Thread class is one of those callout details on a print that people often accept without fully understanding what it means. In practice, the class you specify has a direct impact on whether your parts assemble freely, whether they hold a load, and whether they can be re-used after disassembly. This article breaks down the UN (Unified National) and metric thread class systems and explains how to choose the right one.
What Thread Class Controls
Thread class defines the tolerance and allowance applied to a threaded feature. Tolerance is the total permitted variation in a thread dimension (pitch diameter, major diameter, minor diameter). Allowance is a planned clearance — an intentional offset between the mating part's nominal dimensions that ensures the threads will always assemble even at their worst-case tolerance limits.
Tighter classes have smaller tolerances and, in the case of external threads, less allowance. This produces a closer, more precise fit. Looser classes have larger tolerances and more allowance, making assembly easier under adverse conditions like dirty threads or protective coatings.
UN Thread Classes: The A/B System
Unified National threads use a number (1, 2, or 3) paired with a letter (A for external, B for internal) to define the class. So a bolt thread might be called out as 2A and its mating nut as 2B.
Class 1A / 1B — Loose Fit
Class 1 has the largest tolerances and the most allowance. These threads assemble freely even when dirty, rusty, or coated with thick paint or plating. They're used for rough work where precise fit is not important — coarse fasteners in agricultural equipment, temporary fixtures, military applications requiring assembly under field conditions. You'll rarely see Class 1 specified in precision manufacturing.
Class 2A / 2B — Standard Fit
Class 2 is by far the most common. The vast majority of commercial fasteners — bolts, nuts, screws — are Class 2A/2B. It provides enough clearance for easy assembly while maintaining sufficient contact for reliable load transfer. Zinc-plated or lightly coated parts typically work fine at Class 2 because the coating thickness fits within the allowance.
If a print specifies a thread with no class callout, Class 2A/2B is generally the assumed default in commercial work.
Class 3A / 3B — Close Fit
Class 3 has the tightest tolerances and no allowance — the thread dimensions go right to the nominal size with only a tight tolerance band. These threads are used where close fit, minimal slop, or high-load joint integrity is critical. Aerospace fasteners, precision instruments, and threaded connections that must transmit torque accurately are typical applications.
Class 3 threads are harder to make and inspect, and they're more sensitive to contamination, plating buildup, and damage. When a print calls for 3A/3B, plan for tighter process controls and inspection of pitch diameter.
| Class | Tolerance | Allowance | Typical Use |
|---|---|---|---|
| 1A / 1B | Large | Large | Rough, field-assembled, coated parts |
| 2A / 2B | Medium | Medium | General commercial fasteners (default) |
| 3A / 3B | Small | None | Precision, aerospace, high-load joints |
The Role of Allowance
The allowance on external threads (A classes) is what makes Class 1 and Class 2 threads forgiving. The bolt is made slightly smaller than nominal — intentionally. This creates guaranteed clearance when it's combined with the internal thread, which is held at or near nominal size. Class 3A removes this allowance: the pitch diameter of the bolt goes right to the maximum limit with only tolerance variation below it.
This matters in practice when you're plating parts. Electroplating adds thickness uniformly. A Class 2A thread with 0.003" of allowance on a ¼-20 might absorb a 0.0005"–0.001" zinc coating and still assemble cleanly. A Class 3A thread with no allowance may not — the plating needs to be accounted for in the pre-plate thread dimensions, or the specification needs to say "after plating."
Metric Thread Classes: The ISO System
Metric threads use a different notation defined by ISO 965. The class is expressed as a number (tolerance grade) followed by a letter (tolerance position). External threads use lowercase letters, internal threads use uppercase.
Full callout may show both diameters: e.g., 6h6g (minor · pitch for external)
Tolerance Grade (Number)
The number (4, 5, 6, 7, 8…) is the tolerance grade — higher numbers mean wider tolerances. Grade 6 is the standard general-purpose grade, equivalent in spirit to UN Class 2.
Tolerance Position (Letter)
The letter defines where the tolerance zone sits relative to the nominal thread profile. For external threads:
- e — large allowance (equivalent to a loose fit)
- f — medium allowance
- g — small allowance (most common for precision external threads)
- h — no allowance (tolerance goes right to nominal, like Class 3A)
For internal threads:
- G — small positive allowance
- H — no allowance (tolerance starts at nominal; most common)
Common Metric Classes
| Class | Type | Equivalent UN | Use |
|---|---|---|---|
| 6g | External | ~Class 2A | General commercial (standard) |
| 6H | Internal | ~Class 2B | General commercial (standard) |
| 4g6g | External | ~Class 3A | Precision, close-tolerance |
| 4H5H | Internal | ~Class 3B | Precision, close-tolerance |
| 8g | External | ~Class 1A | Coated parts, coarse work |
Three-Wire Measurement
When you need to verify pitch diameter on a precision thread — particularly Class 3 or metric 4g/4H — three-wire measurement is the standard shop method. Three precision wires of the correct size are placed in the thread grooves, and the measurement over the wires is converted to pitch diameter using a formula. This is more reliable than thread gauges alone for catching gradual pitch diameter drift during a production run.
Choosing the Right Class
In practice, the decision is usually straightforward:
- General fastener, no special requirements → 2A / 2B (UN) or 6g / 6H (metric)
- Precision fit, aerospace, or high-load application → 3A / 3B (UN) or 4g6g / 4H5H (metric)
- Heavily coated, rough-service, or quick assembly needed → 1A / 1B (UN) or 8g (metric)
- Part will be plated → account for coating thickness in pre-plate dimensions; call out "after plating" if needed
When in doubt, ask your customer or engineer what fits in their assembly. A class is meaningless without knowing what it's mating with.
Summary
Thread class is fundamentally about controlled fit. The UN A/B system and ISO metric system both use the same core idea — tolerance grade plus allowance — expressed in different notation. Class 2A/2B and 6g/6H are the workhorses of commercial manufacturing. Class 3A/3B and 4g/4H step up when precision matters. Class 1A/1B is there when the environment or coating demands extra clearance. Know which one your print specifies, understand why, and hold it.