I borrowed this from Unit74 from KTM Talk:
"If you are running a 13 tooth CS sprocket, check your upper swing arm near the sprocket. I found a nasty surprise when I unbolted the motor. The chain had worn 2 grooves into the swing arm about 3-5 mils deep!! The plastic chain guide was toast and I never noticed it due to the fact that where the damage occured is directly behind the frame/subframe joint and there is not enough slack in the chain to lift it up enough to inspect.
Drop your chain and take a look. Remove the 8mm bolt near the rear of the guide closest to the rear sproket, and lift up the guard. Check the thickness left and hope there is not a disheartening groove in your swing arm.
The new guard runs about $16 and will only require you remove the swing arm mounting bolt to replace"
"If you are running a 13 tooth CS sprocket, check your upper swing arm near the sprocket. I found a nasty surprise when I unbolted the motor. The chain had worn 2 grooves into the swing arm about 3-5 mils deep!! The plastic chain guide was toast and I never noticed it due to the fact that where the damage occured is directly behind the frame/subframe joint and there is not enough slack in the chain to lift it up enough to inspect.
Drop your chain and take a look. Remove the 8mm bolt near the rear of the guide closest to the rear sproket, and lift up the guard. Check the thickness left and hope there is not a disheartening groove in your swing arm.
The new guard runs about $16 and will only require you remove the swing arm mounting bolt to replace"