I have my blower taken apart to clean and change coupling. I was going to steam blast it but I don't want to strip the grease from the needle bearings. I am just going to wipe it down with carb cleaner. I have read not to touch the rotors, just leave them grimy. I noticed some scratching on the rotors and case. is this normal? there seems to be no play in the bearings. Anything I should look for while I am in there????