UNC Thread Size Chart: Dimensions, TPI & Tap Drill Sizes

Contents

1 1. What Is a UNC Thread?
2 2. UNC Thread Terminology and Dimensions
3 3. Complete UNC Thread Size Chart
4 4. How to Read a UNC Thread Designation
5 5. UNC Thread Classes and Tolerances
6 6. UNC Tap Drill Size Chart and Thread Engagement
7 9. How to Select the Correct UNC Thread Size
8 10. UNC Thread Standards, FAQs, and Conclusion

UNC threads are among the most widely used inch-based screw threads in mechanical, structural, automotive, agricultural, and general industrial applications. The abbreviation UNC stands for Unified National Coarse, one of the principal thread series defined by the Unified Thread Standard. UNC threads use a 60-degree thread profile and have fewer threads per inch than corresponding UNF sizes, resulting in a larger and more widely spaced thread form.

The coarse thread profile offers several practical advantages. UNC fasteners are generally quicker to assemble, more resistant to cross-threading, and better able to tolerate dirt, minor damage, and surface imperfections. They are also commonly preferred for threaded connections in relatively soft or brittle materials, such as aluminium, cast iron, and some plastics, because their deeper threads provide improved resistance to stripping. These characteristics make UNC threads suitable for general-purpose bolts, screws, nuts, machinery, construction equipment, and maintenance applications.

A typical UNC designation, such as 1/2-13 UNC-2A, provides several important pieces of information. The value 1/2 identifies the nominal thread diameter in inches, 13 indicates the number of threads per inch, UNC identifies the coarse thread series, and 2A specifies a standard commercial tolerance class for an external thread.

Selecting or identifying a UNC thread requires more than matching its nominal diameter. Engineers, machinists, technicians, and maintenance personnel must also consider the threads per inch, pitch, major diameter, pitch diameter, minor diameter, tolerance class, and required tap-drill size. Using the wrong thread series or drill size can cause cross-threading, poor engagement, excessive assembly force, or premature joint failure.

This guide provides a comprehensive UNC thread size chart, covering common numbered and fractional sizes, thread pitches, dimensional terminology, thread classes, tap-drill recommendations, and measurement methods. It also explains how to read UNC designations, compare UNC with UNF and metric threads, and select the correct thread size for a particular application. For precision manufacturing or inspection, the chart should be used together with the applicable limits and tolerances specified in ASME B1.1.

1. What Is a UNC Thread?

UNC Thread Size Chart: Dimensions, TPI & Tap Drill Sizes

A UNC thread, or Unified National Coarse thread, is a general-purpose inch-based screw thread belonging to the Unified Thread Standard. It is one of the three primary Unified thread series:

UNC — Unified National Coarse
UNF — Unified National Fine
UNEF — Unified National Extra Fine

UNC threads have a 60-degree symmetrical V-shaped profile and relatively large spacing between adjacent threads. For a given nominal diameter, a UNC thread has fewer threads per inch than its UNF or UNEF equivalent. For example, a 1/2-inch UNC fastener has 13 threads per inch, while the corresponding UNF fastener has 20 threads per inch:

1/2-13 UNC
1/2-20 UNF

The number of threads per inch determines the axial distance between adjacent thread forms. Fewer threads per inch produce a larger pitch, deeper thread form, and generally more robust thread.

UNC thread characteristics

The principal characteristics of UNC threads include:

Inch-based nominal sizes
A 60-degree included thread angle
Relatively large thread pitch
Deeper threads than corresponding fine-thread series
Faster assembly and disassembly
Greater resistance to dirt and minor thread damage
Lower risk of cross-threading
Good stripping resistance in softer materials

Because of their wider spacing, UNC threads are easier to manufacture and less sensitive to small amounts of contamination or surface damage. They are frequently selected for equipment that must be assembled and disassembled in field conditions.

UNC threads are also commonly used in materials such as aluminium, cast iron, brass, and some engineering plastics. Their deeper thread form can provide stronger engagement in these materials, although the required engagement length must still be calculated according to the material strength and applied load.

Understanding a UNC thread designation

A UNC thread designation normally contains the nominal size, threads per inch, thread series, and tolerance class. Consider the following example:

$\text{1/2-13 UNC-2A}$

Each part has a specific meaning:

1/2 — Nominal major diameter in inches
13 — Number of threads per inch
UNC — Unified National Coarse thread series
2 — Thread fit or tolerance class
A — External thread

Internal threads use the letter B. Therefore, the matching internally threaded nut or tapped hole would normally be designated:

$\text{1/2-13 UNC-2B}$

For numbered screw sizes, the designation follows the same general format. For example:

$\text{No. 10-24 UNC-2A}$

This identifies a No. 10 external screw thread with 24 threads per inch and a Class 2 fit.

If a thread is left-handed, the letters LH are added to the designation:

$\text{1/2-13 UNC-2A-LH}$

When the direction is not stated, the thread is assumed to be right-handed.

External and internal UNC threads

UNC threads can be divided into two basic categories:

External threads are formed on bolts, screws, studs, and threaded shafts.
Internal threads are formed inside nuts, tapped holes, and threaded components.

External threads are identified by the letter A, while internal threads are identified by B. A properly selected external and internal thread pair must have the same nominal diameter, TPI, thread series, and compatible tolerance classes.

For example, a 3/8-16 UNC-2A bolt is typically assembled with a 3/8-16 UNC-2B nut. A 3/8-24 UNF nut cannot be used because it has a different pitch, even though the nominal diameter is the same.

Advantages of UNC threads

UNC threads offer several practical advantages in general engineering applications:

Faster assembly

A coarse thread advances farther with each complete rotation, so fewer turns are required to install or remove the fastener.
Improved resistance to thread damage

The larger, deeper thread form is less likely to be damaged by rough handling, minor corrosion, or contamination.
Lower risk of cross-threading

The wider pitch makes it easier to align the external and internal threads during assembly.
Good performance in soft materials

Coarse threads generally provide improved resistance to stripping in aluminium, cast iron, and plastics because each thread has a larger cross-section.
Better suitability for field service

UNC fasteners are convenient for equipment exposed to dirt, repeated maintenance, and less-controlled assembly conditions.
Reduced sensitivity to manufacturing variation

Coarse threads are generally easier to manufacture and inspect than very fine threads.

Limitations of UNC threads

UNC threads are not automatically the best choice for every application. Compared with a fine thread of the same nominal diameter, a UNC fastener normally has a smaller tensile-stress area because its threads are deeper. It may therefore provide lower tensile capacity when fastener material, diameter, and grade are otherwise identical.

UNC threads may also provide less precise axial adjustment and may be less resistant to loosening under certain vibration conditions. Fine threads can sometimes produce greater clamping force for a given tightening torque because of their smaller helix angle. However, actual joint performance depends on friction, preload, material, tightening method, engagement length, and service conditions.

Common UNC thread applications

UNC threads are commonly found in:

General-purpose bolts and screws
Structural and construction equipment
Industrial machinery
Agricultural machinery
Automotive and off-road equipment
Pumps and compressors
Machine frames and guards
Maintenance and repair assemblies
Cast and machined components
Woodworking and fabrication equipment
Threaded connections in aluminium and cast iron

For critical structural, pressure-retaining, lifting, aerospace, or high-temperature applications, selection should not be based on thread size alone. Fastener grade, material compatibility, preload, engagement length, fatigue, corrosion, and the applicable engineering standard must also be evaluated.

