Shoulder bolts, also known as shoulder screws, are a type of machine screw that are designed to act as a shaft or dowel for the rotation or sliding of attached parts, including pulleys, gears, and mechanical assemblies. They have an unthreaded cylindrical section, or “shoulder”, between the head and the threaded section. This shoulder allows for precise alignment and spacing of components. Shoulder bolts are available in a variety of materials, including alloy steel, stainless steel, and brass. They are also available in different finishes, such as black oxide, passivated, and thermal black oxide. Shoulder bolts are used in a wide range of applications, including automotive, aerospace, and industrial machinery. They are often used in applications where precise alignment and spacing of components is critical, such as in the assembly of engines, transmissions, and other mechanical systems.