Guide
What is the difference between thread class 2B and 3B, and when is 3B worth it?
Thread class is the number-and-letter suffix on a Unified inch thread callout, such as the “2B” in 1/4-20 UNC-2B, that sets how much the thread may vary from its basic size. The number picks the tolerance grade, from 1 (loosest) to 3 (tightest), and the letter says which side of the joint it applies to: A for the external thread on the screw or stud, B for the internal thread in the tapped hole.
By Saola Manufacturing engineering team · Published · Updated
Class 2B and 3B holes start at exactly the same size. What changes is how far the hole is allowed to grow, how the parts are gauged, and what it takes to make them consistently. This guide works from the Unified thread standard as published for US federal use, shows the actual limits for two common sizes, explains why the A side carries an allowance that matters for plating, and gives a practical rule for when a tighter class earns its cost on a machined part. The limits quoted are the standard’s own values, not the capability of any shop.
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What do the numbers and letters in 2A, 2B, 3A and 3B mean?
The Unified system defines six classes: 1A, 2A and 3A for external threads and 1B, 2B and 3B for internal threads, and the classes differ only in how much tolerance and allowance they carry. Class 2 is the general-purpose grade for bolts, screws, nuts and tapped holes, and class 3 is reserved for applications where closeness of fit and accuracy of lead and flank angle matter.
Every class of a given size shares one basic profile. What the class changes is the width of the band around that profile. The federal edition of the standard sets the pitch diameter tolerance for every class as a multiple of one formula, using a factor C of 1.0 for 2A, 0.75 for 3A, 1.30 for 2B and 0.975 for 3B (NBS Handbook H28, Part I). Two consequences follow from those factors. First, a class 3 thread gets three quarters of the pitch diameter tolerance of the class 2 thread on the same side. Second, an internal thread always gets more tolerance than an external thread of the same class, because the standard recognizes that a thread inside a hole is harder to make than one on the outside of a bar.
Class 1 still exists, for ordnance and other uses where parts must assemble quickly even when dirty or slightly bruised, but it is rare on commercial drawings. The practical choice on a machined part is between class 2 and class 3, and the class on each side is chosen separately.
How much tighter is class 3B than class 2B?
Per ASME B1.1, class 3B and class 2B share the same minimum pitch diameter, which is the basic size, but the 3B maximum sits closer to it, so the band is roughly three quarters as wide. For a 1/4-20 UNC hole, that means a pitch diameter band of 0.0036 in for 3B against 0.0049 in for 2B, per the standard table reproduced below.
| Thread and class | Allowance | Pitch diameter, min | Pitch diameter, max | PD tolerance | Minor diameter (internal) |
|---|---|---|---|---|---|
| 1/4-20 UNC-2A | 0.0011 | 0.2127 | 0.2164 | 0.0037 | — |
| 1/4-20 UNC-3A | 0.0000 | 0.2147 | 0.2175 | 0.0028 | — |
| 1/4-20 UNC-2B | — | 0.2175 | 0.2224 | 0.0049 | 0.196 to 0.207 |
| 1/4-20 UNC-3B | — | 0.2175 | 0.2211 | 0.0036 | 0.1960 to 0.2067 |
| 1/2-13 UNC-2A | 0.0015 | 0.4435 | 0.4485 | 0.0050 | — |
| 1/2-13 UNC-3A | 0.0000 | 0.4463 | 0.4500 | 0.0037 | — |
| 1/2-13 UNC-2B | — | 0.4500 | 0.4565 | 0.0065 | 0.417 to 0.434 |
| 1/2-13 UNC-3B | — | 0.4500 | 0.4548 | 0.0048 | 0.4170 to 0.4284 |
Read the table in pairs. The 2B and 3B rows for each size begin at the same minimum pitch diameter, which is why a 3B hole never assembles more tightly than a 2B hole at its best; it simply cannot drift as far toward loose. The 3A row has no allowance and ends exactly at basic, while the 2A row stops short of basic by the allowance.
