Unified Inch Screw Threads: Designation, Classes and Tolerances: Internal and External Screw Thread Design...

Internal and External Screw Thread Design Profiles

This is where the practitioner's education really began. The Design Profiles define the maximum material condition for threads—the tightest they can possibly be while still being in spec.

UN External Screw Thread (Bolt)

                    ┌── 0.125P flat ──┐
                    │   (crest)       │
         ───────────┘     Optional    └───────────
        ╲            rounded crest            ╱
         ╲              60°                  ╱
          ╲                                 ╱
           ╲    Flanks straight beyond     ╱
            ╲   0.25H from sharp root    ╱
             ╲                          ╱
              ╲    0.375H depth       ╱
               ╲                    ╱
                └──────────────────┘
                ┌── 0.25P flat (root) ──┐
                   Rounded root optional

Key specifications:

• Root contour: A flat root contour is specified (0.25P flat width), but a rounded root cleared beyond the basic flat width is optional
• Crest: Flat at 0.125P, with optional rounding tangent to the flat
• Thread depth: $0.54127P$ (same as internal thread = depth of engagement)

UNR External Screw Thread (Fatigue-Resistant Bolt)

The UNR thread form is the fatigue-strength upgrade. Instead of a flat root, the UNR profile specifies:

• A smooth, continuous, non-reversing root contour with a radius of curvature not less than 0.108P at any point
• The root blends tangentially into the flanks and any straight segment
• At maximum material condition, the tangency point sits at a distance not less than 0.625H below the basic major diameter
• Thread depth: $0.59539P$ (deeper than UN because of the rounded root geometry)

Why it matters: That rounded root eliminates stress concentration. If you're making bolts for cyclic loading—think bridge connections, engine head bolts, rotating equipment—UNR is the standard you want to specify for external threads.

UN Internal Screw Thread (Nut)

• The root of the internal thread design profile is rounded and cleared beyond the 0.125P flat width of the Basic Profile
• This accommodates threading tool crest wear during production
• There is no internal UNR screw thread. The UNR designation applies only to external threads.

Design Profile Dimensional Summary

Diameter-Pitch Combinations: The Master Selection Matrix

Thread series are groups of diameter-pitch combinations, and the Unified system defines eleven standard series. Three have graded pitches (pitch changes with diameter), and eight have constant pitches.

Series with Graded Pitches

Series with Constant (Uniform) Pitches

Selection priority: When choosing from constant-pitch series, the standard recommends giving preference to the 8-, 12-, or 16-thread series wherever possible.

The Complete Diameter-Pitch Combination Table

This table shows which thread series applies at each nominal size. Sizes in parentheses are secondary sizes—use primary sizes first.

Note: Where a constant-pitch series entry shows "UNC," "UNF," or "UNEF," that diameter-pitch combination already exists in that graded series, and the graded series symbols and tolerances apply.

Standard Series Combinations: Basic Dimensions

the practitioner's Second Lesson: Why Basic Dimensions Matter

After the bolt rejection, the practitioner sat down with the ASME standard and realized something: the limits of size in Table 3 of the standard are derived from the basic dimensions. If you don't know the basic pitch diameter, the basic minor diameter, and the lead angle for your thread, you cannot verify whether your thread gages are appropriate, calculate tensile stress areas, or troubleshoot why a GO gage fails.

The following tables present the basic dimensions for every standard series.

Extra-Fine-Thread Series (UNEF / UNREF) — Basic Dimensions

The UNEF series is for applications where even finer pitches are required—short engagement lengths, thin-walled tubes, nuts, ferrules, and couplings.

Constant-Pitch Series Quick Reference

8-Thread Series (8-UN): Originally for high-pressure-joint bolts and nuts, now widely used as a substitute for UNC in diameters above 1 inch. Covers 1″ through 6″ diameter.

12-Thread Series (12-UN): Originally for boiler practice. Now used as a continuation of UNF for diameters above 1½ inches. Covers 9/16″ through 6″ diameter.

16-Thread Series (16-UN): Suitable for adjusting collars and retaining nuts. Continuation of UNEF for diameters above 1-11/16″. Covers 3/8″ through 6″ diameter.

4-Thread Series (4-UN): Heavy-duty large-diameter bolting. Covers 2½″ through 6″ diameter.

6-Thread Series (6-UN): Large-diameter general purpose. Covers 1⅜″ through 6″ diameter.

Thread Classes: The Tolerance and Allowance System

This is the section that cost the practitioner his contract. Thread classes define how tight or loose a thread fit will be, and misunderstanding them means parts that look right but fail gaging.

The Three External Classes (Bolts/Screws)

The Three Internal Classes (Nuts/Tapped Holes)

Critical Rules of the Class System

Rule 1: Internal thread tolerances are 30% larger than external. For every class pair (1A/1B, 2A/2B, 3A/3B), the internal thread pitch diameter tolerance is 30% greater than the external thread pitch diameter tolerance. This is a deliberate design decision—it's harder to inspect and control internal threads, so more tolerance is allocated there.

