How many unified screw thread series have been established?

• A. Three
• B. Four
• C. Five
• D. Six

Answer: (C)

Unified screw thread series are the different pitch families defined under the Unified Thread Standard to fit a wide range of sizes and loading conditions, while keeping parts interchangeable. Five series have been established to cover the spectrum from coarse to very fine pitches and to address specialized applications. The coarse series makes assembly quick and forgiving in softer materials; the fine and extra-fine series allow more precise adjustment and better load distribution along the threads; the remaining two series handle special diameters or manufacturing needs. This structure ensures that bolts and nuts can be interchanged across a broad set of Navy and industrial components.