Machine Screw Thread Sizes

Looking for a guide to machine screw sizes? You’ve found it. In this article, we’ll be covering the machine screw sizes for the Unified Screw Thread System, commonly used across the United States and Canada, including minimums and maximums for major and pitch diameters and nominal sizes for both fine- and coarse-thread screws. First, however, we’ll do a quick recap on the basics around machine screws.

  1. Metric Screw Thread Sizes

Metric External (Fasteners) Thread and Fastener Sizes M20 - M50; Screw Thread Designations, Minor Diameter, Major Diameter, Pitch Diameter; ANSI/ ASME EXTERNAL Screw Thread Size Chart. All units are in inches. Unified Screw Threads per. ANSI/ASME B1.1-1989 (R2001), R2001) Nomenclature, are used. Machine Screw Size: Threads Per Inch: Minor Dia: Tap Drills: Clearance Hole Drills: Alum, Brass, & Plasitcs. 75% Thread: Stainless Steel.

What is a Machine Screw?

Machine screws are a specialized type of screw that joins metal parts together, used in electronics, machinery, and other similar applications. They can come with coarse or fine thread; coarse threaded machine screws are also commonly called stove bolts. Most machine screws are designed to be drilled into pre-existing holes, although some self-tapping varieties can drill their own.

Metric Screw Thread Sizes

The two most commonly used machine screw types have either coarse or fine threads. Coarse threads are better suited to quick and dirty applications, while fine threaded screws are better for precise products. Coarsely threaded screws are stronger than finely threaded ones, since their threads are higher, and due to the lower number of threads, they screw in faster. They’re also best for thick materials and coatings. Finely threaded screws, on the other hand, are better for harder and thinner materials. They’re also stronger than coarse threaded screws, both in terms of tension (thanks to their larger stress areas) and shear (thanks to their larger minor diameters, the distance between the grooves on each side).

There are two major screw thread systems used to classify machine screws. The Unified Screw Thread System is most commonly used across the U.S. and Canada, and is based on inches, while the ISO Metric Screw Thread System (based on millimeters) is used across the rest of the globe, although it’s also gaining ground in the United States. Other thread designation systems include the BSW, BA, and BSF, but they have fallen out of use except in the case of replacement parts.

Unified Screw Thread System Sizes

The chart below provides information on machine screw sizes from size 000 to ½ inch diameter (past size 12, screws are called by their measurements in inches). Diameter is the nominal size of the screw, measured from the farthest points of the threads. Although there are several types of machine screw designations in the Unified Screw Thread System, we’re providing information on the two most commonly used types: UNF (Unified Fine) and UNC (Unified Coarse). Often screw sizes will also tell you the number of threads a screw has by adding a dash and the number of threads (for example: 4-40 is a size four screw with 40 threads), but that can vary, so we haven’t included it here. A screw’s major diameter shows the screw’s diameter as measured from tip to tip of its threads, and in this table, we’ve included the minimum and maximum measurements a screw can be to be classified as that size. The pitch diameter is the average of the screw’s width between its threads and grooves. A screw’s tensile stress area is the thinnest part of the screw, in the grooves, where it is most likely to snap under pressure.

Fine Screws (UNF)

Coarse Screws (UNC)

Size

Diameter (inches)

Stress Area (inches2)

Major Diameter

Pitch Diameter

Stress Area (inches2)

Major Diameter

Pitch Diameter

Max.

Min.

Max.

Min.

Max.

Min.

Max.

Min.

000

.034

--

.0340

.0325

.0286

.0272

--

--

--

--

--

00

.047

--

.0470

.0450

.0402

.0386

--

--

--

--

--

0

.060

.00180

.0595

.0563

.0514

.0496

--

--

--

--

--

1

.073

.00278

.0724

.0689

.0634

.0615

.00263

.0724

.0686

.0623

.0603

2

.086

.00394

.0854

.0816

.0754

.0733

.00370

.0854

.0813

.0738

.0717

3

.099

.00523

.0983

.0942

.0867

.0845

.00487

.0983

.0938

.0848

.0825

4

.112

.00661

.1113

.1068

.0978

.0954

.00604

.1112

.1061

.0950

.0925

5

.125

.00830

.1243

.1195

.1095

.1070

.00796

.1242

.1191

.1080

.1054

6

.138

.01015

.1372

.1321

.1210

.1184

.00909

.1372

.1312

.1169

.1141

8

.164

.01474

.1632

.1577

.1452

.1424

.0140

.1631

.1571

.1428

.1399

10

.190

.0200

.1891

.1831

.1688

.1658

.0175

.1880

.1818

.1619

.1586

12

.216

.0258

.2150

.2085

.1918

.1886

.0242

.2150

.2078

.1879

.1845

¼

.25

.0364

.2490

.2425

.2258

.2225

.0318

.2489

.2408

.2164

.2127

5/16

.3125

.0580

.3114

.3042

.2843

.2806

.0524

.3113

.3026

.2752

.2712

3/8

.375

.0878

.3739

.3667

.3468

.3430

.0775

.3737

.3643

.3331

.3287

1/2

.5

.1599

.4987

.4906

.4662

.4619

.1419

.4985

.4876

.4485

.4435

Conclusion

Now that we’ve covered the basics of the Unified Screw System sizes for machine screws, we hope this will help you to more easily find the fasteners you’ll need. Feel free to visit our technical guide section for more technical guides and insights. If you’re more interested in sourcing, you can also check out our Supplier Discovery page, which features over 700 machine screw suppliers.

Sources:

Us machine screw thread sizes

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