Fork Conversion notes? (YZ and early 90s KX)

Questions and comments about converting to beefier forks..
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Chopperpilot
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Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

Been reading the forum here, pretty new to posting. I have a 96' KLX650R and a 2001 KDX 200. I rebuilt the KLX forks last year with gold valves and am happy with what they are/what I use the bike for (mostly dual sporting). I'm currently in the middle of a 93 KX250 fork swap on the KDX. I really dislike the flex in the factory setup (even if I can't ride close to it's limits, I can still feel the flex).
I live in Fairbanks Alaska, so getting cheap used parts up here is hit or miss. I've lucked out on Ebay with these KX fork legs. I've also lucked out with a 2003' YZ250F triple tree/fork setup too. So here's what I've found/confirmed so far in my fork swapping journey. I used KLX650 triple clamps, they use the same bearing and stem setup as the KDX and will fit the KX (blue anodized) fork legs. I'm using the KX wheel and axle as well. Because these legs are 93, I had to get an appropriate year caliper because the KDX caliper doesn't bolt up to these (I think 94 + KX forks can use the 2001 KDX caliper). The fork stops aren't quite enough to keep the KX forks from hitting the bike before the stop, but really close. I may epoxy something on to take up the difference.
I pulled apart the valving on the 93 KX250 forks and the compression valving is as follows:
22 x .15 (x2)
16 x .10
14 x .10
11 x .25 clamp shim
spacers
the rebound stack was a 2 stage stack as follows:
21 x.10 (x2)
15 x .10
20 x .10
18 x .10
17 x .10
14 x .10
11 x .20
for my first go at it, I'm removing one of the 21 x .10 from the rebound. I'm using it to replace one of the 22 x .15 shims on the compression side. I've never ridden the forks before, so this is just an educated 'guess' because MX forks are 'always too stiff'. Plus the fact that there really isn't much to do with these stacks anyway, possibly good candidates for Gold Valves.
I got a spare set of base/compression valves from a 92 KX250 and it's interesting the differences between these two years. Besides the stack, the 93 forks have noticeably larger holes for the oil to flow through the compression stack compared to the 92 valves. I can pin them later and try to get some measurements. We're not talking gold valves here, but visibly larger holes none the less. The compression stack on the 92 KX250 is as follows :
22 x .10
19 x .10
18 x .10
16 x .10
14 x .10
11 x .10
spacer/clamp shim
So a pretty simple pyramid type stack with thinner shims, but also smaller holes. I figure if my mod on the 93s are close to what I want, but not quite, I can use the 92 shims to get the feel I want. If they're no where in the ballpark, I'll gold valve the 92 base valves and swap them out with the 93s.
Now, onto the YZ forks. I noticed not much on these as people usually swap late model KXF forks, but again, I have to look for shipping deals on ebay. I forgot what I got these 2003 YZ250 forks and triple trees for, but it was a song. I've taken some measurements and think these yamaha forks might be an easy swap. The stems (YZ and KDX) are very similar in height as far as the bearing journals and where the nut threads are. The bottom journal on the YZ stem is 30mm so a standard KDX bearing should work. The upper journal is 28mm..... which was a problem until I looked on Amazon and found a 28 x 47 x 12mm bearing (still waiting for it to get here though). So that's on order and on the way. You might ask, why bother? Well the fact that my KLX and my KDX use the same front end, means that I can improve both bikes with the same setup for the triple trees. So this coming winter I'll probably swap the KX setup to the KLX (for the adjustable rebound) and fit the YZ to the KDX for more adventurous riding. I'll keep this updated in the future.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

Ok, my pin set is for main jets and isn't nearly big enough to pin the compression valves. I sorta got a crappy set of calipers in there and roughly the difference in hole diameter from the 92 to 93 KX base valve is 3mm (92) to 3.5mm (93). Like I said, visibly larger on the 93. Kind of a bummer because I was hoping to make different stacks on these spare base/compression valves and just swap the whole bottom 'bolt' out of the fork legs, but the significantly smaller holes in the 92 valves would make that pointless.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by KDXGarage »

Thanks for the info and good luck on the swapping.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

