2-3/16" wide opening. Highest quality of craftsmanship and materials with unique features. Innovative adjusting knob's six surfaces are numbered for visual reference, allowing a quick set to your ideal pressure. The knob is fixed in relationship to the frame, making it easier to use and allows the lowest profile when stored behind the nut. Clean, precise fit of all components ensures the screw's fit will not become loose or sloppy. Inert silicone rubber neck pad, rubber bumpers, neatly rounded edges. An ergonomic treat!