I was asked to provide a little more info on the YZ fork build. The pictures of my build are posted here:
http://s34.photobucket.com/user/ken-powell/library/?view=recent&page=1All YZ125, YZ250, YZ250F and YZ450F front ends from 2004 thru 2010 will fit the 2003 thru 2005 KX125 and KX250 chassis. 2011 to present might fit but Yamaha started playing with the triple clamp offsets about this time and I haven't kept up with what bikes got which clamps and when - you would have to research the use of the 2011 and later clamps for offset. The 2005 KX250 MIGHT have changed chassis rake and triple clamp offset - there are conflicting reports on this change and I have never owned one - you are OK with the 2004 chassis. Note that both the KX and YZ of these years used a 25mm triple clamp offset and are compatible. Also, the fork length seems to be either the same or within 10mm of each other and any difference in fork length can be accommodated by moving the forks up in the clamps. I am running my '08 YZ250F forks flush with the top of my '04 triple clamps.
I did not do any machine work to make this fork conversion work. I added a 3mm spacer (big washer) to the bottom of the stem so the seal would sit flush with the bottom of the KX frame. Note that the bearings are the same part number for the YZ and the KX. The top clamp does tighten against the top bearing properly but sits proud of the frame ~ 3mm. I used a rope seal (no kidding, just a small piece of cotton rope greased like off-road trucks use) to fill the space between the YZ seal and the frame.