The minor diameter column matters for the drill. On the 1/2-13 size, the 3B minor diameter band is noticeably narrower than the 2B band per the same table, which narrows the choice of tap drill and makes the drill size a controlled item rather than whatever the shop has on hand. On many sizes the two minor diameter bands are almost identical, so check the table for the size you actually use.
Why does class 2A have an allowance and 3A does not?
The 2A allowance is a deliberate gap below basic size that eases assembly, reduces galling in repeated wrench assembly and leaves room for plating. Class 3A has none, so its maximum diameters are basic whether the thread is bare or coated, and any plating must be planned for before the thread is cut.
The federal edition of the standard explains how the 2A allowance works with coatings. A plain 2A thread must meet the 2A maximums; after plating it may grow up to basic, so it is accepted after coating by a basic-size GO gauge. As a general guide, the allowance covers a coating whose minimum thickness on the flanks is up to one-sixth of the 2A pitch diameter allowance. For 1/4-20 UNC, with its allowance of 0.0011 in per the table above, that one-sixth rule (NBS Handbook H28, par. 9.1) works out to just under 0.0002 in (about 0.00018 in) of plating on the flanks. The factor is large because a coating on a 60° flank adds to the pitch diameter several times over.
Three situations need special handling instead of relying on the allowance: a design that needs the allowance to survive plating (the standard marks these 2AG), any 3A thread that will be coated, and any coating too thick for the allowance. In those cases the drawing should give two sets of limits, marked “BEFORE COATING” and “AFTER COATING”, as the standard’s designation examples show.
Internal threads have no allowance in any class, so plating a 2B or 3B hole always eats into its tolerance. The same handbook recommends cutting the hole oversize before coating, either by the amount of the 2A allowance or by raising the minimum pitch and minor diameters by six times the minimum coating thickness and the maximums by four times, per NBS Handbook H28 par. 9. In practice, most shops mask tapped holes during surface finishing or chase them afterward; say on the drawing which you want. The guide to anodizing, powder coating and zinc plating covers how each finish builds up.
Can a class 2A screw go into a class 3B hole?
Yes: the standard treats each side independently, and a 2A external thread can be used with a 1B, 2B or 3B internal thread. The classes are chosen per part, not per joint.
This is the most useful fact for a designer of a machined housing that takes catalog fasteners. Commercial screws are almost always 2A, and you rarely control them. If your hole must be closer to size, for example to limit radial play on a locating screw or to keep a thin wall from being wedged by an oversize thread, you can call the hole 3B and still buy standard 2A screws. The 2A allowance still guarantees clearance at maximum material.
The reverse case is also common: a turned stud that must fit a customer’s tapped hole you do not control. Calling the stud 3A removes the allowance and puts it at basic size at maximum material, so the fit depends on the hole. If the hole might be plated or undersize, 2A is the safer call for the stud.
What makes a 3A or 3B thread cost more to machine?
Mostly process control and gauging, not cutting time. The federal standard itself warns that class 3 threads are obtainable consistently only with high quality production equipment backed by an efficient gauging and inspection system, and it tells designers to ask whether the extra production cost is justified before choosing 3A or 3B.
In a machined part, the extra cost typically shows up in four places:
- Method. A narrower band typically pushes external threads toward single-point turning or thread milling with measured offsets, rather than a die head or a roll that simply runs to size. On CNC turning this means a finishing pass that is checked and adjusted during the run.
- Tapping. A tapped 3B hole leaves less room for tap wear and for the tap cutting oversize in soft or gummy material, so taps are typically changed earlier and some holes move to thread milling on a CNC milling setup, where the size can be adjusted with a cutter offset.
- Gauges. Thread plug and ring gauges are made to the limits of a specific class, per ASME B1.2, so a 2B NOT GO plug does not reject an oversize 3B hole. Every 3B size on the drawing needs its own GO and NOT GO set, and gauges wear and need calibration.
- Inspection share. With less margin, more parts are typically gauged per lot and first-article checks go deeper, sometimes to measured pitch diameter instead of attribute gauging.
Which tap produces a class 2B or 3B thread?
