What are the two series of threads on aircraft bolts and screws according to the American National System?

The two series of threads on aircraft bolts and screws according to the American National System are known as National Coarse and National Fine. This classification reflects the standard threading systems established for fastening methods.

National Coarse threads are designed with a larger pitch, allowing for quicker assembly and disassembly, while National Fine threads provide greater tensile strength and fit, making them ideal for applications requiring a more secure connection. Each series serves specific applications based on the mechanical requirements and types of loads they will encounter in service.

Understanding these classifications is crucial for ensuring the correct fastener is selected for an application, thereby maintaining safety and operational integrity in aircraft design and maintenance.