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

When drilling and tapping a ½" bolt hole for a heavy cast iron housing, using national coarse thread is the appropriate choice. National coarse threads are designed with a larger pitch compared to fine threads, which allows for greater engagement of the threads, making them ideal for use with materials like cast iron that require strength and durability under load.

Coarse threads provide better resistance to stripping and are more effective at accommodating the potential for expansion and contraction within heavy materials. This is particularly important in applications involving heavy cast iron housings, where the bolted connection needs to withstand significant stress and ensure structural integrity.

Additionally, national coarse threads facilitate easier assembly and disassembly compared to finer threads, which can be especially beneficial during maintenance or repair activities. Overall, for drilling and tapping in this context, national coarse is the optimal choice to achieve a secure and reliable fit.