After assembling and installing the first encoder I decided to assemble and install a second encoder. I can't emphasize enough that the circular Lexan base must be flush to the back of the motor. That means notching the bottom of the base to clear the motor power terminal bosses and flattening the raised lettering on the back of the motor. If you do this your encoder will mount perfectly flat to the back of the motor. US Digital states that the encoder disk should stand off of the encoder detector by 0.070 in which is hard to accomplish without the alignment tool they provide in their kit. All I can say is place the encoder disk on a flat, hard surface, align the motor shaft with the hole in the disk, and with reasonable pressure press the motor shaft into the disk until it bottoms out on the hard surface. Now find a tool, like a nut driver, that can slide over the motor shaft and CAREFULLY begin to push the disk towards the encoder detector constantly checking the clearance. You might try to eyeball the correct spacing and be successful. I used a caliper, a magnifying glass, and came within 0.01 in of the correct spacing which worked for me. Take your time and you will prevail.