Hi all,
Just an FYI for you, there are various posts here stating that the xr50 parts are the same, this seems not to be the case. It seems the Z50 parts are more interchangeable. I just put on a Z50 countershaft sprocket(13T) and it made a noticeable difference on bottom end. You will have to buy the retainer clip also as using the stock the splines line up when you line up the bolt holes in the z50 sprocket!!
Worth a try for those of you that want a little more zip on the bottom end...