You really need to be posting in the truggy thread as the Hellfire is a truggy and this is the stadium truck thread. How much are you willing to spend on a servo? Good servos are $85 to $130. I recommend at least 200oz of torque and a speed of .15 or faster, metal gears and digital. If you run digital it is mandatory to run a radio with end point settings which most RTR radios do not have.