I'm pretty sure they won't. The KX500 frame stayed pretty much the same from 85 up, while the 250 frame changed dramatically. The KDX frame is very similar to the 500 frame (the 500 clamps bolt on), and you can't use the 250 clamps on those bikes without swapping the steering stems. I think you can press the steering stem out of the 500 clamps and into the 250 clamps, but there is an easier way to get better forks. The KX450F forks will slide right in to the stock 500 clamps. I'm not sure if all years will, but I know the early to middle year 450 forks will.
