This one really sucks... On most four stroke bikes, and all aftermarket countershaft sprockets, the rimmed edges on either side of the sprocket [all the way around] that the outside link plates ride on are made of metal - that is, these rimmed edges are simply CNC'ed out of original stock piece of metal. For whatever reason, Yamaha has glued plastic rims on either side of the stock WR426 countershaft sprocket. This was a bad idea. The other day, a rock [or some other piece of debris] got in between the inside of the sprocket and the case. The rock completely chewed up this plastic lip - pieces of which then shimmied under the countershaft sprocket seal which of course caused it to fail. Next, most of the oil drained out of the bike to the point where the motor started running rough. Fortunately, I caught the problem before the motor seized but I will still do the perfunctory head/valve inspection just in to be safe. The point of this post... Spend the $25 on an all steel countershaft sprocket and eliminate the possibility this could happen to you! Also, order a countershaft sprocket seal from your dealer and throw it in your spares kit as they are normally not carried in dealer stock.