I replaced my steering servo on my avalanche with the HiTec HS645 servo and the arms sent do not fit the screws that i already have. so i put back on the arm from the stock servo but now the servo barely turns the wheels. does the stock arm fit the HiTec Servo or do i have to purchase one made for a HiTec Servo. any info would be greatly appreciated.

You should have gotten some silver screws with the servo that you can use to screw the steering arm on the servo arm. Also did you turn you truck on before you put the servo on the servo? You should do that because it centers the servo gear. If not the servo gear might be turned all the way to one side and not let you steer the wheels in one direction.