KX500 USING YZ250F YZ450F FORKS

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The question has been asked what it takes to install newer YZ250F or YZ450F forks on a KX500.
First things first, I grabbed a set of 05 YZ250F triple clamps and measured the distance between the fork tubes and they measure 190mm.
The KX500 fork tubes also measure 190mm so were good there.
Next I placed the 250F triple clamps into the head tube of a 94 KX500 frame.
Just about any year from 86 to o4 will have the same measurements.
Here is what I see.
The bottom bearing isn't big enough.

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The upper bearing is too big

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We seem to have plenty of top nut adjustment if we can find a smaller upper bearing.

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The steering stops should work just fine.

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KX500 TRIPLE CLAMP TOP BEARING MEASUREMENTS
ID 25MM
OD 47MM
WIDTH 15MM

KX500 TRIPLE CLAMP BOTTOM BEARING MEASUREMENTS
ID 30MM
OD 55MM
WIDTH 17MM

YZ250F AND YZ450F 2001-2013 TRIPLE CLAMP TOP BEARING MEASUREMENTS
ID 28MM
OD 52MM
WIDTH 16MM

YZ250F AND YZ450F 2001-2013 TRIPLE CLAMP BOTTOM BEARING MEASUREMENTS
ID 30MM
OD 52MM
WIDTH 16MM

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Here is a reply I received from a guy who has done the conversion:

You don't need to change bearings. Swap the steering stems. The original bottom seal doesn't fit exactly, but it does work good enough for me. The upper bearing will need a thin spacer to shim between KX stem and the upper YZ triple clamp. Then the upper clamp needs a slight material removal done so the stem nut on the upper clamp will have 100% thread engagement. Additionally the YZ forks are about 3/8" shorter than the KX forks. I have found the handlebar ergonomics are much better for me as I used to pull hard on the bars with the stock KX setup. I don't pull on the bars anymore. What also helped with rider comfort a lot was the wider footpegs.
The YZ forks has help me with handling as also the modified rear shock. The high/low speed adjustor modification stopped the pitching to the side of the rear tire in choppy sections and now when you adjust the shock, it actually helps. I no longer have to compromise for one type of terrain and suffer on the other.

[Zerofear66]

Any info out there regarding using a newer model YZ450 or 250F ??
Im looking to use a set of the SSS forks.

TIA!!

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I have a customer we built a engine for and he is running a 2013 YZ450F fork.
Here is his comment:
For those who have ridden with that SSS fork....its unbelievable when set up right! P L U S H!!
He said the tube went right into the 97-04 KX500 triples.
He is using a YZ wheel and brake.

Generally speaking, the Chrome diameter follows the outside clamping diameters.
IF that is true, the seals for the chrome tube 17D-23145-00-00 fit 2010-2016 YZ250F and YZ450F so it stands to reason that 10-16 would all bolt right up.
However, the only year I have actually measured is the 2013's and... the 2006 YZ450F.
What you need is something close to the following: UPPER 53.8MM - LOWER 58.9MM
The 2006 UPPER 55.82MM - LOWER 59.06
Or, .080" difference between the upper KX500 and the 06 YZ450F forks
The lower is .006" difference.

Here is some info from another YZ fork guy.
Just some info...SSS forks and clamps
All have 59mm lower clamping surface
'06-'09 450f 56mm top
'10- current 450f 54mm top
'06-'12 250f 56mm top
'13-current 250f 54mm top
two strokes 56mm top
Hope this helps anyone planning a fork swap.
I'm installing new '09 450f forks on a '01 500. Needed to machine out the stock KX top clamp to 56mm to fit these forks.
The fork spacing of the KX clamps is slightly narrower than the YZ clamps, so I'll have to fiddle with the wheel spacers a bit. Otherwise it should be a clean set up.
I own a 2016 YZ450FX, so did some measuring and research before I even bought a 500 and learned that the Yamaha stuff is real close. I used a caliper to measure the diameter of both sets of forks, but since it is just a cheap Harbor Freight caliper, I doubt it is exact, but I figure the difference in measurements between the two forks should be pretty close. The top clamping area was on a difference of 0.01 mm, while the lower was 0.3 mm difference. Just used the stock KX triple clamps, and the YZ stuff slid right in, also used the front wheel and brake from the YZ