Rule 2: The allowance lives on the external thread only. Classes 1A and 2A external threads have their maximum diameters reduced below basic by the allowance. This creates a designed gap that serves two purposes: it prevents galling and seizing during high-cycle wrench assembly, and it provides space for plating or coating.

Class 3A has no allowance—its maximum diameter equals the basic size.

Rule 3: Internal threads have no allowance. All three internal thread classes (1B, 2B, 3B) have their minimum diameters set at the basic size. There is no built-in gap on the nut side.

Rule 4: Classes can be mixed. A Class 2A external thread may be used with a Class 1B, 2B, or 3B internal thread. The fit depends on the combination. The standard explicitly allows this flexibility.

How Classes 2A and 2B Handle Coatings

This is where many shops get burned. Class 2A is the only class with a built-in coating allowance.

• Before coating: The 2A major and pitch diameters are within the standard 2A limits
• After coating: The maximum major and pitch diameters may increase up to the basic size (effectively consuming the allowance)
• Gaging after coating: The thread must pass a basic Class 3A GO gage and a Class 2A NOT-GO gage

If the coating consumes the allowance and you still need the allowance for assembly clearance (e.g., high-temperature applications, lubricated joints), specify Class 2AG—the "G" means the allowance is maintained even after coating.

How Class 3A Handles Coatings

Class 3A provides no allowance. When a 3A thread must be coated, the standard suggests reducing the limits of size before plating by the amount of the 2A allowance. After plating, the thread should not exceed basic size.

Classes 1A and 1B Special Notes

Class 1A carries the same allowance as Class 2A, but the allowance is not available for coating. It exists solely for assembly clearance with rough or damaged threads. If a 1A thread is to be coated, special provisions must be made.

the practitioner's Diagnosis: What Went Wrong

Armed with this knowledge, the practitioner finally understood his rejection. His ¾–10 UNC-3A bolts were the problem. Here's what happened:

Class 3A means no allowance. The maximum pitch diameter is the basic pitch diameter: 0.6850″.

the practitioner had been cutting threads to Class 2A dimensions out of habit. Class 2A for ¾–10 UNC has a maximum pitch diameter of 0.6832″ (basic minus the 0.0018″ allowance) and a minimum of 0.6773″.

But his customer specified Class 3A. The Class 3A maximum pitch diameter is 0.6850″ and the minimum is 0.6806″. The tolerance band is tighter and sits higher.

When the practitioner machined his bolts, some came in at 0.6840″—perfectly within Class 2A range but slightly below the Class 3A minimum of 0.6806″. Wait—that's not quite right. Actually, the 3A range sits between 0.6850 (max) and 0.6806 (min). His threads at 0.6840 were within this range.

The real issue? the practitioner's threading process had some pitch diameter variation from lead and flank angle deviations. Class 3A has no extra "breathing room" from an allowance. The cumulative effect of tiny lead errors pushed the functional pitch diameter beyond the GO gage limit.

The lesson: Class 3A demands not just tighter numbers but tighter process control. Every tenth of a thousandth matters. Every lead deviation is amplified.

Screw Thread Designation: Reading and Writing the Callout

The designation system follows a strict sequence:

[Nominal Size] – [Threads per Inch] [Series Symbol] – [Class Symbol] ([Gaging System])

Standard Designation Examples

Decimal Equivalents

Decimal equivalents may be used instead of fractions or screw numbers:

• 0.250–20 UNC-2A (21) is the same as ¼–20 UNC-2A (21)
• 0.190–32 UNF-2A (22) is the same as 10–32 UNF-2A (22)

Important: Decimal equivalents are nominal size designations only—they have no dimensional significance beyond the fractional size or number.

Optional Pitch Diameter Limits

For uncoated standard series threads, the designation may include pitch diameter limits:

¼–20 UNC-2A (21)
PD 0.2164–0.2127 (Optional for uncoated threads)

Designating Coated (Plated) Threads

For coated Class 2A external threads, the callout includes both before and after coating dimensions:

¾–10 UNC-2A
Major dia 0.7500 max    }  AFTER COATING
PD 0.6850 max           }
Major dia 0.7482–0.7353  }  BEFORE COATING
PD 0.6832–0.6773         }

Designating UNS (Special) Threads

UNS threads use non-standard diameter-pitch combinations with Unified tolerance formulations. The basic form designation is always followed by the limits of size.

Designating Multiple-Start Threads

Multiple-start threads specify the nominal size, pitch, and lead:

[Nominal Size] – [Pitch] lead [Lead], [Start Type]

For example: "1 inch lead, 1/12 inch pitch, triple thread."

