Metric Thread Callouts: Diameter, Pitch and Tolerance Class

A metric thread designation identifies more than a hole diameter. Read the thread family, nominal diameter, pitch and any tolerance-class information separately. For example, M10 × 1.25 identifies a metric thread with a 10 mm nominal diameter and 1.25 mm pitch. It does not identify the drill size for producing that thread. Carr Lane callout explanation

The quantities are different

Sandvik’s guide separates the thread diameters and explains that tolerance notation contains a grade and a position. Its examples also distinguish pitch-diameter and crest-diameter classes when both are shown. This introduction does not reproduce the corresponding limit tables. Sandvik, pp. 4–5

Pitch is an axial distance

In M10 × 1.25, the 1.25 value is the pitch in millimetres: the axial spacing between corresponding points on neighbouring threads. The 10 value is the nominal diameter, while required threaded length describes how far the thread must extend. Those three dimensions answer different questions. Metric pitch examples

A metric pitch value is a distance per thread spacing. Threads per inch reports how many threads occur in an inch. Read the convention before comparing numbers.

Do not turn an omitted pitch into a guess

Carr Lane describes omission of pitch for the applicable coarse metric series. That convention requires the thread family and governing specification to be clear. When transferring a requirement between a drawing, model and purchase request, confirm the intended pitch rather than treating every omitted item as permission for the supplier to choose. Metric designation convention

As an editorial handover checklist, retain the full approved designation, whether the feature is internal or external, the intended usable threaded length, and whether the hole is blind or through. Keep total hole depth separate from usable threaded length so the recipient can identify which requirement must be checked.

A callout is not a manufacturing recipe

Do not derive a tap-drill recommendation, tapping cycle or load capacity from the nominal diameter alone. Those are separate tasks with additional inputs. Metric and inch thread descriptions also encode pitch differently; converting units does not establish mating compatibility.

Keep the permitted size variation separate from pitch: the dimensional tolerance introduction explains how requirements differ from nominal dimensions.

Limits and next steps

Exact tolerances, default conventions and inspection requirements need the applicable standard edition and approved design. Include the complete requirement in the CNC quotation package. This reference does not cover pipe threads, multi-start design or joint-strength calculations.