How do you write a thread callout on a drawing for UNC 2B and metric 6H threads?

What does each part of a UNC 2B thread callout mean?

A Unified inch callout such as 1/4-20 UNC-2B reads, in order, nominal size, threads per inch, thread series and thread class, where the digit sets the tolerance grade and the letter B marks an internal thread (A marks an external one).

That sequence is the basic designation method in the federal screw-thread handbook (NBS Handbook H28, Part I), which ASME B1.1 carries forward for the UN, UNR and UNJ forms. Taking the example apart:

• 1/4 is the nominal size, the basic major diameter. H28 writes it as a decimal (.250); fraction and number sizes such as 1/4 and 10 still appear on US drawings.
• 20 is threads per inch, not pitch in inches.
• UNC is the series: coarse. UNF is fine, UNEF extra fine, UN a constant-pitch series, and UNS any special diameter-pitch combination. UNR and UNJ add a controlled root radius on the external thread.
• 2B is the class. Per H28, classes 1A, 2A and 3A apply only to external threads and 1B, 2B and 3B only to internal threads, and classes 2A and 2B are described as the most commonly used for general applications.

Per the handbook, every thread is right-hand unless the callout says otherwise, so a right-hand thread needs no suffix. A left-hand thread adds LH at the end, as in 1/4-20 UNC-2B-LH.

How do Unified and metric thread callouts compare side by side?

Both systems carry the same four pieces of information (size, pitch, internal or external, and a tolerance band), but they order and encode them differently, as the table shows element by element.

The classes are not conversions of each other: 2B and 6H play the same role, but their limits come from different formulas, so a 2B plug gauge cannot accept a metric hole. A drawing can still mix systems, as long as each callout is complete on its own.

How should you call out threads on plated or anodized parts?

State whether the class applies before or after the coating, and for metric threads pick a position with an allowance (6G inside, 6e or 6f outside) when the coating is thick enough to consume the standard allowance.

Coatings add material to the thread flanks, and because of the thread angle the build-up is multiplied on the pitch diameter: H28 derives its coating relief by dividing the flank build-up by the sine of the half angle and doubling it, which works out to four times the flank thickness before any plater’s tolerance. The handbook’s aim is that finished threads land inside the limits of their class, and its method for inch threads is explicit:

• Class 2A external threads carry an allowance that can absorb a thin coating. Per H28, the 2A maximum diameters apply before plating and the basic diameters apply after plating.
• Class 2AG means the 2A dimensions must still be met after coating, so the allowance survives the finish. The callout then adds the major and pitch diameter limits followed by the words AFTER COATING, plus the before-coating limits followed by BEFORE COATING.
• Internal threads (1B, 2B, 3B) have basic minimum diameters and no allowance. Per H28, where a coating thickness is required inside, the hole is relieved so a basic-size GO plug still enters after coating.

Metric practice solves the same problem through the tolerance position. An H or h position leaves no room, so an electroplated M-thread is typically specified with a G internal or an e, f or g external position; the ISO 965-1 bibliography points to ISO 4042, the standard for electroplated fastener coatings, for that case. Whichever system you use, the note that prevents most disputes is the word “after”: for example, “M8x1.25-6H after anodize” or “gauge after plating.” The finish comparison covers build-up by finish, and surface finishing can sit in the same quote as the machining.

Which thread callout gaps slow down an overseas RFQ?

Missing class, missing depth, and an undefined coating state are the three gaps that most often send a drawing back with questions, and each can be closed with a few characters on the print.

When a part moves from a domestic shop to a supplier in Vietnam, the drawing loses the hallway conversation that used to fill its gaps. A checklist:

• Class present on every thread. “1/4-20” or “M6” alone leaves the gauge undefined. The assumption should be yours, not the supplier’s.
• Case and suffix right. A metric class in the wrong case (6h on a hole) names a gauge that does not exist for that feature.
• Blind depth as full thread. Say full-thread depth, and say whether a thread relief or a bottoming tap is acceptable.
• Coating state stated. Before or after plating or anodizing, and whether threads are masked.
• Pipe threads kept separate. NPT and NPTF are tapered threads under ASME B1.20.1 and B1.20.3; they do not take a 2B or 6H class. Our tolerance guide covers pipe-thread callouts next to the general tolerance note.
• Standard edition named where it matters. ISO 965-1 was technically revised in 2026, its fifth edition. A drawing that cites a dated edition removes any doubt about which tables govern.

On a threaded feature the thread class, not a general turned tolerance, governs fit, so it is the class that should be on the print. The RFQ package guide covers the rest of what to send, and when the drawing is ready you can send it for a quote or email rfq@saolamfg.com.