2. UNC Thread Terminology and Dimensions

Understanding a UNC size chart requires familiarity with the dimensions used to describe a thread. Although a thread may be identified by a simple designation such as 1/4-20 UNC, its complete geometry includes the major diameter, minor diameter, pitch diameter, pitch, lead, crest, root, flank, and thread depth.

Nominal thread size

The nominal thread size is the size used to identify the thread. For fractional UNC threads, it approximately corresponds to the basic major diameter of the external thread.

For example:

A 1/4-20 UNC thread has a nominal diameter of 0.2500 inch.
A 1/2-13 UNC thread has a nominal diameter of 0.5000 inch.
A 3/4-10 UNC thread has a nominal diameter of 0.7500 inch.

The actual measured major diameter of a manufactured external thread is normally slightly smaller than the nominal value because allowance and tolerances must be provided.

Small UNC threads are commonly identified by numbered screw sizes rather than fractions. The nominal diameter of a numbered thread can be calculated using:

$D = 0.060 + 0.013N$

where:

$D$ = nominal diameter in inches
$N$ = screw number

For a No. 10 screw:

$D = 0.060 + (0.013 \times 10) = 0.190\text{ in}$

Therefore, a No. 10-24 UNC thread has a nominal diameter of 0.190 inch and 24 threads per inch.

Major diameter

The major diameter is the largest diameter of the thread.

On an external thread, it is measured across the thread crests.
On an internal thread, it is measured across the thread roots.

For a 1/2-13 UNC thread, the basic major diameter is:

$D = 0.5000\text{ in}$

However, the maximum and minimum permissible major diameters depend on whether the thread is internal or external and on its tolerance class.

The major diameter is often the first dimension checked when identifying an unknown external thread, but it cannot identify the thread by itself. The TPI must also be measured because UNC, UNF, and UNEF threads may share the same nominal diameter.

Minor diameter

The minor diameter is the smallest diameter of the thread.

On an external thread, it is measured across the thread roots.
On an internal thread, it is measured across the thread crests.

The minor diameter influences the remaining cross-sectional area of a bolt and the amount of material removed when producing a tapped hole. For an internal thread, the tap-drill diameter is closely related to the minor diameter, but the two values are not always identical.

A larger tap drill creates a larger internal minor diameter and reduces the percentage of thread engagement. A smaller drill produces greater engagement but also increases tapping torque and the risk of tap breakage.

Pitch diameter

The pitch diameter is the diameter of an imaginary cylinder that passes through the thread profile at the point where the thread thickness equals the width of the space between adjacent threads.

Pitch diameter is one of the most important dimensions for controlling thread fit. It determines how tightly the internal and external threads engage and is commonly inspected using:

Thread plug gauges
Thread ring gauges
Thread micrometers
The three-wire measurement method
Optical or coordinate measuring equipment

Two threads can have acceptable major diameters but still fail to assemble correctly if their pitch diameters are outside the permitted limits.

Threads per inch

Threads per inch, abbreviated TPI, is the number of complete thread pitches contained in one inch of axial length.

Examples include:

No. 6-32 UNC: 32 TPI
1/4-20 UNC: 20 TPI
3/8-16 UNC: 16 TPI
1/2-13 UNC: 13 TPI
3/4-10 UNC: 10 TPI
1-8 UNC: 8 TPI

As nominal diameter increases, the standard UNC thread normally uses fewer threads per inch and therefore has a larger pitch.

Thread pitch

The thread pitch is the axial distance from one point on a thread to the corresponding point on the next thread. For inch threads, pitch is calculated from TPI:

$P = \frac{1}{T P I}$

where:

$P= thread pitch in inches$

TPI = threads per inch

For a 1/4-20 UNC thread:

$P=\frac{1}{20}=0.0500\text{ in}$

For a 1/2-13 UNC thread:

$P=\frac{1}{13}=0.07692\text{ in}$

To convert the pitch to millimetres:

$P_{\mathrm{mm}}=\frac{25.4}{\mathrm{TPI}}$

For a 1/2-13 UNC thread:

$P_{\mathrm{mm}}=\frac{25.4}{13}=1.9538\text{ mm}$

The metric result is a conversion of the UNC pitch, not a metric thread designation. A 1/2-13 UNC thread is therefore not interchangeable with a metric thread having a similar pitch.

Lead

The lead is the axial distance a screw advances during one complete revolution.

For a standard single-start UNC thread:

$L=P$

where:

$L= Lead$

$P = pitch$

Most UNC fasteners are single-start threads, so lead and pitch are equal. For a multi-start thread, the lead would equal the pitch multiplied by the number of thread starts. Multi-start configurations are not typical for standard UNC fasteners.

Thread angle and flanks

UNC threads use a symmetrical 60-degree included angle. Each flank is therefore positioned at 30 degrees relative to a plane perpendicular to the thread axis.

The flanks are the angled sides connecting the crest and root. Contact between the flanks of the internal and external threads transfers the assembly load. The crest and root are normally truncated or rounded according to the standardized Unified thread profile rather than forming perfectly sharp V-points.

Crest and root

The crest is the outermost surface of a thread:

On an external thread, it is located at the major diameter.
On an internal thread, it is located at the minor diameter.

The root is the innermost surface between adjacent thread flanks:

On an external thread, it is located at the minor diameter.
On an internal thread, it is located at the major diameter.

Root geometry is especially important in fatigue-loaded fasteners because sharp transitions can create stress concentrations. Rolled threads often have smoother root contours and improved fatigue performance compared with poorly formed cut threads.

Thread depth

The thread depth is the radial distance between the crest and root. It must not be confused with the difference between major and minor diameters, which represents twice the radial depth:

$h=\frac{D-d}{2}$

where:

h=radial thread depth
D= major diameter
$d= minor diameter$

The theoretical height of a sharp 60-degree thread form is:

$H=\frac{\sqrt{3}}{2}P=0.866025P$

However, actual Unified threads have truncated crests and specified root forms. Their finished thread depth is therefore less than the theoretical sharp-V height. Exact dimensions should be obtained from the appropriate standard rather than calculated solely from the theoretical profile.

Basic dimensions versus tolerance limits

A UNC thread chart may provide basic dimensions, which represent the theoretically exact geometry used as the starting point for applying allowances and tolerances. Basic dimensions are not necessarily the actual acceptable manufacturing limits.

The final permissible dimensions depend on:

Nominal thread size
Threads per inch
Internal or external thread
Thread tolerance class
Allowance
Manufacturing tolerance
Coating or plating requirements

For example, 1/2-13 UNC-2A and 1/2-13 UNC-3A have the same nominal size and basic thread geometry, but their acceptable dimensional limits differ. Class 3A provides a closer fit and generally has a narrower tolerance than Class 2A.

For machining, inspection, or acceptance of precision threads, dimensional limits should be taken from the applicable edition of ASME B1.1 or the governing project specification.

