UN Thread Size Chart: UNC, UNF, UNEF Dimensions

5. UN Thread Classes: 1A, 2A, 3A, 1B, 2B, and 3B

UN thread classes define the amount of dimensional tolerance and clearance between mating internal and external threads. Selecting the correct class is important because it affects assembly ease, thread fit, interchangeability, manufacturing cost, and positional accuracy.

A complete thread designation includes a class number followed by the letter A or B:

• A: External thread, such as a bolt or screw
• B: Internal thread, such as a nut or tapped hole

For example:

• 1/2-13 UNC-2A: External Class 2 thread
• 1/2-13 UNC-2B: Internal Class 2 thread

When the external and internal parts are assembled, a Class 2A bolt is normally paired with a Class 2B nut or tapped hole.

Class 1A and 1B Threads

Class 1 threads provide the loosest fit and the largest amount of assembly clearance. They are used where rapid assembly and easy disassembly are more important than precise positioning.

Typical characteristics include:

• Generous dimensional tolerance
• Easy manual assembly
• Better tolerance of dirt and minor thread damage
• Reduced risk of interference
• Lower positional accuracy
• Noticeable clearance between mating threads

Typical applications include:

• Temporary assemblies
• Adjustment mechanisms
• Low-precision equipment
• Components exposed to dirt or corrosion
• Assemblies that require frequent removal

Class 1 threads are less common than Class 2 threads in modern general-purpose manufacturing.

Class 2A and 2B Threads

Class 2 threads provide a medium fit and are the most commonly used UN thread classes. They balance manufacturing economy, interchangeability, strength, and assembly performance.

Typical characteristics include:

• Standard commercial fit
• Reliable interchangeability
• Moderate manufacturing tolerance
• Easy assembly without excessive looseness
• Suitable for mass-produced fasteners
• Widely available gauges and tooling

Typical applications include:

• Commercial bolts and nuts
• Machinery and industrial equipment
• Automotive assemblies
• Structural connections
• Hydraulic and pneumatic equipment
• General maintenance fasteners

A designation such as 3/8-16 UNC-2A identifies a standard external coarse thread. The matching internal thread is normally specified as 3/8-16 UNC-2B.

Class 2A external threads normally include an allowance that creates intentional clearance between mating components. Class 2B internal threads generally use the basic profile as their maximum-material reference, with tolerance extending toward additional clearance.

Class 3A and 3B Threads

Class 3 threads provide a close and accurate fit. Their dimensional tolerances are tighter than those of Classes 1 and 2.

Typical characteristics include:

• Small clearance between mating threads
• High positional accuracy
• Tight dimensional tolerance
• Higher manufacturing and inspection cost
• Greater sensitivity to dirt, damage, and plating thickness
• Careful alignment required during assembly

Typical applications include:

• Aerospace fasteners
• Precision instruments
• High-performance machinery
• Critical mechanical assemblies
• Applications requiring accurate alignment
• Components subjected to demanding fatigue conditions

Class 3A external threads typically have no intentional allowance at maximum material condition. Consequently, plating, coatings, dimensional variation, or contamination can interfere with assembly if they are not properly considered.

Comparison of UN Thread Classes

The thread class does not change the nominal diameter or TPI. A 1/2-13 UNC thread remains a 1/2-inch, 13-TPI thread whether it is Class 1, 2, or 3. The class changes its dimensional limits and fit.

Allowance and Tolerance

Although allowance and tolerance are related, they have different meanings.

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

Tolerance is the permitted dimensional variation during manufacturing. It defines the acceptable range between the maximum and minimum sizes.

External Class 1A and 2A threads normally have an allowance. Class 3A threads generally have no allowance, which creates a closer fit but demands tighter production control.

Coated and Plated Threads

Surface treatments can increase the dimensions of external threads and reduce the available space inside internal threads. Zinc plating, nickel plating, paint, dry-film lubricant, and other coatings must therefore be considered when selecting thread limits.

If an external thread is manufactured to its maximum permissible pre-plating size, adding a coating may cause it to exceed the final allowable dimension. The machining or rolling dimensions may need to be adjusted so the finished coated thread remains within specification.

