An 18 tap bit has a smaller thread pitch than a 24 tap bit, making it better suited for drilling holes in materials with a finer thread (.003 inch or .086 mm). It is also commonly used to create pilot holes before using a larger diameter drill bit. A 24 tap bit has a bigger thread pitch, making it better suited for drilling through tougher materials (.017 inch or 4. Elizabethan) without the need for a pilot hole. Additionally, it’s also used to create larger holes in hardwood and other non-ferrous materials.