Which Unified thread pitch series is used in place of UNC threads for diameters over 1 inch?

The correct choice is based on the standard practices in engineering related to Unified thread specifications. The Unified thread pitch series is categorized into several pitches, and for diameters greater than 1 inch, the 8-UN series is specifically utilized instead of the more commonly used UNC series.

The 8-UN series has a coarser thread pitch compared to UNC and is designed to provide better performance in high-torque situations, which is particularly important when dealing with larger diameters. This designation helps ensure that the threads have appropriate load-bearing capabilities and are able to withstand the stresses of applications typically found in larger machinery and equipment.

Larger thread diameters often encounter different operational and mechanical requirements, allowing the 8-UN thread series to fulfill those needs more effectively than the UNC series would. Thus, whenever you encounter a Unified thread application involving diameters over 1 inch, the 8-UN series is the standard to follow.