Just tried it without the pads, the gap isn't big enough to slide the 'outer' pad into. It seems the outer pad is stopping the caliper going back far enough.Take the pads out again , then bolt the pad carrier on . Make sure the piston is wound in enough to put the pads back in , clockwise direction . Use a screwdriver or small some square bar . Also make sure the piston is lined up properly so the notch lines up with the with the bit on the back of the new pad , or you'll never get it in .