They are rusted on around the hub flange. The best method I've ever tried
of freeing them is a little half-pound ball-peen hammer.
The rotor's shape is a bit like a hat turned sideways. In this instruction,
we are concerned with not the brim (the friction surface) or the crown
(where the bolts are), but the side part that would have the ribbon if it
were a hat.
Take the narrow end of the ball-peen hammer and tap the "ribbon" side of
the rotor as close to the friction surface as you can without actually
striking the friction surface. Tap gently and rapidly, many many times,
turning the rotor by hand as you go. Every so often tap the part of the
rotor where the bolts are. This action will eventually break up the rust
that's holding the rotor to the hub, and you should be able to wiggle the
rotor off by hand.
You will see lots of rust drop to the ground as you tap and turn.
Remember, it's shock and time that will do this, not brute force. Tap the
rotor a bit harder than you would knock on a house door.
You MUST make 100% certain that the flange and rotor hub surface are dead
clean, flat and smooth, otherwise you'll introduce brake vibration with the
new rotors. Use lots of 50-grit emery cloth (NOT sandpaper) and elbow
grease. Do not use a wire brush.
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