What kind of thread should be used to drill and tap a ½" bolt hole for a heavy cast iron housing?

For drilling and tapping a ½" bolt hole specifically for a heavy cast iron housing, national coarse thread is the most appropriate choice. National coarse threads are designed to provide greater strength and holding power, which is especially important when dealing with heavy materials like cast iron. The wider spacing and larger crest help distribute the load more evenly across the threads, reducing the risk of stripping under stress.

National fine threads, while providing a tighter fit and better engagement when precision is necessary, are generally more suited for applications where the materials being fastened are lighter and less subject to heavy loads. They can also be more prone to stripping in materials like cast iron under heavy cyclic loads. Therefore, this makes national coarse the better option for ensuring durability and stability in applications involving heavy components.

In contrast, Acme and square threads are specialized threads more commonly used for applications involving linear motion and power transmission, such as lead screws, rather than for securing heavy sections like a cast iron housing. Thus, choosing national coarse is optimal for achieving a lasting and secure fit in this scenario.