An employee was disassembling hydraulic components to determine the source(s) of a hydraulic system failure. The employee removed the track drive motor assembly from a bulldozer and placed it on a workbench to perform a complete cleanout procedure on the hydraulic assembly. The employee was operating a pneumatic impact driver with the right hand to remove a bolt while supporting/holding a hydraulic servo with the left hand. The servo rotated and pinched the employee s left index fingertip between the machined edges of the servo and the hydraulic motor housing, resulting in a distal amputation (about 3/8 inch) of the finger.