3. Complete UNC Thread Size Chart

The following UNC thread size chart lists the standard diameter–pitch combinations for Unified National Coarse threads. It covers numbered screw sizes from No. 1 through No. 12 and fractional sizes from 1/4 inch through 4 inches.

All diameters shown are basic dimensions, representing the theoretically exact thread geometry before allowances and manufacturing tolerances are applied. They should not be treated as maximum or minimum inspection limits.

UNC thread size and basic dimensions chart

UNC thread size	Major diameter (in)	Major diameter (mm)	TPI	Pitch (in)	Pitch (mm)	Basic pitch diameter (in)	External minor diameter (in)	Internal minor diameter (in)
No. 1-64 UNC	0.0730	1.854	64	0.01562	0.397	0.0629	0.0538	0.0561
No. 2-56 UNC	0.0860	2.184	56	0.01786	0.454	0.0744	0.0641	0.0667
No. 3-48 UNC	0.0990	2.515	48	0.02083	0.529	0.0855	0.0734	0.0764
No. 4-40 UNC	0.1120	2.845	40	0.02500	0.635	0.0958	0.0813	0.0849
No. 5-40 UNC	0.1250	3.175	40	0.02500	0.635	0.1088	0.0943	0.0979
No. 6-32 UNC	0.1380	3.505	32	0.03125	0.794	0.1177	0.0997	0.1042
No. 8-32 UNC	0.1640	4.166	32	0.03125	0.794	0.1437	0.1257	0.1302
No. 10-24 UNC	0.1900	4.826	24	0.04167	1.058	0.1629	0.1389	0.1449
No. 12-24 UNC	0.2160	5.486	24	0.04167	1.058	0.1889	0.1649	0.1709
1/4-20 UNC	0.2500	6.350	20	0.05000	1.270	0.2175	0.1887	0.1959
5/16-18 UNC	0.3125	7.938	18	0.05556	1.411	0.2764	0.2443	0.2524
3/8-16 UNC	0.3750	9.525	16	0.06250	1.588	0.3344	0.2983	0.3073
7/16-14 UNC	0.4375	11.113	14	0.07143	1.814	0.3911	0.3499	0.3602
1/2-13 UNC	0.5000	12.700	13	0.07692	1.954	0.4500	0.4056	0.4167
9/16-12 UNC	0.5625	14.288	12	0.08333	2.117	0.5084	0.4603	0.4723
5/8-11 UNC	0.6250	15.875	11	0.09091	2.309	0.5660	0.5135	0.5266
3/4-10 UNC	0.7500	19.050	10	0.10000	2.540	0.6850	0.6273	0.6417
7/8-9 UNC	0.8750	22.225	9	0.11111	2.822	0.8028	0.7387	0.7547
1-8 UNC	1.0000	25.400	8	0.12500	3.175	0.9188	0.8466	0.8647
1 1/8-7 UNC	1.1250	28.575	7	0.14286	3.629	1.0322	0.9497	0.9704
1 1/4-7 UNC	1.2500	31.750	7	0.14286	3.629	1.1572	1.0747	1.0954
1 3/8-6 UNC	1.3750	34.925	6	0.16667	4.233	1.2667	1.1705	1.1946
1 1/2-6 UNC	1.5000	38.100	6	0.16667	4.233	1.3917	1.2955	1.3196
1 3/4-5 UNC	1.7500	44.450	5	0.20000	5.080	1.6201	1.5046	1.5335
2-4.5 UNC	2.0000	50.800	4.5	0.22222	5.644	1.8557	1.7274	1.7594
2 1/4-4.5 UNC	2.2500	57.150	4.5	0.22222	5.644	2.1057	1.9774	2.0094
2 1/2-4 UNC	2.5000	63.500	4	0.25000	6.350	2.3376	2.1933	2.2294
2 3/4-4 UNC	2.7500	69.850	4	0.25000	6.350	2.5876	2.4433	2.4794
3-4 UNC	3.0000	76.200	4	0.25000	6.350	2.8376	2.6933	2.7294
3 1/4-4 UNC	3.2500	82.550	4	0.25000	6.350	3.0876	2.9433	2.9794
3 1/2-4 UNC	3.5000	88.900	4	0.25000	6.350	3.3376	3.1933	3.2294
3 3/4-4 UNC	3.7500	95.250	4	0.25000	6.350	3.5876	3.4433	3.4794
4-4 UNC	4.0000	101.600	4	0.25000	6.350	3.8376	3.6933	3.7294

The diameter–pitch combinations and basic dimensions follow the standardized Unified coarse-thread series described in the National Bureau of Standards thread handbook. The current contractual requirements should be confirmed against the applicable edition of ASME B1.1. NIST Handbook H28

How the chart dimensions are determined

The thread pitch in inches is calculated using:

$P=\frac{1}{n}$ $P = n 1$

where:

$p= pitch in inches$
$n= threads per inch$

The metric equivalent of the pitch is:

$P_{m m} = \frac{25.4}{n}$

For example, a 3/8-16 UNC thread has 16 threads per inch:

$P = \frac{1}{16} = 0.0625 in$

P_{\mathrm{mm}}=\frac{25.4}{16}=1.5875\text{ mm}

For the basic Unified thread profile, the pitch diameter can be calculated approximately as:

$E = D - 0.649519 P$

where:

$E= basic pitch diameter$
D= basic major diameter
$P= thread pitch$

The basic minor diameters of the internal and external thread are different because the crest and root truncations are not identical.

For an external thread:

$d_{1} = D - 1.226869 P$

For an internal thread:

$D_{1} = D - 1.082532 P$

These equations describe the basic profile. They do not include the allowance or tolerance associated with Classes 1A, 2A, 3A, 1B, 2B, or 3B.

Numbered versus fractional UNC sizes

UNC sizes smaller than 1/4 inch are generally designated by screw numbers. The nominal diameter can be estimated from:

$D = 0.060 + 0.013 N$

$where N$ is the screw number.

For example, the major diameter of a No. 8 screw is:

$D = 0.060 + (0.013 \times 8) = 0.164 in$

Its complete coarse-thread designation is therefore No. 8-32 UNC, indicating a 0.164-inch nominal diameter and 32 threads per inch.

Fractional designations begin at 1/4 inch and express the nominal major diameter directly. For example:

1/4-20 UNC: 0.250-inch major diameter and 20 TPI
3/8-16 UNC: 0.375-inch major diameter and 16 TPI
1/2-13 UNC: 0.500-inch major diameter and 13 TPI
1-8 UNC: 1.000-inch major diameter and 8 TPI

Important chart limitations

The chart should be used as a dimensional reference, not as a complete inspection specification.

The actual acceptable thread dimensions depend on:

Internal or external thread
Thread tolerance class
Length of engagement
Manufacturing method
Surface treatment or plating
Required thread engagement
Applicable standard edition

Tap-drill diameters are also not identical to the listed basic internal minor diameters. The selected drill size depends on the desired percentage of thread engagement, workpiece material, tap type, and machining conditions. Recommended tap-drill sizes are covered separately in the UNC tap-drill chart.

