Understanding Bolt Thread Sizes

To understand more about UNC and UNF bolt sizes, we must first break down the differences between TPI and thread pitch. TPI, or threads per inch, is how bolts and other fasteners are measured under the imperial measuring system (which is based off of length and thread count). Metric bolts, on the other hand, are measured by their length and pitch, which is the distance between the threads measured in millimeters.

Starting a new bolt threading project? Refer to the chart below for UNC & UNF size conversions or browse our selection of bolt threading machines

What is The Difference Between UNC Threads & UNF Threads?

Unified Thread Standard

The Unified Thread Standard was established in 1949 by the United States, Canada, and Great Britain to define standards for fastener threads based off thread form, allowances, tolerances, and specific designations. The designations provided under this series factor in both TPI and thread pitch, with UNC & UNF being two of the most common designations used.

UNC VS UNF

UNC stands for unified course pitch threads whereas UNF stands for unified fine pitch threads. UNF threads are distinguished by having fewer threads per distance than UNC threads. While both UNC and UNF bolts are used for fastening, they are often applied in different fields due to their unique advantages.

For example, the coarse threads on UNC bolts are incredibly versatile and resistant to damage, making them the standard choice for most lower-tolerance, general purpose applications. On the other hand, the fine threads on UNF bolts give them higher tensile strength, making them useful for machine part assembly and other high precision applications.

Measuring Bolt Dimensions & Diameters

The size of a bolt is determined by the diameter of the bolt’s shank (separate from the bolt head). Bolt diameter is typically separated into three categories:

  • Major diameter: The largest and most outer diameter of a bolt’s shank.
  • Pitch diameter: The diameter where the thread thickness = the space between threads.
  • Minor diameter: The smallest and most inside diameter of a bolt’s shank.

Bolt Size Chart

Refer to the table for dimensions of commonly used bolt sizes. Measurements on top row represent UNC threads and measurements on bottom row represent UNF threads. Class 2A is acceptable for the majority of applications and Unified Thread Standard (UTS) bolt classes.

Size

Threads Per Inch

Class Thread

Major Diameter

Min

Max

Pitch Diameter

Min

Max

Minor Diameter

1/4

20

2A

0.2489

0.2408

0.2164

0.2127

0.1876

28

2A

0.2490

0.2425

0.2258

0.2225

0.2052

5/16

18

2A

0.3113

0.3026

0.2752

0.2712

0.2431

24

2A

0.3114

0.3042

0.2843

0.2806

0.2603

3/8

16

2A

0.3737

0.3643

0.3331

0.3287

0.2970

24

2A

0.3739

0.3667

0.3468

0.3430

0.3228

7/16

14

2A

0.4361

0.4258

0.3897

0.3850

0.3485

20

2A

0.4362

0.4281

0.4237

0.3995

0.3749

1/2

13

2A

0.4985

0.4876

0.4485

0.4435

0.4041

20

2A

0.4987

0.4906

0.4662

0.4619

0.4374

9/16

12

2A

0.5609

0.5495

0.5068

0.5016

0.4587

18

2A

0.5611

0.5524

0.5250

0.5205

0.4929

5/8

11

2A

0.6234

0.6113

0.5644

0.5589

0.5119

18

2A

0.6236

0.6149

0.5875

0.5828

0.5449

3/4

10

2A

0.7482

0.7353

0.6832

0.6773

0.6255

16

2A

0.7485

0.7391

0.7079

0.7029

0.6718

7/8

9

2A

0.8731

0.8592

0.8009

0.7946

0.7368

14

2A

0.8734

0.8631

0.8270

0.8216

0.7858

1

8

2A

0.9980

0.9830

0.9168

0.9100

0.8446

12

2A

0.9982

0.9868

0.9441

0.9382

0.8960

NC Thread Profiles

Unified

Unified NC Thread Profiles

American National

American National

 

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Oster Bolt Threading Capabilities

No matter your bolt threading specifications, we have an extensive selection of industrial grade pipe and bolt threading machines to meet the requirements for any industry, including construction, military & defense, manufacturing, and fabrication. Our reliable 792ALX bolt threading machine can produce Class 2A threads ranging from ¼” to 2”—no skilled operator necessary.

We also manufacture high-performance dark thread cutting oil to ensure your threading machine works efficiently and effectively. Browse our selection of high quality, easy-to-use bolt threading machines and additional threading necessities. Read more about the applications and benefits of bolt threading here.  

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