Alright! got the early model KX caliper installed and bled (I think all the way). I've heard on forums that the early KX calipers/brakes worked good enough but felt mushy. I think that's where I'm at (also waiting on new pads as the KDX/KLX ones I have are not the same).
Review:
I have a cool 1.5-2 track to ride down to check my mail on and I took the KDX today. I immediately liked the front end better. I'm not a seasoned enough rider to say if it's exactly sprung or valved correctly yet, but it's close enough for this summer (short up here!). The increased stiffness and decreased deflection were just what the doctor ordered, for me anyway. My trail is a slow speed one on a side hill, mostly using first gear, quite a few roots and a flat faster area with baseballish sized rocks. I think the front end kicked back at me just a slight bit more than the factory KDX forks, but the increased control more than made up for it. I guess I'll start playing with clickers now. They are currently in the middle of travel, I'm thinking I'll open them up all the way and ride, then work the clicks in a couple at a time.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by KDXGarage »

Cool. Keep at it and write down your settings.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

I noticed that compared to the KLX650 (a significantly heavier bike, but pretty much same forks/shock/frame), the KDX kicked the rear wheel sideways a few times in the faster flat rocky area. The KLX just motored straight through it, both bikes on the gas and both about the same speed. I'm thinking rear shock clickers ........ and I'm thinking increase low speed rebound dampening some? is that right? I know on mountain bikes when I need a little slower rebound because the rear will have a tendency to 'kick up' coming off of jumps and is very unsettling. I'm guessing under power on flat but rocky terrain this is what too fast rebound feels like on a motorcycle.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by KDXGarage »

How long has it been since the KDX shock oil was changed?
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chuck78 »

That's great info to know that the 93 KX forks have a much larger base valve piston. I never looked into these and was unaware of any differences 91-95 other than color, & that mid valves came on I know the 94+ forks. Did the 93 have mid valves also? 96 model year bumped the forks up to 46mm except for the KX500 which didn't get those forks until '97 (through the end of the model in '04).

FYI from what I gather, please verify, but it sounds like all 91-95 KX forks and 96 KX500 were 43 mm with the same clamping diameters. 91 was the only year of the KX 125 and 250 that shared the old steering stem bearing size that the KDX also uses. All KX500 retained the older bearing size & steel steering stem, FYI, through the end of the KX500 model run in 2004. 1989 or maybe 88 all the way through 2004 were largely unchanged on the KX500 aside from suspension and graphics.

Additionally, all of the 43 mm KLX forks should have the same diameter on the triple clamp tube clamping areas, although I'm not certain about the newer models from the past 5 or 10 years yet. The KDX 250 1991 - 1994, all water-cooled models, also uses the same clamping diameter on the triples for their 43 mm forks. I believe those have aluminum steering stems, so that's the triple clamp to seek out if you are running any of these KX forks and one an extra upgrade to shave a slight bit of weight off.

So:
91-95 KX125/250 43mm forks
91-96 KX500 43mm forks
all KLX 43mm forks
***I believe ALL have the same clamping diameters regarding the triple clamps. So any of those KX years particularly 93-95 KX125/250 and the 96 KX500, can bolt into a KDX using of the triple clamps mentioned, 91 KX, 91-96 KX500, KDX250 water cooled years 91-94. No stem swap necessary.
I'm going to guess that the blue forks all have the smaller base valve piston and no mid valve stack, and the later forks that are silver I believe or maybe gold tinted all will have the larger pistons and probably all have mid valves. 93 might have been a split here, the KX 500 may not have gotten the mid valve fork until '94 as it was much like the kdx, based on the previous year's KX125/250. I previously had read that 94 got the mid valve on all KX 125 and 250 but it could have been as early as 93, I'm interested to hear what you have to say on that.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

KDXGarage: No clue on the last time it was changed. I'll probably send the shock off to Racetech this winter though. It'll have to do for now with the short time to ride!
The 93 KX forks I'm using are blue anodized, so the color isn't a for sure about the base valve holes. The KLX650 triple clamps I used to mount them on the KDX have an aluminum stem, so there's that benefit as well. The KLX clamps also have all the mounting bosses/tapped holes for mounting the headlight etc like the KDX triples. I believe all the KX clamps use 2 lower clamping bolts per leg though, which is a more rigid/stiffer/less stress in one area setup than the single lower clamping bolt per leg on the KLX clamps. Real world effect?? who knows. The 93 KX forks that I have do not have a mid-valve, only a check plate setup as most of the internet says. (Again though, we're relying on people knowing exactly what they have when posting on ebay)

Possibly the 93 forks got the larger base valve holes, MXers didn't like the softness on jumps, and in 94' they added the mid-valve to help control fork movement on landings?? So 93 KX 250 (and maybe 125) forks are the secret year for woods riding?? Hahaha, maybe?
Also, the holes in the 93 KX rebound valves (that would be for the mid-valve or check-plate) had the same size holes as the 92 KX base valves, about 3mm