4. How to Read a UNC Thread Designation

UNC Thread Size Chart

A UNC thread designation communicates the essential characteristics needed to manufacture, select, or inspect the thread. A complete designation normally contains the nominal size, threads per inch, thread series, tolerance class, and external or internal thread identification.

The general format is:

$Nominal size–TPI UNC–Class$

A typical example is:

$1/2-13 UNC-2A$

This designation can be divided into four elements:

Designation element	Example	Meaning
Nominal size	1/2	Basic major diameter in inches
Threads per inch	13	Number of complete pitches per inch
Thread series	UNC	Unified National Coarse
Thread class	2A	Standard commercial external thread

Nominal thread size

The first element identifies the nominal major diameter. Fractional sizes are stated directly in inches.

For example:

1/4-20 UNC has a nominal diameter of 0.250 inch.
3/8-16 UNC has a nominal diameter of 0.375 inch.
1/2-13 UNC has a nominal diameter of 0.500 inch.
3/4-10 UNC has a nominal diameter of 0.750 inch.

For numbered screws, the first element is a number rather than a fraction:

$No. 10-24 UNC-2A$

A No. 10 thread has a nominal major diameter of 0.190 inch. The number does not represent the diameter in millimetres or tenths of an inch.

Threads per inch

The number following the diameter indicates the thread count in threads per inch.

In the designation:

$3/8-16 UNC$

the value 16 means that 16 complete thread pitches occur over one inch of axial length. It does not indicate the thread depth or fastener length.

The pitch is found by taking the reciprocal of the TPI:

$P = \frac{1}{16} = 0.0625 in$

A 3/8-inch fine thread may be designated 3/8-24 UNF. Although both threads have the same nominal diameter, their pitches are different. They cannot be assembled together safely.

Thread-series abbreviation

The letters UNC identify the thread as part of the Unified National Coarse series.

Other common Unified designations include:

UNF — Unified National Fine
UNEF — Unified National Extra Fine
UN — Unified constant-pitch series
UNS — Unified Special thread

The series abbreviation is essential because diameter alone cannot identify the thread. A 1/2-inch fastener, for example, may use 1/2-13 UNC, 1/2-20 UNF, or another permitted diameter–pitch combination.

Thread tolerance class

The number following the thread series identifies the thread class. Unified threads normally use Classes 1, 2, or 3:

Class 1 provides the loosest fit.
Class 2 provides a general-purpose commercial fit.
Class 3 provides the closest and most accurate fit.

The class number must be followed by either A or B:

A identifies an external thread.
B identifies an internal thread.

Common examples include:

1A — Loose-fit external thread
1B — Loose-fit internal thread
2A — General-purpose external thread
2B — General-purpose internal thread
3A — Close-fit external thread
3B — Close-fit internal thread

The most common commercial combination is a Class 2A bolt or screw assembled with a Class 2B nut or tapped hole.

For example:

$1/2-13 UNC-2A$

identifies an external thread, while:

$1/2-13 UNC-2B$

identifies its corresponding internal thread.

The A and B letters do not mean that the two components have identical dimensional limits. External threads require clearance from internal threads, and the applicable allowances and tolerances determine the permitted fit.

Right-hand and left-hand designations

UNC threads are assumed to be right-handed unless otherwise specified. A right-hand thread tightens when turned clockwise when viewed from the fastener end.

A left-hand thread is identified by adding LH:

$1/2-13 UNC-2A-LH$

This identifies:

1/2-inch nominal diameter
13 TPI
UNC thread series
Class 2 external thread
Left-hand direction

Left-hand threads are used where normal rotation could loosen a right-hand fastener, or where opposite thread directions are required for adjustment.

Examples of UNC thread designations

Example 1: No. 8-32 UNC-2A

No. 8: 0.164-inch nominal major diameter
32: 32 threads per inch
UNC: Unified National Coarse series
2A: Class 2 external thread

This designation typically applies to a machine screw or externally threaded component.

Example 2: 1/4-20 UNC-2B

1/4: 0.250-inch nominal diameter
20: 20 threads per inch
UNC: Coarse thread series
2B: Class 2 internal thread

This designation normally applies to a nut or tapped hole.

Example 3: 3/4-10 UNC-3A

3/4: 0.750-inch nominal diameter
10: 10 threads per inch
UNC: Coarse thread series
3A: Close-tolerance external thread

Example 4: 1-8 UNC-2B-LH

1: 1.000-inch nominal diameter
8: 8 threads per inch
UNC: Coarse thread series
2B: General-purpose internal thread
LH: Left-hand thread

Thread callouts on engineering drawings

A drawing callout for an external thread may appear as:

$3/8-16 UNC-2A \times 1.00 LONG$

This specifies the thread form and the required threaded length.

A tapped-hole callout may appear as:

$1/4-20 UNC-2B \times 0.50 DEEP$

The drawing may separately state:

Tap-drill depth
Full-thread depth
Through or blind hole
Countersink or chamfer
Quantity of holes
Surface coating
Inspection requirements

For blind holes, the drilling depth must normally exceed the required full-thread depth because the tap has a chamfered lead and the drill point creates a conical bottom.

Coated and plated thread designations

Plating, galvanizing, anodizing, or other coatings can change the effective thread dimensions. A coating increases the external thread size and reduces the available space in an internal thread. Because the coating is applied to both flanks, a small coating thickness can produce a considerably larger change in functional pitch diameter.

The drawing or purchasing specification should therefore state:

Whether dimensional limits apply before or after coating
Required coating type and thickness
Whether allowance has been provided
Applicable thread-gauging condition
Any oversize tapping requirement

A standard Class 2A designation should not be assumed to provide adequate room for every coating. Heavy coatings, especially hot-dip galvanizing, may require special allowances or oversize internal threads.

Avoiding designation errors

Common mistakes when reading UNC callouts include:

Treating TPI as thread pitch
Confusing No. 10 with a 10 mm diameter
Omitting the A or B thread classification
Assuming UNC and UNF threads are interchangeable
Confusing a converted metric pitch with a metric thread
Ignoring an LH suffix
Using basic dimensions as final inspection limits
Failing to account for plating or coating thickness

The complete designation must be checked before selecting the fastener, tap, die, or gauge. Diameter and visual appearance alone are not enough to confirm thread compatibility.

5. UNC Thread Classes and Tolerances

UNC thread dimensions are controlled through a system of classes, allowances, and tolerances. These controls ensure that separately manufactured bolts, screws, nuts, and tapped components can assemble correctly while providing the required amount of clearance.

A thread class does not change the nominal diameter, TPI, or basic thread profile. Instead, it determines how far the manufactured dimensions may vary from the basic profile and how loosely or closely the internal and external threads fit together.

Unified threads use three principal classes:

Thread class	External thread	Internal thread	General fit
Class 1	1A	1B	Loose
Class 2	2A	2B	General-purpose
Class 3	3A	3B	Close and accurate

The letter following the class number identifies the thread type:

A — external thread, such as a bolt, screw, or stud
B — internal thread, such as a nut or tapped hole

Allowance and tolerance

Although the terms are sometimes used interchangeably, allowance and tolerance have different meanings.

Allowance is the intentional difference between the maximum-material sizes of mating internal and external threads. It creates a minimum guaranteed clearance between the components.

