The part of the fitting that allows it to rotate, should be internal to the knurled part of the fitting, and shouldn't effect the o-ring which seals the fitting to the radiator. However, and im just speculating here, there should be some form of sealing between the internal part of the fitting that allows it the rotate, and the inner wall of the knurled part of the fitting, i'm not sure if this is an o-ring or not. I guess that perhaps excessive rotating of the fitting could effect this internal seal, and that the fitting could begin to leak from the internal wall of the knurled fitting. Although I have no idea how much rotating would be neccessary, and again, i'm just speculating.