Eye nuts (eyenuts) are fasteners with a looped head. They are designed to receive a hook or rope. Eye nuts are rated for different types of applications and are oval or round in shape.  Load-rated eye nuts are designed for vertical lifting, but are not recommended for use with angular lifts. Eye nuts that are not load-rated are designed for use with tie downs and guiding wires. They are not rated for lifting applications. Specifications for eye nuts include maximum load capacity, bell diameter, threaded length, inside diameter or width, eye section, and weight. Maximum load capacity is the maximum load which an eye nut can handle. Bell diameter is a measurement which refers to the part of the eye nut that contains the tap. Although most eye nuts are threaded, untapped eye nuts are also available. The inside diameter or width (bail size) is defined as the bail diameter for round eye nuts, and the bail width for oval eye nuts. 

Eye nuts differ in terms of material and finish. Metallic eye nuts can be made of aluminum, brass, bronze, steel, hardened steel, stainless steel, titanium, or proprietary alloys. Aluminum is lighter than steel, but not as strong. Brass and silicon bronze are common copper-base alloys. Many different grades of carbon steel and alloy steel are used as fastener materials. Stainless steel may not require anti-corrosion coatings, but cannot be hardened to the same degree as carbon steels. Titanium, a metal that is used widely in the aerospace industry, is suitable for specialized applications. In terms of finish, eye nuts are often anodized, galvanized, or plated with gold, silver, tin, or zinc. Black oxide is an eye nut coating that causes virtually no dimensional change. Phosphate coatings provide corrosion resistance and a better surface for the adhesion of primers and paints. Eye nuts with zinc chromate finishes are also available.

Thread sizes for eye nuts are measured in English units such as inches (in) or metric units such as millimeters (mm). Measurements for inch-based eye nuts include: 1/4, 5/16, 3/8, 7/17, 1/2, 5/8, 3/4, 7/8, 1, 1 1/8, 1 1/4, 1 3/8, 1 1/2, 1 3/4, 2, 2 1/4, 2 1/2, and 2 3/4 inches. Fasteners designated as #16, #18, #20, #24 eye nuts are also commonly available. Designations for metric eye nuts include: M6, M8, M10, M12, M14, M16, M18, M20, M22, M24, M25, M26, M27, M28, M30, M32, M33, M35, M36, M38, M40, M42, M48, M56, and M54. As a rule, the numbers which follow the letter “M” indicate the number of millimeters in diameter. For example, an M6 eye nut is 6 mm in diameter.