The same site that has the rebound kit has 20mm bottoming cones. I don't know if these would work on the KDX's, but if anyone calls about the rebound kit, it would be something to ask about. I think all the KDX cartridge forks use a 20mm cartridge.
Here is a picture of the kit. It is too wide to politely be in the thread.
http://www.racingsuspensionproducts.com ... ntibtm.jpg
