Thread callout basics for standard fasteners When writing a thread note for a typical bolt or tapped hole, which specific dimension always appears as part of the basic callout?

Introduction / Context:
Thread notes must unambiguously identify the thread so mating parts assemble correctly. Standards such as ISO, ASME, and UNC/UNF rely on a concise callout that includes the thread size (major diameter), pitch (or TPI), series, class/fit, and hand if not right-hand.

Given Data / Assumptions:

• We are specifying a typical external bolt or internal tapped hole.
• The callout format must uniquely identify the thread.
• Material is documented elsewhere (BOM/spec), not inside the thread size string.

Concept / Approach:
The major diameter is the “size” of the thread (for example, M12 or 1/2-13). For metric, the callout reads like M12 x 1.75-6g; for inch UNC/UNF, 1/2-13 UNC-2A. Without the major diameter, the callout is incomplete. Center lines and offset distances are drawing constructs, not part of the thread note.

Step-by-Step Solution:

Verification / Alternative check:
Confirm mating part specifications; a M12 x 1.75 external must mate with an M12 x 1.75 internal, with compatible tolerance classes.

Why Other Options Are Wrong:

• Material: Important but managed via BOM/spec, not the thread size string.
• Center line / offset distance: Not thread parameters; these are drawing geometry annotations.

Common Pitfalls:
Omitting class/fit or pitch, leading to mismatched fasteners; ensure the major diameter is always present as the starting point.

Final Answer:
major diameter of the thread