Tolerance is the permitted variation in a manufactured thread dimension. It represents the difference between the maximum and minimum acceptable sizes.

For Unified threads, allowance is normally applied to Classes 1A and 2A external threads. Class 3A external threads have no allowance at the basic pitch diameter, which helps produce a closer fit.

The actual fit between mating threads is primarily controlled by their pitch diameters. Major and minor diameters are also controlled, but pitch diameter has the greatest influence on whether the parts assemble freely, tightly, or not at all.

Class 1A and 1B threads

Classes 1A and 1B provide the loosest fit and the greatest assembly clearance among the standard Unified thread classes.

They may be selected where:

Quick and easy assembly is important
Dirt or contamination may be present
Threads may experience minor damage
Components require frequent assembly and disassembly
Accurate alignment is not critical
A large clearance is acceptable

Class 1 threads are less common in modern commercial fasteners than Class 2 threads. Their greater clearance can permit more movement between assembled components, making them unsuitable for applications requiring precise positioning or minimum backlash.

A typical Class 1 pairing is:

$1/2-13 UNC-1A with 1/2-13 UNC-1B$

Class 2A and 2B threads

Classes 2A and 2B provide the standard general-purpose commercial fit. They are the most widely used thread classes for UNC bolts, screws, nuts, and tapped holes.

Class 2 threads provide a practical balance between:

Manufacturing cost
Ease of assembly
Interchangeability
Dimensional control
Strength and load distribution
Allowance for light coatings or surface variation

A typical designation for a commercial bolt is:

$1/2-13 UNC-2A$

The corresponding nut or tapped hole is normally:

$1/2-13 UNC-2B$

Typical applications include:

General industrial machinery
Structural assemblies
Automotive equipment
Pumps and compressors
Agricultural machinery
Maintenance fasteners
Equipment frames and supports
General fabricated components

Unless a drawing or specification requires another class, Class 2A for external threads and Class 2B for internal threads are generally the expected choices for standard commercial UNC fasteners.

Class 3A and 3B threads

Classes 3A and 3B provide a close and accurate fit. Their tolerances are narrower than those of Class 2 threads, and Class 3A has no pitch-diameter allowance.

Class 3 threads may be used where:

Accurate positioning is required
Thread alignment is important
Minimum looseness or backlash is desired
High-quality controlled assembly is available
Close dimensional control can be maintained
The application specification explicitly requires a close fit

Applications may include:

Precision machinery
Aerospace components
Measuring instruments
High-quality tooling
Critical positioning assemblies
Specialized high-strength joints

Class 3 threads require more accurate manufacturing and inspection. They are also more sensitive to burrs, contamination, distortion, plating thickness, and dimensional changes caused by heat treatment.

A Class 3A external thread should not automatically be paired with any internal thread of the same diameter and TPI without checking the specified fit. Manufacturing capability and coating requirements must also be considered.

Comparison of UNC thread classes

Characteristic	Class 1	Class 2	Class 3
Fit	Loose	Medium	Close
Assembly clearance	Greatest	Moderate	Smallest
Manufacturing accuracy	Lowest	Standard	Highest
Manufacturing cost	Generally lowest	Moderate	Generally highest
Sensitivity to contamination	Lowest	Moderate	Highest
Commercial availability	Limited	Very common	Specialized
Typical use	Rough or quick assembly	General engineering	Precision assemblies

Basic dimensions versus limits of size

The basic dimensions in a UNC thread chart define the theoretical thread geometry. They do not represent the complete acceptable size range for a manufactured part.

For example, the basic pitch diameter of a 1/2-13 UNC thread is approximately:

$E = 0.4500 in$

However, the acceptable pitch-diameter limits for the following threads are not identical:

1/2-13 UNC-1A
1/2-13 UNC-2A
1/2-13 UNC-3A
1/2-13 UNC-1B
1/2-13 UNC-2B
1/2-13 UNC-3B

Each class has its own maximum and minimum values. Therefore, a machinist or inspector cannot determine compliance using only the basic-dimension chart.

The Unified thread standard establishes separate dimensional limits for external and internal threads. The allowable dimensions should be obtained from the applicable edition of ASME B1.1 or the governing product specification.

Effect of plating and coatings

Plating and coatings can significantly affect thread fit. When a coating is added to an external thread, material accumulates on both thread flanks. This increases the functional pitch diameter by more than the coating thickness measured perpendicular to the surface.

Similarly, coating an internal thread reduces its available clearance. If coating thickness is not considered, the finished components may bind or fail to assemble.

Common surface treatments affecting thread dimensions include:

Zinc plating
Nickel plating
Cadmium plating
Chromium plating
Hot-dip galvanizing
Phosphate coatings
Anodizing
Paint and polymer coatings
Dry-film lubricants

The drawing or specification should clarify whether thread dimensions apply:

Before coating
After coating
Before coating with a specified allowance
After coating using specified GO and NO-GO gauges

Light coatings may sometimes be accommodated within the allowance of a Class 2A external thread. Heavy coatings, such as hot-dip galvanizing, normally require special consideration and may require oversize internal threads or modified external-thread dimensions.

Inspection of UNC threads

UNC threads are commonly inspected using functional gauges rather than relying only on direct diameter measurements.

Typical inspection equipment includes:

GO thread plug gauges for internal threads
NO-GO thread plug gauges for internal threads
GO thread ring gauges for external threads
NO-GO thread ring gauges for external threads
Thread micrometers
Three-wire measuring systems
Optical comparators
Coordinate measuring machines

A GO gauge verifies that the thread can assemble under maximum-material conditions. A NO-GO gauge helps confirm that the thread has not exceeded the permitted minimum-material limit.

Thread gauges must match the complete thread designation. A gauge marked 1/2-13 UNC-2B is intended for a different thread condition from a gauge marked 1/2-13 UNC-3B.

6. UNC Tap Drill Size Chart and Thread Engagement

A tap drill creates the hole that will subsequently be threaded with a tap. Its diameter has a direct effect on thread engagement, tapping torque, tool life, and the risk of damaging the workpiece.

If the drilled hole is too small:

Thread engagement becomes excessive
Tapping torque increases
The tap may wear or break
Threads may tear or gall
Thin-walled parts may distort

If the hole is too large:

Thread engagement decreases
The internal thread becomes weaker
Stripping resistance may be reduced
The finished hole may fail inspection

The following chart provides commonly used cutting-tap drill sizes for standard UNC threads. These are practical starting recommendations, not universal requirements.

