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We used a 2013 PSF-1 air fork which is a tad on the firm side for him now, but it fits up like any other KYB KX450F fork or 2010+ YZ450F fork.
The fork lug offset is important. You need to know total offset from stem to axle. That's the clamp offset plus the lug offset. I know the YZ stuff is all 32mm and I believe the KX 450F stuff is 33mm.
You can run the original 97 front wheel if you come up with a modern KX axle and the right spacers for the seals.
I am a machinist, so I made him a bag of several sets to last well into the future with his 2013 forks.
His clamps were Gary Emig from back in the 90's when he bought that bike new.
I machined about 0.010" out of the diameter of the bottom clamp for a better fit.
The old Emig clamps were machined using a tiny end-mill (1/4" or 6mm) and probably a lighter-duty machine like a Haas or a Fadal based on the appearance of the tool marks.
My friend had an oversized rotor back in those days, so we kept it on the original '97 wheel and I milled about 4mm off the back of a modern EBC 270mm rotor bracket. That allows the old-school rotor location to work with the new forks. I'd have to go look at his bike, but i think we just swapped an OEM 2013 KX450F front brake hose on the bike to get the "CR style" routing. I think he had that laying around the shop from his upgrade to stainless.
My suggestion would be to use a set of 2012 KX450F spring forks. The advantage to the KX450F forks is that they have the same axle diameter as your original KX wheel and all you'll need is a milled down (4mm) brake mounting bracket and some axle spacers.
The YZ450F SSS forks have excellent valving and match the same upper tube diameters, but they use a larger axle diameter through the fork lugs and have a 1mm less offset than most of the later KX stuff.
I believe the 97 KX250 has the same stem diameters as a modern KX or YZ motorcycle, but if you start swapping clamps around, be prepared to do some welding and modification to the steering stop on your steel frame. Most of the modern clamps have a different interface at the steering stop and they aren't just a simple bolt-on.
Hope that info helps you out. You can probably check with Emig for some plug-and-play clamps... They make a few options (Stems/clamps) for the 500cc guys. I built my own, but I don't have time to do one-off clamps at the moment.
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NHtoWA421, that was on the way to the Dream Race.
He said something when we got to Washougal about someone pointing out the bike at a stoplight.
I can post up a thread about the 500 build later tonight after I workout and do some homework.
https://www.vitalmx.com/forums/Bike-Builds,46/1990-KX500-Classic-Looks-…
Not trying to be the guy to kick up a thread from years ago but....
Why not just just Yamaha SSS forks, ft wheel and brake setup?
Looking to do something different on my 96 kx250.
If you follow the link above, that's what I've done on my KX500 with custom clamps.
Another buddy did a similar setup on his KX500. He used a 2022 YZ250F front end with some clamps from a KX250F (maybe a 2008 model?).
I gave him some steering stem measurements and he machined a stem to fit the KX500 headset. That's not an issue on the KX250.
The thing you have to take into consideration is the alignment of the front wheel in the chassis and the distance between fork tubes.
Some years of KX and YZ are the same, some are not. I don't have all that data fresh in my mind, but I believe there are fork centers on 190mm and 192mm depending on the year of Yamaha and I believe the Kawi is all the same from the mid or late 1990s on up to current.
If you mix and match and you get it wrong, the wheel will be offset about 1mm.
You can correct that with a pair of custom seal collars (wheel spacers) but then you need to check that your front brake caliper and bracket clears the brake rotor properly.
When you mix and match components from different years and brands, you just have to take careful measurements to make sure you are getting what you want. Axle offset at the fork lug and triple clamp offset should be compared. You can mess up the way a bike corners pretty easily with the wrong clamps.
Steerings stops are another issue. Some frames and some clamps work together, others don't.
Steel perimeter frame Kawasakis cannot accept super short clamp offsets in some cases.
The forks will hit the frame, radiators, tank, etc. It varies by year and model, so it's worth doing a little check before you settle on some components.
There's a lot of little details to track, but if you find the right parts combination, it's a big upgrade.
Staying close to the original geometry (fork length, total axle offset, handlebar position, steering stop angles) is the best starting point.
YZ250F and YZ450F fork tubes from 2014 up to current are the same clamp diameters as a Kawasaki four stroke.
YZ125 and YZ250 fork tubes from 2006 up to current share the same bottom clamp diameter as the Kawi, but the YZ top clamp is a larger diameter.
Besides all the obvious problems associated with swaps. I know for a fact the aluminum frame KXs have a 10mm longer steer tube.
That's a good point.
For most of the swaps I've encountered, you have to retain the original steering stem for the frame and press it into the new lower triple clamp.
Most of the Japanese bikes from the mid 1990s on up use matching diameters at each location on the steering stem, so they work out.
The older Kawasaki stuff like the KX500 is an example of an oddball, outdated bearing and stem diameter.
Looking up the beearing diameters or measuring is the best way to solve any particular puzzle.
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