Unified Inch Screw Threads: Designation, Classes and Tolerances: Thread Series (16-UN / 16-UNR)

Thread Series (16-UN / 16-UNR)

Application: Uniform-pitch series for large diameters requiring fine-pitch threads. Suitable for adjusting collars and retaining nuts. Also serves as a continuation of the UNEF series for diameters larger than 1-11/16 inches.

Selected sizes from the 16-Thread Series:

Thread Series (20-UN / 20-UNR)

Application: Fine-pitch adjustments, instruments, and precision mechanisms across a wide diameter range.

Selected sizes from the 20-Thread Series:

Thread Series (28-UN / 28-UNR)

Application: Very fine-pitch threads for precision instruments, thin-walled fittings, and applications demanding extremely tight adjustment.

Selected sizes from the 28-Thread Series:

Thread Classes: The Tolerance and Allowance System

the practitioner's Second Lesson: Class Matters

After the pressure vessel incident, the practitioner dove deep into thread specifications. He discovered that two bolts can have the same nominal size and pitch but wildly different fit characteristics depending on their thread class.

Thread classes are distinguished by the amounts of tolerance (how much size variation is permitted) and allowance (the intentional clearance between mating threads at maximum material condition).

The Six Standard Classes

How Tolerances and Allowances Interact

Classes 2A and 2B are the most commonly used classes for general applications. Here's how they work:

• Class 2A external thread: The maximum major and pitch diameters are less than basic by the amount of the allowance. This allowance minimizes galling and seizing during wrench assembly, and it can accommodate plated finishes.
• Class 2B internal thread: The minimum diameters are basic — no allowance is applied. All clearance comes from the external thread.
• Combined effect: At maximum material condition, there is always a gap equal to the 2A allowance between mating threads.

Classes 3A and 3B provide closer tolerances with no allowance on the external thread. The maximum diameters of 3A threads are basic. This means:

• Tighter fits
• Better thread alignment
• But: no room for plating unless the thread is undercut before coating

Classes 1A and 1B are intended for ordnance and special uses where quick and easy assembly is necessary, even with slightly bruised or dirty threads. The 1A allowance is maintained for both coated and uncoated product.

Key Tolerance Relationships

• Internal thread PD tolerance is 30% greater than external thread PD tolerance (same class)
• Pitch diameter tolerances for UNC, UNF, 4-UN, 6-UN, and 8-UN series are based on a length of engagement equal to the basic major diameter, applicable for engagements up to 1-1/2 diameters
• Pitch diameter tolerances for UNEF, 12-UN, 16-UN, 20-UN, 28-UN, and 32-UN series are based on a length of engagement of 9 pitches, applicable for 5 to 15 pitches

Class Combinations: You Can Mix and Match

The Standard allows mixing classes for specific fit requirements:

A Class 2A external thread may be used with a Class 1B, 2B, or 3B internal thread.

This flexibility lets you optimize cost (2A bolt in a 3B hole for precision alignment) or ease of assembly (1A bolt in a 2B hole for field maintenance).

Screw Thread Designation: Reading the Code

Every Unified thread designation follows a strict sequence. Understanding this code is non-negotiable for anyone who reads or creates engineering drawings.

The Standard Format

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

Examples Decoded

Critical Rules

• Nominal size is the basic major diameter — fractional, screw number, or decimal equivalent
• Decimal equivalents are nominal size designations only — they have no dimensional significance beyond the fractional size
• LH after the class symbol indicates a left-hand thread — omit for right-hand (default)
• UNR replaces UN in the series symbol to specify a rounded-root external thread
• Thread designations may optionally include pitch diameter limits of size

Designating Coated Threads

For plated Class 2A external threads, the designation includes both before-coating and after-coating dimensions:

3/4–10 UNC–2A (21)
Major dia 0.7500 max           } AFTER COATING
PD 0.6850 max
Major dia 0.7482–0.7353        } BEFORE COATING
PD 0.6832–0.6773

When the 2A allowance must be maintained after coating (not consumed by the coating), the class symbol is qualified with G:

3/4–10 UNC–2AG (21)

