Doesn't have to be. Use a plastic rod/sleeve and make very sure the carb isn't opened all the way to the end point, just a bit below so vibration won't be transferred to the servo. Even a small nylon geared servo will last a lifetime this way, however a digital is preferred for better resolution and centering. I one time had a HS81 RIP on a small nitro engine when the piston ring broke causing the engine to shake so heavily, it killed the servo with it.