KX Guru Racing

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

The steering stops should work just fine.

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

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.