Pitch Diameter Tolerances: Where the Precision Lives

For UNC, UNF, 4-UN, 6-UN, and 8-UN Series

Pitch diameter tolerances are based on a length of engagement equal to the basic major (nominal) diameter and are applicable for lengths of engagement up to 1½ diameters.

For UNEF, 12-UN, 16-UN, 20-UN, 28-UN, 32-UN, and UNS Series

Pitch diameter tolerances are based on a length of engagement of 9 pitches and are applicable for engagement lengths from 5 to 15 pitches.

This is not the same as thread length on the part. The thread length on a bolt may far exceed the length of engagement. Don't confuse the two.

Internal Thread Minor Diameter Tolerances

Minor diameter tolerances for internal threads are based on a length of engagement equal to the nominal diameter and are suitable for engagement lengths up to 1½ diameters.

For applications with engagement lengths greater than 1½ diameters or less than the nominal diameter, the tolerance may be increased or decreased respectively, as covered in the Tapping section of the standard.

Hole Sizes for Tapping

Hole size limits for tapping Classes 1B, 2B, and 3B threads at various engagement lengths are specified in the Tapping section of the standard. The tap drill size must produce a minor diameter that allows the tap to form threads within the specified tolerance band while accounting for material pushback and tap geometry.

Thread Selection: A Decision Framework

the practitioner's Final Transformation

By the time the practitioner finished studying the standard, he had built himself a decision framework that he laminated and hung in the shop:

Step 1: Choose the Series

START HERE
    │
    ├── General fastening, soft materials, rapid assembly?
    │   └── UNC (Coarse)
    │
    ├── Higher tensile strength needed? Short engagement?
    │   Thin walls? Fine adjustment?
    │   └── UNF (Fine)
    │
    ├── Even finer pitch? Ferrules, couplings, thin tubes?
    │   └── UNEF (Extra-Fine)
    │
    ├── Large diameter (>1″) and need a standard pitch?
    │   └── 8-UN (preferred), 12-UN, or 16-UN
    │
    ├── Very large diameter (>2½″) heavy bolting?
    │   └── 4-UN or 6-UN
    │
    └── None of the above fit?
        └── UNS (Special) — calculate from standard formulas

Step 2: Choose the Class

START HERE
    │
    ├── Standard commercial fastener?
    │   └── 2A/2B (most common, has allowance for coatings)
    │
    ├── Precision fit, no assembly clearance needed?
    │   └── 3A/3B (no allowance, tightest tolerances)
    │
    ├── Quick assembly, rough conditions, bruised threads?
    │   └── 1A/1B (liberal allowance and tolerance)
    │
    └── Coated thread that must maintain assembly clearance?
        └── 2AG/2B (allowance preserved after coating)

Step 3: Verify the Selection

1. Check Table 2 for the diameter-pitch combination — is it a standard series?
2. Check Table 3 for the limits of size at your chosen class
3. Verify that your gage set matches the gaging system number in the callout
4. If no standard series fits, use selected combinations from Table 3, or as a last resort, calculate from the formulas in the standard

Quick-Reference Formula Card

Clip this section for your toolbox:

Note: The exact tensile stress area formula accounts for the helix angle and thread form. See ASME/ANSI B1.1-1989 for the precise formulation. The approximation above is sufficient for most engineering calculations.

Improvement method and result

Six months after the rejection, the practitioner got another call from the same defense subcontractor. Same part: ¾–10 UNC-3A bolts. Same quantity.

This time, the practitioner:

1. Verified the class requirements — Class 3A means no allowance, basic maximum diameters, and the tightest tolerance band
2. Calculated the exact pitch diameter limits — Max 0.6850″, Min 0.6806″
3. Set up his CNC with process capability in mind — targeting the middle of the tolerance band at 0.6828″
4. Checked his threading insert geometry — confirmed it produced a root form compliant with the design profile
5. Ran first-article inspection with calibrated thread gages — GO gage assembled smoothly, NOT-GO gage rejected correctly
6. Documented everything — attached the inspection report to the shipment

Every bolt passed. The subcontractor placed a standing order.

The difference wasn't his machine. It wasn't his tooling. It was his understanding.

Your Next Step

You now have the same reference that transformed the practitioner from a machinist who cut threads into an engineer who understood them. But knowledge without action is just trivia.

Here is your challenge:

Pick the most common thread you cut or specify in your work. Pull the basic dimensions from the tables above. Look up the class limits. Then ask yourself:

• Do you know the exact pitch diameter tolerance band you're working within?
• Do you know whether your process has the capability to stay inside it?
• If your customer changed the class from 2A to 3A tomorrow, would your parts still pass?

If the answer to any of those is "no" — you just found your next project.

Print this guide. Laminate the formula card. Tape the coarse-thread table to your machine. And the next time someone hands you a thread callout, you won't just read it. You'll own it.