UNC tap drill size chart

UNC thread	TPI	Recommended drill	Drill diameter (in)	Drill diameter (mm)	Approx. thread engagement
No. 1-64 UNC	64	No. 53	0.0595	1.511	65%
No. 2-56 UNC	56	No. 50	0.0700	1.778	67%
No. 3-48 UNC	48	No. 47	0.0785	1.994	74%
No. 4-40 UNC	40	No. 43	0.0890	2.261	69%
No. 5-40 UNC	40	No. 38	0.1015	2.578	71%
No. 6-32 UNC	32	No. 36	0.1065	2.705	76%
No. 8-32 UNC	32	No. 29	0.1360	3.454	67%
No. 10-24 UNC	24	No. 25	0.1495	3.797	73%
No. 12-24 UNC	24	No. 16	0.1770	4.496	70%
1/4-20 UNC	20	No. 7	0.2010	5.105	74%
5/16-18 UNC	18	Letter F	0.2570	6.528	75%
3/8-16 UNC	16	5/16 in	0.3125	7.938	75%
7/16-14 UNC	14	Letter U	0.3680	9.347	73%
1/2-13 UNC	13	27/64 in	0.4219	10.716	76%
9/16-12 UNC	12	31/64 in	0.4844	12.303	70%
5/8-11 UNC	11	17/32 in	0.5313	13.494	77%
3/4-10 UNC	10	21/32 in	0.6563	16.669	70%
7/8-9 UNC	9	49/64 in	0.7656	19.447	74%
1-8 UNC	8	7/8 in	0.8750	22.225	75%
1 1/8-7 UNC	7	63/64 in	0.9844	25.003	74%
1 1/4-7 UNC	7	1 7/64 in	1.1094	28.178	74%
1 3/8-6 UNC	6	1 7/32 in	1.2188	30.956	70%
1 1/2-6 UNC	6	1 11/32 in	1.3438	34.131	70%

The engagement percentages are approximate because actual engagement depends on the finished minor diameter, tap geometry, thread tolerance class, material behavior, and hole quality.

Calculating an approximate tap-drill diameter

A commonly used approximation for a 75% UNC thread is:

$D_{drill} \approx D_{major} - \frac{1}{T P I}$

Because:

$P=\frac{1}{\mathrm{TPI}}$

the equation can also be written as:

$D_{drill} \approx D_{major} - P$

For a 1/2-13 UNC thread:

$D_{drill} \approx 0.5000 - \frac{1}{13}$ $D_{\text{drill}}\approx0.5000-0.07692$

$D_{drill} \approx 0.4231 in$

The nearest commonly selected fractional drill is 27/64 inch:

$\frac{27}{64} = 0.421875 in$

This approximation is useful for workshop estimates, but a verified tap-drill chart should be used for final tool selection.

Percentage of thread engagement

Thread engagement percentage describes how much of the theoretical thread height is produced in the tapped hole. Increasing the percentage does not increase connection strength proportionally.

For many general-purpose tapped holes, approximately 60% to 75% thread engagement provides a useful balance between strength and machinability. Beyond this range, tapping torque may increase substantially while the improvement in stripping strength remains relatively small.

Approximate engagement for a 60-degree Unified thread can be estimated using:

$Thread engagement (%) \approx 75 (\frac{D_{major} - D_{drill}}{P})$

For a 1/4-20 UNC thread drilled with a No. 7 drill:

$D_{\text{major}}=0.2500\text{ in}$

$D_{\text{drill}}=0.2010\text{ in}$

$P=\frac{1}{20}=0.0500\text{ in}$

Therefore:

$Engagement \approx 75 (\frac{0.2500 - 0.2010}{0.0500})$

Engagement \approx 73.5 %

This is an approximate workshop calculation. It does not replace the exact dimensional limits for a specified internal-thread class.

Selecting drill size by workpiece material

The standard drill size may be adjusted according to the material being tapped.

Carbon and alloy steel

A conventional tap-drill size producing approximately 70% to 75% engagement is normally suitable. Adequate cutting fluid and correct tap geometry are important, especially for deep or blind holes.

Stainless steel

Stainless steel can work-harden and create high tapping torque. A slightly larger drill may reduce friction, galling, and tap breakage while still providing adequate thread strength.

Sharp tools, rigid alignment, suitable cutting fluid, and controlled tapping speed are particularly important.

Aluminium

Aluminium is relatively soft, so adequate engagement length is important for stripping resistance. However, an excessively small tap drill can cause material pickup or loading on the tap.

A standard or slightly larger drill may be appropriate depending on the aluminium alloy, tap design, and required strength.

Brass and bronze

Free-machining brass is generally easy to tap, but tap geometry must prevent grabbing. Engagement requirements vary with alloy strength and whether the tapped hole will be repeatedly assembled.

Cast iron

Cast iron is commonly tapped dry or with application-specific lubrication. Its brittleness and chip formation should be considered when selecting the tap style and engagement percentage.

Plastics

Plastic materials may deform, recover elastically, or creep after tapping. Drill size, tap geometry, engagement length, and assembly torque should be established through material-specific guidance or testing.

Cutting taps versus forming taps

The tap-drill chart above primarily applies to cutting taps. A cutting tap removes material to create the thread form.

Common cutting-tap styles include:

Taper taps
Plug taps
Bottoming taps
Spiral-point taps
Spiral-flute taps

A forming tap, also called a roll tap, displaces material instead of cutting it. Forming taps require a larger pre-drilled hole because material flows inward to create the thread crests.

Using a cutting-tap drill size with a forming tap can cause:

Extremely high torque
Tap breakage
Oversized or malformed threads
Workpiece cracking
Machine overload

The drill size specified by the forming-tap manufacturer should therefore be used. Forming taps are generally best suited to ductile materials and are not appropriate for brittle materials such as grey cast iron.

Through holes and blind holes

The correct tap style also depends on whether the hole passes completely through the workpiece.

For a through hole, a spiral-point or gun tap pushes chips forward and out of the hole. This reduces chip accumulation in the flutes.

For a blind hole, a spiral-flute tap pulls chips upward and away from the bottom. The drilled hole must be deeper than the required full-thread depth to provide space for:

Drill-point length
Tap chamfer
Chip accumulation
Bottom clearance

Thread depth and drill depth are not the same. A drawing should clearly distinguish between:

Total drilled-hole depth
Tapped depth
Minimum full-thread depth
Usable thread engagement length

Thread engagement length

Thread engagement length is the axial length over which the internal and external threads contact each other. It should not be confused with the percentage of thread height created by the tap drill.

A high percentage of thread height cannot compensate for inadequate engagement length. The required engagement depends on:

Fastener tensile strength
Internal-thread material strength
Nominal diameter
Thread pitch
Applied tensile and shear loads
Safety factor
Repeated assembly requirements

A tapped hole in aluminium, plastic, or cast iron may require a greater engagement length than a tapped hole in high-strength steel.

For critical connections, tap-drill selection and engagement length should be verified by engineering calculation rather than relying exclusively on general rules of thumb.

9. How to Select the Correct UNC Thread Size

Selecting a UNC thread requires more than choosing a diameter that fits within the available space. The fastener and engaged threads must safely carry the applied load without tensile fracture, shear failure, thread stripping, fatigue failure, excessive deformation, or loosening.

The selection process should consider:

Type and magnitude of applied load
Fastener diameter and tensile-stress area
Fastener material and strength grade
Strength of the internally threaded material
Required engagement length
Available wall thickness
Joint preload and tightening method
Vibration and fatigue
Temperature and corrosion
Coatings and surface treatments
Availability of compatible tools and fasteners

Determine the applied load

Begin by identifying the forces acting on the connection. A threaded joint may experience:

Axial tensile load
Transverse shear load
Combined tensile and shear loading
Static loading
Cyclic or fatigue loading
Impact or shock loading
Thermal expansion and contraction
Vibration

The external service load is not necessarily equal to the load carried by the bolt. A properly preloaded joint behaves as a system involving the fastener and the clamped components.

