to get the back to ride higher and successfully made the front feel like it would take out my fillings. To some of you this may be rudimentary suspension knowledge, but there are a lot of riders who never touch their clickers and don't understand what they do.

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