If it is a common problem on these quads, which it sounds like it might be, then it could be a vibration issue that yamaha overlooked. What year quad do you have? Mine is an 87. It could be a problem on the earlier models that they fixed later. I work on this type of stuff almost weekly at work. Basically, the natural frequency of the guard could be near an engine speed which would cause it to fail. I am going to check and see if they changed the part number for later years later today and get back to you.