No tap is “a 3B tap” by itself; the tap marking only places the tap’s own pitch diameter in a band above basic, and whether the finished hole lands in 2B or 3B depends on the tap, the material, the machine and the tap’s wear. The class is proved by the gauge, not by the tap label.
Ground taps for inch threads carry an H-limit marking under ASME B94.9, the standard for taps. The number after the H steps the tap’s pitch diameter further above basic: a higher number cuts a larger hole. Tap makers publish charts that pair H-limits with thread classes, and those charts are a reasonable starting point. They assume a typical material and a sharp tap, though. Aluminum and some stainless grades tend to cut oversize, and a worn tap cuts undersize, so the chart choice is confirmed on the first parts with the plug gauges for the class on the print.
For a buyer, the practical point is to ask the supplier how the class will be verified, not which tap will be used. A reply that names GO and NOT GO plug gauges for the stated class, and how often they are applied, answers the question that matters.
When is class 3B worth specifying on a machined part?
Specify 3B only where the fit of the thread itself does a job: locating, limiting play, carrying high or cyclic load, or protecting a thin wall. Everywhere else, 2B is the standard’s recommended default and the cheaper one to make and gauge.
A short checklist helps. Choose 3B, or 3A on the external side, when:
- the thread positions a part and radial play would show up as a functional error;
- the joint sees vibration or reversing load and the design relies on thread contact, not only on preload;
- a thin boss or wall could be split by an external thread at the loose end of the 2B band;
- the customer or a governing specification already requires class 3.
Stay with 2B when the thread only clamps with a standard fastener, when the hole will be plated without masking, or when you are not sure. The federal standard for Unified threads puts it plainly: consider 2A with 2B first, because those classes are designed for general use.
If you send parts to a supplier overseas, write the class and the gauging system in the note, state whether plating limits are before or after coating, and list class 3 threads in the RFQ so the supplier prices the gauges on purpose. That applies to a shop in Vietnam, Mexico or anywhere else. The RFQ package guide covers the rest of the drawing set. When the drawing is ready, send it for a quote; the reply flags any thread class that deserves a second look.
Frequently asked
What does 1/4-20 UNC-2B mean on a drawing?
A quarter-inch nominal thread, 20 threads per inch, Unified coarse series, class 2 internal. The size, series and class together tell the shop which limits apply and which gauges to use.
Is class 3B a tighter fit than class 2B?
It has a narrower tolerance, not a smaller starting size. Both classes start at the same basic minimum, so 3B removes the loose end of the range rather than making every hole tighter.
Do I need to specify a gauging system?
It is good practice. The federal standard requires the gauging system to be specified, and ASME B1.3 defines the systems in use. Naming one removes an argument about how the thread will be accepted.
Is there a metric equivalent of 2B and 3B?
Not one to one. ISO 965-1 uses tolerance classes such as 6H for internal and 6g for external threads, with finer and coarser grades on either side. Pick the metric class from ISO 965-1 directly rather than converting from the Unified class.
Can you quote a part with class 3A or 3B threads?
Yes. Send the drawing with the class, the gauging system and any plating requirement marked; the quote lists each class 3 thread and how it will be verified.
Sources
- Screw-Thread Standards for Federal Services, Handbook H28, Part I: Unified, UNJ, Unified Miniature Screw Threads, National Bureau of Standards, 1969 (sections 4 to 10 and Table 2.21)
- FED-STD-H28/2B, Screw-Thread Standards for Federal Services, Section 2: Unified Inch Screw Threads, UN and UNR Thread Forms, US General Services Administration, 20 August 1991 (paragraphs 4.2 and 5.2)
- ASME B1.2, Gages and Gaging for Unified Inch Screw Threads, ASME, 2025 edition
- NASA Reference Publication 1228, Fastener Design Manual, R. T. Barrett, NASA Lewis Research Center, 1990
- ASME B1.1, Unified Inch Screw Threads (UN, UNR and UNJ Thread Forms), ASME, 2019 edition (the federal documents above incorporate earlier editions; FED-STD-H28/2B cites ASME B1.1-1989)