For safety-critical assemblies, the required fastener size should be established by engineering calculation using the applicable design code and safety factor.

Select the fastener diameter

Increasing the nominal diameter generally increases:

Tensile-stress area
Shear area
Thread-stripping resistance
Available clamp load
Resistance to bending
Required installation torque

The bolt should be sized using its tensile-stress area rather than its nominal circular area because the threads reduce the effective cross-section.

The approximate tensile-stress area of a Unified thread can be calculated as:

$A_t=\frac{\pi}{4} \left(D-\frac{0.9743}{n}\right)^2$ $A_{t} = 4 π (D - n 0.9743)^{2}$

where:

$A_{t}$

= tensile-stress area in square inches
$D$

= nominal major diameter in inches
$n$

= threads per inch

For a 1/2-13 UNC thread:

$A_{t} = \frac{π}{4} {(0.500 - \frac{0.9743}{13})}^{2}$

A_t\approx0.1419\text{ in}^2

This area can be used with the allowable tensile stress or proof strength specified for the selected fastener material and grade.

Select the fastener material and grade

Two fasteners with the same UNC size can have significantly different load capacities because of differences in material and heat treatment.

Common fastener specifications include:

SAE Grade 2, Grade 5, and Grade 8
ASTM A307
ASTM A325 or ASTM F3125 structural fasteners
ASTM A193 alloy and stainless steel bolting
ASTM F593 stainless steel fasteners
Proprietary high-strength fasteners

The selected material should be compatible with:

Mechanical loading
Operating temperature
Corrosive environment
Clamped-component materials
Required service life
Applicable project specification

The nut or tapped material must also be strong enough to develop the required fastener load without stripping.

Evaluate thread engagement length

Thread engagement length is the axial distance over which the external and internal threads are in contact.

If the internal material is weaker than the bolt, a longer engagement length is generally required. Typical preliminary guidance is:

Internal-thread material	Preliminary engagement guidance
High-strength steel	Approximately 0.8D–1.0D
Carbon steel	Approximately 1.0D
Cast iron	Approximately 1.0D–1.5D
Brass or bronze	Approximately 1.0D–1.5D
Aluminium alloy	Approximately 1.5D–2.0D
Engineering plastic	Approximately 2.0D–3.0D or inserts

Here, $D$ is the nominal thread diameter.

These values are only preliminary rules of thumb. Actual engagement must account for the shear strength of the internal and external threads, tolerance class, thread form, loading, and safety factor.

A connection can fail by thread stripping even when the bolt itself is strong enough. Conversely, excessive engagement does not always provide a useful increase in strength because load is not distributed equally across every engaged thread.

Consider parent-material strength

The correct UNC thread size depends heavily on the material containing the internal thread.

A high-strength steel bolt installed in a soft aluminium component may strip the aluminium threads before reaching the bolt’s intended preload. Possible solutions include:

Increasing the nominal thread diameter
Increasing engagement length
Using a lower-strength fastener
Installing a steel threaded insert
Using a through bolt and nut
Increasing the local wall thickness
Reducing assembly torque where permitted

Threaded inserts can provide stronger and more wear-resistant internal threads in aluminium, magnesium, plastics, and repaired components.

Check edge distance and wall thickness

A tapped hole must have enough surrounding material to resist cracking, breakout, and local deformation.

Check:

Distance from the hole centre to the component edge
Material thickness below a blind hole
Wall thickness around the tapped hole
Distance between adjacent threaded holes
Proximity to welds and geometric discontinuities
Local casting or machining defects

A thread that is strong in pure axial loading may still fail if installed too close to a free edge.

Account for preload and tightening torque

Most bolted joints rely on preload to keep the components clamped together. Preload is generated by tightening torque or another controlled installation method.

The approximate relationship is:

$T = K F d$

where:

$T$

= tightening torque
$K$

= nut factor
$F$

= desired preload
$d$

= nominal fastener diameter

This equation is only an approximation because friction under the head, beneath the nut, and within the threads strongly affects the achieved preload.

Lubrication, plating, anti-seize compound, surface finish, and reuse can substantially change the torque–preload relationship. A torque value intended for a dry fastener should not automatically be used on a lubricated or coated fastener.

Consider vibration and loosening

UNC threads are robust and resistant to damage, but thread pitch alone does not prevent loosening.

For vibration-prone assemblies, consider:

Adequate preload
Prevailing-torque nuts
Locking adhesives
Wedge-locking washers
Safety wire where specified
Mechanical locking plates
Double-nut arrangements
Regular inspection

The locking method must be compatible with temperature, chemicals, disassembly requirements, and the applicable standard.

Consider fatigue loading

In cyclic service, fastener fatigue is often controlled by fluctuating stress rather than maximum static load.

Fatigue performance can be improved by:

Maintaining sufficient preload
Avoiding joint separation
Reducing load eccentricity
Using rolled rather than poorly cut threads
Keeping the thread runout away from high-stress planes
Using generous transitions beneath the fastener head
Avoiding corrosion and surface damage
Selecting an appropriate fastener grade

A larger or stronger bolt does not automatically solve fatigue problems if the joint remains poorly designed or improperly preloaded.

Consider corrosion and temperature

Environmental exposure may affect both fastener strength and thread function.

Evaluate:

Atmospheric corrosion
Saltwater exposure
Chemical compatibility
Galvanic corrosion
Hydrogen embrittlement
Oxidation at high temperature
Loss of strength at elevated temperature
Low-temperature toughness
Thermal expansion differences
Galling of stainless steel threads

Stainless steel fasteners may gall during tightening, especially when mating threads are of similar material. Appropriate lubrication, material pairing, tightening speed, and surface treatment may be required.

Confirm availability and maintainability

A technically acceptable thread may still be impractical if compatible fasteners, taps, dies, gauges, and replacement components are difficult to obtain.

Standard UNC sizes such as 1/4-20, 3/8-16, 1/2-13, 5/8-11, and 3/4-10 are widely available. Special diameter–pitch combinations may increase procurement cost and maintenance difficulty.

The final selection should therefore balance strength, manufacturability, availability, inspection, and long-term service requirements.

10. UNC Thread Standards, FAQs, and Conclusion

Standards applicable to UNC threads

The primary standards associated with UNC thread geometry, tolerances, and inspection include the following.

ASME B1.1 — Unified Inch Screw Threads

ASME B1.1 is the principal American standard for Unified inch screw threads. It specifies:

Basic thread form
Standard thread series
Diameter–pitch combinations
Thread designations
Tolerance classes
Allowances and tolerances
Dimensional limits
UN, UNR, and UNJ thread forms within its stated scope

UNC is one of the standard thread series controlled through this standard. ASME B1.1

ASME B1.2 — Gages and Gaging for Unified Inch Screw Threads

ASME B1.2 provides requirements for gauges and measuring equipment used to inspect Unified inch threads. It addresses the specifications, dimensions, and basic use of thread gauges, including functional inspection methods. ASME B1.2

ISO 68-2 — ISO General Purpose Screw Threads

ISO 68-2:2023 specifies the basic and design profiles for ISO inch screw threads using UN and UNR forms. The 2023 edition replaced the withdrawn 1998 edition. ISO 68-2:2023

ISO 725 — ISO Inch Screw Threads: Basic Dimensions

ISO 725 specifies basic dimensions for ISO inch screw threads. It may be referenced where Unified inch-thread requirements are controlled through ISO documentation.