Oh yeah, one more thing on the KX front wheel. I got a 99' KX front wheel and it fits on my 93 forks no issues (brake rotor and everything). It uses a 20mm axle though, and a 20mm axle was easily found that clamps into the 93 forks (not 17mm like I've seen).
Last edited by Chopperpilot on 06:23 pm Jun 06 2024, edited 1 time in total.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

Notes on the 2003 YZ250 front end:
I don't have it mounted yet, but I did get the 28 x 47 x 15 bearing in from amazon. I also got a YZ wheel from ebay. Here's what I've found so far.
1) The KDX caliper mounts perfectly on the forks and is aligned L/R with the wheel rotor. The YZ wheel I got has a rippled/wave rotor and there is the slightest scuffing from one part of the rotor on the metal clip that holds the brake pads in the KDX caliper. The scuffing is in rhythm with the waved edge of the rotor but only on about 90 -120 degrees of it. Touch up with a file should get rid of it easily.
2) The KDX stem measures 6" from the top of the bottom bearing to the middle of the threads on the stem, the YZ250 stem measures 6" to the bottom of the threads...... so that may or may not be a problem solved by a spacer?

I entered into a Hare Scramble this weekend so I'll see how the KX front end does on that. I ordered fork guards and wheel bearings for the YZ front end. When that stuff comes in maybe I'll take the KDX down and see about putting it on this summer. This YZ front end is the 2003 version that is 46mm tubes. These have Racetech stickers on them, so hopefully Gold Valves are already installed!
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chuck78 »

The stem on the YZ triples being slightly longer will effectively lower the fork same as sliding it up in the triples if it had the KDX stem in place. Just make sure that you have enough clearance between the tire at full suspension compression bottomed out up to the fender / fender mounting bolts. You don't want any sudden surprises on a hard landing, having your front tire momentarily lock up and potentially toss you end over end.

When comparing fork swap steering stems, I measure from the bottom of the lower bearing up to the top of the upper bearing area / bottom of threads. The upper bearing seat area is the more important one to have it's reference located within the confines of the KDX frame.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

Chuck, no doubt about better places to measure. I just had a quick look while assembling the YZ front end with a spare KDX triple I got for mock up purposes. I kind of figured the threads would be a good place to measure to because that'll bring the nut to a stop, and the bearing journal is immediately below the threads.
Are you sure the effect is the same as sliding the forks up in the triples? The overall height of the YZ triples is taller and some of that is due to the clamps being thicker than either the KLX or KDX clamps (especially the top clamp). There appears to be plenty of journal to position the top bearing on. I would think the vertical position of the clamps relative to the frame wouldn't affect the geometry like sliding the tubes up or down would because that actually moves the front axle relative to the steering head. (Of course all of this is speculative, because I don't know how much rake/trail/offset is built into each front end on the stock bikes anyway)
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chuck78 »

If the steering stem is moderately taller and causes the upper triple clamp to sit higher than the KDX upper triple clamps' top fork clamping areas relative to the KDX frame, then it absolutely is the same as sliding the triples down over the fork tubes as I call it, most people call it sliding the forks up in the triples - although it's actually almost the entire bike that's moving down aside from the forks and wheels.

Having a thicker upper triple clamp could also have the same effect but it just depends on how the triple clamp is machined or cast, or shaped rather.


None of this is a bad thing whatsoever though, as most fork swaps are taller than what the KDX actually should have.
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
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'88 Toyota 4x4 pickup
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

I guess what I'm saying is that where the front axle is relative to the frame steering head is what makes the difference in whether the forks are 'slid up' relative to stock or not. I see what you're saying about if you move the upper clamps away from the lower clamps, that would move the fork legs up 'higher' to meet the upper clamps, but if the yamaha triple is just a little taller anyway, that doesn't (by itself) do the same thing as sliding the fork legs up in the triple clamps. Like you said though, most USD forks are 'too long' so I'll make sure to add whatever offset is needed for the taller clamps to the top side of the frame. I'll get additional 'bar riser' effect too!
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by Chopperpilot »

These 2003 YZ forks are the 46mm KYBs, which have some benefits for ease of re-valving for woods riding and rebuilding over the later 48mm SSS KYBs. But I am curious if the clamping diameters are the same, if anyone has of the SSS 48mm fork legs we can compare clamping diameters. Obviously people would rather try the better KYBs if this setup ends up working.
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Re: Fork Conversion notes? (YZ and early 90s KX)

Post by KDXGarage »

Look for aftermarket clamps information to see compatibility.
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