An employee strapped an 8-inch mud motor on a pipe rack, then went to trace the high side of the mud motor from the adjustable kick off (AKO) to the upper connection of the motor. To better position the motor on the pipe rack, the employee went to roll the motor over. They inserted a rock between the motor and the pipe rack as a chock to prevent it from rolling. The motor rolled back crushing the employee's left thumb and resulting in amputation of the tip.