I've had my 2000 WR400F since 2003. I fiqure I have at least 12000 miles of trail riding on it. I consider my bike well maintained. I have the original cam chain still running the original valvetrain. I was warned by a fellow rider one day at the trailhead this fall that I need to replace the cam chain after every 25 hours of riding. I was shocked, I have never heard of this for a WR400F. I have heard that the '03,'04 YZs have had issues but not the WRs. I will be replacing the cam chain before the next ride but I was wondering what everyone else's opinion is about this? How often should I be replacing the chain? It is a very cheap cost to avert a very expensive problem! How often do you replace yours?