1/4-28 Cap Nut 18-8 SS . Typically manufactured from either low carbon steel or stainless steel, cap nuts, also referred to as acorn nuts are used to cover exposed bolt ends and add a finished appearance. ;These stainless steel cap nuts are composed of a hex nut and top cap consisting of 18% chromium and 8% nickel for corrosion resistance. Good for exterior and most marine applications.