Coated 60-Degree Threads: The Plating Problem

The Core Principles

1. To keep finished threads within limits, external threads should not exceed basic size after plating, and internal threads should not be below basic size after plating
2. Class 2A provides both a tolerance and an allowance — the allowance provides adequate undercut for most electro-plating processes
3. After plating, Class 2A threads should be accepted by a basic Class 3A size GO gage and a Class 2A gage as a NOT-GO gage
4. Class 1A allowance is maintained for both coated and uncoated product — it is not available for coating
5. Class 3A has no allowance — reduce limits before plating by the amount of the 2A allowance when that allowance is adequate
6. Internal threads generally don't need coating provisions — depositing significant coating thickness on internal thread flanks is very difficult
7. Hot-dip galvanizing may not maintain these limits — special considerations apply

the practitioner's Coating Decision Matrix

The Thread Selection Decision Framework

This is the decision logic the practitioner and the practitioner wish they'd had from day one. Use it every time you specify a thread.

Step 1: Determine the Diameter Range

Step 2: Apply the Selection Priority

1. Start with UNC — unless you have a specific reason to deviate
2. Move to UNF if you need more tensile stress area, finer adjustment, shorter engagement, or thinner wall accommodation
3. Move to UNEF if UNF still isn't fine enough for your application
4. Use constant-pitch series (prefer 8-UN, 12-UN, or 16-UN) only if the graded-pitch series don't meet design requirements
5. Use UNS (special) only if no standard series meets requirements

Step 3: Select the Thread Class

Step 4: Verify the Engagement Length

The pitch diameter tolerances in the standard are based on specific engagement lengths:

• UNC, UNF, 4-UN, 6-UN, 8-UN: Engagement = basic major diameter; applicable up to 1.5 × diameter
• UNEF, 12-UN, 16-UN, 20-UN, 28-UN, 32-UN, UNS: Engagement = 9 pitches; applicable for 5 to 15 pitches

If your engagement length falls outside these ranges, you may need to recalculate tolerances from the Standard's formulas.

The Unified Thread Form Data Card

Keep this card in your toolbox, your desk, or your phone. It contains every critical formula for the Unified thread form at any pitch:

Worked Example: 1/2–13 UNC

Given: $n = 13$, therefore $P = 1/13 = 0.07692$ in.

Quick-Reference: Inch Screw Thread Form Data

The following table provides pre-calculated form dimensions for every standard pitch:

All dimensions in inches.

Engineering takeaway

the practitioner's 2 a.m. pressure vessel failure and the practitioner's year of maintenance frustration both point to the same truth: thread selection is a design decision with real consequences.

Here's what their combined experience distills into:

1. Start with UNC for general work. It's forgiving, it's available, and it resists damage. Move to finer pitches only when you have a specific engineering reason.

2. Know your tensile stress areas. The difference between UNC and UNF can be 10–15% more load capacity at the same nominal size. That margin matters.

3. Specify the thread class, not just the size. A 1/2–13 UNC–2A bolt in a 2B hole is a different animal from a 1/2–13 UNC–3A bolt in a 3B hole.

4. Plan for coating before you thread. If the fastener will be plated, Class 2A gives you room. Class 3A does not.

5. Use the right series for the diameter range. Above 1-1/2 inches, you're in constant-pitch territory. The 8-thread series is your best friend for large-diameter general purpose. The 12-thread series replaces UNF. The 16-thread series replaces UNEF.

6. Keep a reference. The tables in this guide are worth bookmarking, printing, or laminating. They'll be accurate today, next year, and a century from now.

Your Next Step

Pull out the last set of engineering drawings you approved or reviewed. Find the thread callouts. Ask yourself three questions:

• Is the thread series justified for the application?
• Is the class appropriate for the fit and coating requirements?
• Does the engagement length support the pitch diameter tolerances assumed by the standard?

If you can answer all three with confidence, you're ahead of 90% of your peers. If you can't—you now have every tool you need to fix that.

The threads hold the world together. Make sure yours are right.

