That's the problem with steering setups in RC cars. The linkage attached to the servo horn travels in an arc, so every degree your horn rotates, the actual steering linkage moves exponentually less. Why this industry hasn't seriously adapted linear actuators for steering servos is beyond me. It...
There is a thing called form taps, or roll form taps, or roll taps (depends on which machine shop you work in). They don't cut threads into material. They form them in. Makes for a much stronger thread in soft materials like aluminum and plastic. I have them for everything up to 4mm screws...