A bolt with an X inside a triangle on the head indicates it is classified as what type of bolt?

The presence of an "X" inside a triangle on the head of a bolt signifies that it is classified as a NAS (National Aerospace Standard) close tolerance bolt. This specific marking indicates adherence to stringent manufacturing specifications that ensure precise dimensional tolerances, which are critical for aerospace applications where the integrity of components is paramount.

Close tolerance bolts are designed to have tighter fits and greater precision in their dimensions compared to standard commercial bolts. This precision is essential in applications that demand a high level of safety and reliability, typically found in the aerospace industry. The NAS bolts are often used in environments where performance under stress, vibration, and load are significant concerns, ensuring that the fasteners can withstand the unique demands placed upon them.

While other types of bolts, such as corrosion-resistant steel bolts or high-strength steel bolts, serve important roles as well, they do not carry the specific marking of an "X" inside a triangle, which is uniquely associated with NAS close tolerance designs. Understanding the identifiers on fasteners helps engineers and technicians select the appropriate components for their specific applications, ensuring safety and effectiveness in construction and maintenance.