Reference Standard: ASME/ANSI B1.1-1989, Unified Inch Screw Threads (UN and UNR Thread Form). All dimensional data sourced from Machinery's Handbook.

The Definitive Engineering Reference That Separates Professionals from Guessers

What You Will Master in This Guide

This is not a summary. This is the complete technical reference for Unified Screw Threads in the Inch Series, extracted from the authoritative ASME/ANSI B1.1-1989 standard and the Machinery's Handbook. You will walk away understanding:

• The Unified thread system—what it is, where it came from, and why it replaced the old American National system
• Thread form geometry—every dimension, truncation, flat, and radius defined by the standard
• The design profiles for UN and UNR external threads and UN internal threads
• The complete Inch Screw Thread Form Data table with all formulas
• Diameter-pitch combinations across all eleven standard series
• Basic dimensions for UNC, UNF, UNEF, and all constant-pitch series (4-UN through 32-UN)
• Thread classes 1A/1B, 2A/2B, and 3A/3B—tolerances, allowances, and when to use each
• Coated thread provisions—how plating interacts with class limits
• Screw thread designation—the complete callout system
• Thread selection logic—a decision framework for choosing the right series and class

Every formula. Every table. Every specification. No shortcuts.

The Unified Thread System: Why It Exists

The Problem That Created the Standard

Before 1948, the United States, Great Britain, and Canada each maintained separate screw thread standards. A ½″-13 bolt made in Detroit would not reliably mate with a nut made in Birmingham. During World War II, this incompatibility cost lives—field mechanics couldn't swap fasteners between American and British equipment.

The Unified Screw Thread system was born from that crisis. It established a common thread form, tolerance structure, and designation system across the three nations, and it remains the foundational inch-series thread standard used throughout North America and many global industries.

What Changed from the Old American National System

If you encounter older drawings or legacy equipment, understanding the differences matters. The Unified system modified four critical areas:

• Application of allowances: In the Unified system, both Class 1A and Class 2A external threads carry an allowance. The old American National system only provided an allowance on Class 1 external threads.
• Tolerance variation with size: Unified tolerances scale with diameter and pitch, rather than applying fixed tolerance bands.
• Pitch diameter tolerance asymmetry: The Unified system gives the internal thread (nut) a pitch diameter tolerance that is 30% greater than the external thread (bolt). The old system made them equal.
• Designation differences: The letter-number system (1A, 2B, 3A, etc.) replaced the old numeric "classes of fit."

Key takeaway: Unified threads are mechanically interchangeable with former American National threads of the same diameter and pitch. The geometry is the same. The tolerance and allowance structure is what changed.

The governing standard is ASME/ANSI B1.1-1989. Where the letters U, A, or B do not appear in a thread designation, the threads conform to the outdated American National system.

Thread Form: The Geometry That Governs Everything

The Basic Profile

Every Unified screw thread—whether UN or UNR, internal or external—derives from the same Basic Profile. This is the theoretical foundation, the geometric DNA of the thread.

The Basic Profile is a 60-degree symmetrical V-thread with specific truncations at the crest and root:

         ┌─ 0.125P ─┐
         │  (crest)  │
    ─────┘           └─────    ← Crest (truncated)
   ╲    60° included    ╱
    ╲     angle        ╱
     ╲                ╱
      ╲   Pitch     ╱
       ╲   Line   ╱        ← 0.5P at pitch line
        ╲        ╱
         ╲      ╱
          └────┘           ← Root (truncated)
         ┌─ 0.25P ─┐
         │ (root)   │

The master variable is H—the height of a sharp V-thread:

$$H = 0.86603 \times P$$

Where $P$ = pitch = $\frac{1}{n}$ (n = threads per inch)

From $H$, every other dimension in the thread form is derived:

The Complete Inch Screw Thread Form Data Table

This table gives you the exact dimensional values for every thread pitch from 80 TPI down to 4 TPI. These are the numbers you need on the shop floor.

Shop floor tip: Laminate this table. Tape it to the side of your lathe or CNC control. When you're programming a thread cycle, these are the numbers that tell you exactly how deep to go and what your tool geometry needs to produce.