Which of the following is not a standard method for thread identification?

The correct response highlights that "packing series symbol" is not recognized as a standard method for thread identification. In the context of thread identification, standard methods include nominal size, thread class symbol, and thread series symbol, each serving specific roles in identifying the characteristics of a thread.

Nominal size refers to the designated measurements of the diameter of the thread, allowing for a clear understanding of the thread’s size specifications. The thread class symbol provides information about the tolerances and fits of the threads, offering details crucial for ensuring compatibility between threaded components. The thread series symbol distinguishes the different types of threads in use, such as UNC, UNF, or others, which relates to the thread's design and application.

In contrast, the packing series symbol is not utilized in standard thread identification practices. It may pertain to packaging or group classifications but does not fall within the conventional methods used by boilermakers or other professionals for threading specifications. This understanding highlights the distinction between relevant and irrelevant terms in the context of thread identification.