I know a few guys with new 05 or last years 04 YZ 450. Seems like these bikes have the same problem no matter what year they are though. The bikes are almost completely stock(hard to believe, ain't it!). One of the few mods done to these machines has been to replace the stock rubber band chain with a real O-ringer for the trails. The new chains are slightly wider(a few millimeters) but I don't think the width makes a difference. The new chains do not hit the cases but they are carving up the subframe right above the swingarm. The chain has been tensioned perfectly according to the service manual and aligned too. The chain continues to saw into the subframe most noticeably in the off-throttle/cruising mode. Get on the gas and the chain stays reasonably straight. The chain appears to flop sideways too much and without more side clearance the damage continues. I know some chain slap is normal, but that is up and down movement, not side to side. What is going on here? Does the stock thin(and cheap) chain have to go back on the bikes?