Bad Breaks!

I purchased a rotor from autozone, made in china, which will not accept a dust cap. it's like it's out of round, in the casting or something. It doesn't seem to brake shudder or vibrate, but try to put a dust cap on, and no way. I filed on it with a round fil to see if I could get one to fit. It's like the opening is egged. You can measure on one side and it's about 1/4 in, and the other is 3/8. I mean I don't care as long as it doesn't give me bearing or brake trouble later on, but I need a dust cap. Help and thanks! Gerry B.