I'm not the author of the thread here, but when I did the two Acewell gauges I used, I drilled into the front rotor. I drilled a hole that was slightly smaller than the magnet circumference at first, and then drilled the correct circumference to the depth I wanted. I then used a good two part epoxy to keep the little bugger in there. Good news is that it's a magnet and wants to stay put anyways, the epoxy is just to keep it there. Oh and the reason I drilled the smaller hole through was so that I could get some epoxy on from the backside. (note, you do not drill into the brake pad surface! Just one of the "spokes" of the disc. Make sure it doesn't stick up enough to hit anything on it's way around.