I did the same job recently on my 420. The advice about cleaning the crud off is difinitely the right thing to do. I always use a C-clamp and the old pads to open the calipers. I replaced rotors as well and the new pads went on easily.Once complete, the first application of the brakes will snug them where they're supposed to be.

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U. S. Air Force Air Traffic Controller
"U gotta keep 'em separated!!"