ISO 5864 — ISO Inch Screw Threads: Allowances and Tolerances

ISO 5864 specifies allowances and tolerances for standard inch-thread series. Its stated scope covers nominal diameters from 0.06 to 6 inches and pitches from 80 to 4 TPI. ISO 5864:1993

The applicable standard and edition should always be confirmed from the engineering drawing, customer specification, contract, or governing regulatory document. A newer publication does not automatically replace the edition contractually specified for an existing project.

Frequently asked questions

What does UNC stand for?

UNC stands for Unified National Coarse. It is a standard inch-based thread series with a 60-degree thread angle and relatively coarse pitches.

What does 1/4-20 UNC mean?

The designation means:

1/4 — nominal major diameter of 0.250 inch
20 — 20 threads per inch
UNC — Unified National Coarse thread series

If no class is shown, the engineering documentation or product specification should be checked for the required tolerance class.

What does 1/2-13 UNC-2A mean?

It identifies a thread with:

1/2-inch nominal diameter
13 threads per inch
UNC thread series
Class 2 commercial fit
External thread, indicated by A

The corresponding general-purpose internal thread is normally designated 1/2-13 UNC-2B.

What is the most common UNC thread class?

Class 2A is the most common general-purpose external thread, while Class 2B is the corresponding internal thread. This pairing is widely used for commercial bolts, screws, nuts, and tapped holes.

Are UNC and SAE threads the same?

Not exactly. SAE is an organization and a broad system of fastener grades and engineering standards, not a single thread series. UNC and UNF threads are frequently used on fasteners described informally as SAE fasteners, but the thread designation and material grade are separate characteristics.

For example, a bolt may have a 1/2-13 UNC thread and an SAE Grade 8 strength classification.

Are UNC and Whitworth threads interchangeable?

No. UNC threads use a 60-degree thread angle, while British Standard Whitworth threads use a 55-degree profile with different crest and root geometry. Some sizes may have similar diameters and TPI, but they should not be treated as interchangeable.

Are UNC and metric coarse threads interchangeable?

No. UNC threads are specified by nominal inch diameter and TPI, while metric threads are specified by nominal diameter and pitch in millimetres.

Even when their measured dimensions appear close, differences in diameter and pitch prevent correct flank contact.

How is UNC pitch calculated?

Pitch in inches is calculated from TPI:

$P = \frac{1}{T P I}$

Pitch in millimetres is:

$P_{m m} = \frac{25.4}{T P I}$

For 20 TPI:

$P=\frac{1}{20}=0.0500\text{ in}$

$P_{\mathrm{mm}}=\frac{25.4}{20}=1.270\text{ mm}$

What tap drill is used for a 1/4-20 UNC thread?

A No. 7 drill, measuring 0.2010 inch or approximately 5.105 mm, is a common cutting-tap drill for a 1/4-20 UNC thread.

The optimum drill may vary with material, tap type, thread engagement requirement, and machining conditions.

What tap drill is used for a 3/8-16 UNC thread?

A 5/16-inch drill, measuring 0.3125 inch or 7.9375 mm, is a common selection for a 3/8-16 UNC cutting tap.

What tap drill is used for a 1/2-13 UNC thread?

A 27/64-inch drill, measuring 0.4219 inch or approximately 10.72 mm, is commonly used.

Can UNC and UNF fasteners be assembled together?

No. They may have the same nominal diameter but different thread pitches.

For example:

1/2-13 UNC
1/2-20 UNF

Forcing them together will damage the threads and will not produce a safe joint.

Is UNC better than UNF?

Neither series is universally better.

UNC is generally more suitable for:

Rapid assembly
Soft parent materials
Dirty environments
Field maintenance
Resistance to minor thread damage

UNF may be more suitable for:

Fine adjustment
Short engagement lengths
Larger tensile-stress area
Thin-wall applications
Precision assemblies

The correct choice depends on the joint design and service conditions.

Are UNC threads suitable for stainless steel?

Yes, UNC threads are widely used on stainless steel fasteners. However, stainless steel threads may gall during installation.

Galling risk can be reduced through:

Suitable thread lubricant
Controlled installation speed
Compatible material combinations
Clean, undamaged threads
Appropriate surface treatments
Correct tightening procedure

Lubrication changes the relationship between torque and preload, so torque values must be adjusted or obtained from an appropriate specification.

How can a UNC thread be identified?

Measure the major diameter and use a thread-pitch gauge to determine the TPI. Compare both values with the UNC thread chart.

For production or precision verification, use a calibrated plug gauge for internal threads or ring gauge for external threads.

Is the tap-drill diameter the same as the minor diameter?

Not necessarily. The tap drill creates the initial hole, while the finished internal minor diameter depends on the tapping process, material response, tool geometry, and tolerance class.

Tap-drill sizes are selected to produce a practical percentage of thread engagement rather than to reproduce the basic minor diameter exactly.

Why does an external thread measure smaller than its nominal size?

External UNC threads commonly include allowance and manufacturing tolerance. As a result, the measured major diameter may be slightly smaller than the nominal diameter.

Wear, plating removal, and measurement across damaged crests may reduce the measured value further.

Where can exact UNC tolerance limits be found?

Exact maximum and minimum dimensions should be obtained from the applicable edition of ASME B1.1 or another standard specified by the drawing or contract.

A general UNC chart showing basic dimensions cannot replace the class-specific limits required for manufacturing and inspection.

Conclusion

UNC threads provide a durable and practical fastening system for general machinery, construction equipment, automotive assemblies, agricultural machinery, and industrial maintenance. Their coarse pitch promotes rapid assembly, reduces the likelihood of cross-threading, and performs well in softer materials and demanding field conditions.

A UNC designation must be read as a complete system. For example, 1/2-13 UNC-2A identifies the nominal diameter, TPI, thread series, tolerance class, and external-thread condition. Diameter alone is not sufficient because the same nominal size may be produced with UNC, UNF, or special pitches.

The UNC thread size chart can be used to determine:

Nominal major diameter
Threads per inch
Thread pitch
Basic pitch diameter
Basic internal and external minor diameters
Common cutting-tap drill sizes

However, basic dimensions are not final inspection limits. Manufacturing and acceptance must consider the specified thread class, allowance, tolerance, coating thickness, material, and applicable standard.

Before machining or assembling a UNC thread, confirm the complete designation, choose an appropriate tap-drill size, provide sufficient engagement length, and verify the thread using suitable measuring equipment. Critical, pressure-retaining, structural, lifting, or fatigue-loaded connections should always be supported by engineering calculations and the requirements of the governing design standard.

Metric Thread Size Chart: Dimensions, Pitch & Tap Drill Sizes

JIC Fitting Size Chart: Thread Dimensions, Dash Sizes & Hydraulic Fittings Guide