The king pin is the part that the front wheel rotates when the wheels are turned. In the attached image, it is large vertical item (#29). They can "go bad" if they are not greased regularly (3,000 miles). The king pin is normally not the item that normally wear, the upper (#80) or lower bushings (#58).
Jim Villers
190SL, 230SL 5-Speed, 190E 2.3-16 Kompressor