I have ran 13- 49 thru 13-45 on my 200, 250, 500 many times, almost all of the time. Understanding that you can get to the same place with a 14 countershaft, its just personal pref. I mean, in general 13-48 is very close to 14-51. Thanks to Kawasaki sticking with the same sprockets for so many years, ( and Ben from here on kxriders) I have a collection of front and rear sprockets to play with. Worth mentioning, some countershaft sprockets are machined differently. IMO, there is no clearance issues until you want a 12. Now, you may, or probably will, get more wear on the cases from dirt, mud, sticks and rocks passing thru the smaller area there, but I do not believe the chain rubs, o-ring or not. There are some high end chains that are definately wider and still a 520 chain. You may have noticed masterlinks from one chain do not work in other chains. A wider chain can rub subframes, wear-out chain guides, and possibly rub the case, but again I think its about your choices of brands and sticking with the stuff that works for you. Also, a non-oring chain has plenty of tensil strength for the k5, but will not last as long as non o-ring chain.