What thread challenge are you facing right now? Is it a tolerance issue, a series selection question, or a coating compatibility problem? Drop your question below — this is exactly the kind of problem worth solving together.

The Thread Callout That Shut Down a Production Line

A single misread thread designation cost a machine shop 72 hours of downtime. Here's everything you need to know so it never happens to you.

How to Select the Right Unified Screw Thread

Before you can designate a thread, you need to select the correct one. This is where most engineers start — and where surprising numbers of them go sideways.

The Four-Step Selection Hierarchy

The ASME/ANSI B1.1-1989 Standard lays out a clear priority system for selecting threads. Think of it as a decision waterfall — you start at the top and only move down when the level above can't meet your requirements.

The Rule: Never jump to Step 2, 3, or 4 if Step 1 will work. Every step down the hierarchy increases manufacturing cost, tooling complexity, and inspection difficulty.

Understanding Thread Series: Which Pitch Belongs Where

The Unified system organizes threads into eleven standard series — three with graded (variable) pitches and eight with constant pitches. Each series exists for a reason, and understanding those reasons prevents over-engineering and under-specifying.

Graded Pitch Series

Coarse-Thread Series (UNC/UNRC) — The workhorse. This is the most commonly used series for bulk production of bolts, screws, nuts, and general engineering applications. Use it for:

• Threading into lower tensile strength materials — cast iron, mild steel, bronze, brass, aluminum, magnesium, plastics — to obtain optimum resistance to stripping of the internal thread
• Rapid assembly or disassembly
• Applications where corrosion or slight damage is possible

Fine-Thread Series (UNF/UNRF) — The precision option. External threads in this series have greater tensile stress area than comparable sizes of the Coarse series. Use it when:

• The resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member
• The length of engagement is short
• A smaller lead angle is desired
• Wall thickness demands a fine pitch
• Finer adjustment is needed

Extra-Fine-Thread Series (UNEF/UNREF) — The specialist. Use for:

• Short lengths of engagement
• Thin-walled tubes, nuts, ferrules, or couplings
• Same conditions as Fine series, but when even finer pitches are required

Constant Pitch Series

Pro Tip: When selecting from constant-pitch series, the Standard recommends giving preference to the 8-, 12-, or 16-thread series wherever possible.

Selecting the Right Combination of Thread Classes

The fit between a bolt and a nut isn't just about the thread size — it's about the class combination. The thread class determines how much tolerance and allowance exists between mating parts.

Here's the key principle: A Class 2A external thread may be used with a Class 1B, 2B, or 3B internal thread. The classes are mix-and-match by design.

Thread Classes: The Tolerance and Allowance System

the practitioner's aerospace bracket problem came down to class selection and thread form. Understanding the six thread classes — and why they exist — is non-negotiable for anyone working with Unified threads.

The Three External Thread Classes

Thread classes are distinguished from each other by the amounts of tolerance and allowance. Classes identified by a numeral followed by the letter A apply to external threads only, and classes followed by the letter B apply to internal threads only.

Classes 2A and 2B — The General-Purpose Standard

These are the most commonly used classes for general applications, including production of bolts, screws, nuts, and similar fasteners.

• The maximum diameters of Class 2A (external) uncoated threads are less than basic by the amount of the allowance
• The allowance minimizes galling and seizing in high-cycle wrench assembly
• The allowance can be used to accommodate plated finishes or other coatings
• For threads with additive finish, the maximum diameters of Class 2A may be exceeded by the amount of the allowance — meaning the 2A max diameters apply to an unplated part, while the basic diameters apply after plating
• The minimum diameters of Class 2B (internal) threads, whether or not plated or coated, are basic — affording no allowance in assembly at maximum metal limits

Class 2AG — The Coating-Protected Variant

Certain applications require an allowance for rapid assembly, to permit application of a proper lubricant, or for residual growth due to high-temperature expansion. When the thread is coated and the 2A allowance is not permitted to be consumed by such coating, the thread class symbol is qualified by G following the class symbol.

Classes 3A and 3B — Close Tolerance

• May be used when closer tolerances are desired than provided by Classes 2A and 2B
• Maximum diameters of Class 3A external threads and minimum diameters of Class 3B internal threads are basic — affording no allowance or clearance for assembly of maximum metal components
• Class 3A does not include an allowance, so it is suggested that the limits of size before plating be reduced by the amount of the 2A allowance whenever that allowance is adequate

Classes 1A and 1B — Quick Assembly

• Replaced American National Class 1
• Intended for ordnance and other special uses
• Used on threaded components where quick and easy assembly is necessary
• Provide a liberal allowance to permit ready assembly, even with slightly bruised or dirty threads
• Class 1A provides an allowance which is maintained for both coated and uncoated product — meaning it is not available for coating

Thread Class Quick-Reference Decision Matrix