What does 1/4-20 UNC-2A mean?

A 1/4 inch Unified thread with 20 threads per inch, in the coarse series (UNC), class 2, external (A). Class 2A/2B is the general purpose fit.

What does M10x1.5-6H mean?

An ISO metric thread, 10 mm nominal diameter, 1.5 mm pitch, tolerance class 6H. The capital H means an internal thread, such as a tapped hole.

Why does M12 have no pitch written?

When the pitch is left out, the thread has the standard coarse pitch. For M12 that is 1.75 mm. Fine threads always show the pitch, such as M12x1.25.

What does LH mean on a thread?

Left hand. The thread tightens anticlockwise. Without LH, a thread is right hand.

Mechanical prints: lesson 4 of 7

Thread callouts

Read the callout and answer. Get 10 right to finish. ASME practice uses Unified inch threads, ISO uses metric threads.

What is the nominal (major) diameter?

Unified inch threads (US)

1/4-20 UNC-2A reads as size, threads per inch, series, class:

1/4 is the nominal (major) diameter. Small sizes use numbers: #10 is .190 in.
20 is threads per inch.
UNC is the coarse series, UNF the fine series.
2A is the class: 1, 2 or 3 for loose to tight, A for external, B for internal.

Metric threads (ISO)

M10x1.25-6g reads as metric, diameter x pitch, tolerance class:

M10 is a metric thread with a 10 mm nominal diameter.
1.25 is the pitch in mm. Leave it out and the coarse pitch applies (1.5 mm for M10).
6g is the tolerance class. Lowercase is external, capital is internal. 6H/6g is the general purpose fit.

See the thread chart for threads per inch and pitches by size.

Common mistakes

Reading the pitch of a metric thread as threads per mm. 1.5 is the distance between threads.
Mixing up A and B. A goes on a bolt, B in a nut.
Swapping an M8 and a 5/16 bolt. They look close but will not mate.

Questions

What does 1/4-20 UNC-2A mean?: A 1/4 inch Unified thread with 20 threads per inch, in the coarse series (UNC), class 2, external (A). Class 2A/2B is the general purpose fit.
What does M10x1.5-6H mean?: An ISO metric thread, 10 mm nominal diameter, 1.5 mm pitch, tolerance class 6H. The capital H means an internal thread, such as a tapped hole.
Why does M12 have no pitch written?: When the pitch is left out, the thread has the standard coarse pitch. For M12 that is 1.75 mm. Fine threads always show the pitch, such as M12x1.25.
What does LH mean on a thread?: Left hand. The thread tightens anticlockwise. Without LH, a thread is right hand.

Explained in our own words and checked against several textbooks and published tables. For the full rules, see ASME Y14.5, ASME Y14.36, ISO 286, ISO 965, ISO 1302 and ISO 21920-1.