We used the US Digital HB5M. It was very easy to install and fired right up no problems. It has a nice mounting system and is available with the 8 mm thru hole. All we did was make a new cover for the spin box adding two holes for the 4-40 fasteners. We mounted the encoder on the motor shaft and recorded 5310 rpm on startup. The encoder is more expensive than AM but is very easy, durable and reusable.