I had my back-up wheel for my 97 500 mounted up to my 86 kdx 200. It fit without any troubles. I was attempting to convert the Kdx to disc brakes, when I noticed that the disc set-up had lots more drag during a free spin. I am pretty sure that the axle diameter is the same on other years, but not 100%. I think the advantage to an 18" wheel is to run a wide wheel. That way you have alot wider tread patch on the ground. A narrow 18" seems to be the wrong direction to me. The bigger rear wheel would roll better IMO. I know that bigger bikes that are pro hillclimbing will run a 21" rear for the advantage of a flatter tread patch, if that makes any sense. Its just my opinion, as I run stock wheels. I also noticed in my recent experiences with tire changing, that stock wheels corrode very bad on the inside. I don't know, if water gets in there while riding and washing, or the soap and water for easy assembly causes it, but I have seen lots of corrosion on the inside of used wheels. If any bike will destroy a rear hub, its a kx500. Be careful what you mount up to it. Hope I helped.