For coated threads, engineers should define whether the specified dimensions apply:

• Before coating
• After coating
• Before and after coating
• At maximum or minimum coating thickness

Selecting the Correct Thread Class

Class 2A and 2B are appropriate for most general industrial applications. Class 3 should be selected only when tighter fit and accuracy provide a functional benefit, because it increases manufacturing and inspection requirements.

Selection should consider:

• Required assembly clearance
• Manufacturing method
• Expected contamination
• Frequency of assembly
• Surface treatment thickness
• Temperature variation
• Alignment requirements
• Inspection capabilities
• Industry standards

A tighter thread class does not automatically make the joint stronger. Joint strength depends on fastener material, tensile stress area, engagement length, preload, joint geometry, and installation method.

6. UN Thread Drill Size Chart

A tap drill creates the initial hole into which an internal thread is cut or formed. The correct drill size must provide enough material for strong thread engagement while avoiding excessive tapping torque.

If the hole is too small:

• Tapping torque increases.
• The tap may break.
• The thread may tear or gall.
• Tool life is reduced.
• Dimensional accuracy may deteriorate.

If the hole is too large:

• Thread engagement decreases.
• The internal thread may strip under load.
• The finished thread may fail inspection.
• Joint strength may be reduced.

The following chart lists commonly used drill sizes for cutting standard Class 2B internal threads. Exact drill selection may vary with material, required percentage of thread, tap type, hole depth, and applicable standard.

Numbered Screw Tap Drill Sizes

Fractional UNC Tap Drill Sizes

Fractional UNF Tap Drill Sizes

Common UNEF Tap Drill Sizes

Estimating a Tap Drill Size

A simple approximation for a cutting tap is:

Tap drill diameter=D−TPI1​

Where:

• $D$ = nominal major diameter in inches
• $TPI$ = threads per inch

For a 1/2-13 UNC thread:

0.5000−131​=0.4231 in

The nearest commonly used drill is 27/64 inch:

6427​=0.4219 in

This formula provides only a practical estimate. Production applications should use the drill size recommended for the required thread percentage, workpiece material, and tap manufacturer.

Cutting Taps vs. Forming Taps

The drill sizes above are primarily intended for conventional cutting taps. A forming tap displaces material instead of removing it and therefore requires a larger pilot hole.

Using a cutting-tap drill size with a forming tap can produce excessive torque, tap breakage, or an oversized thread crest. Always follow the forming-tap manufacturer’s recommended hole size.

7. How to Read a UN Thread Designation

A UN thread designation communicates the size, pitch, thread series, tolerance class, and whether the thread is internal or external. Additional information may define the thread direction, engagement length, coating, or special inspection requirements.

A typical designation is:

1/2-13 UNC-2A

This callout can be divided into four principal elements:

The corresponding internal thread would be:

1/2-13 UNC-2B

Nominal Diameter

The first value represents the nominal major diameter. Fractional sizes are used for threads 1/4 inch and larger.

Examples include:

• 1/4-20 UNC
• 3/8-24 UNF
• 1/2-28 UNEF
• 1-8 UNC

For small machine screws, the first value is a number:

• #4-40 UNC
• #6-32 UNC
• #8-36 UNF
• #10-32 UNF

A #10 thread, for example, has a nominal major diameter of 0.190 inch.

Threads per Inch

The number following the nominal size identifies TPI. It determines the pitch of the thread.

For example:

• 3/8-16 has a pitch of 1/16 inch.
• 3/8-24 has a pitch of 1/24 inch.
• 3/8-32 has a pitch of 1/32 inch.

Although all three threads have the same nominal diameter, they cannot mate because their pitches are different.

Thread Series

The letters after the TPI identify the thread series:

• UNC: Unified National Coarse
• UNF: Unified National Fine
• UNEF: Unified National Extra Fine
• UNS: Unified National Special
• UN: Unified constant-pitch or general UN-series designation

Examples include:

• 5/16-18 UNC-2A
• 5/16-24 UNF-2A
• 5/16-32 UNEF-2A
• 5/16-28 UNS-2A

The thread series should not be assumed from diameter alone. Both the diameter and TPI must be checked.

Tolerance Class and Thread Type

The class number defines the fit and tolerance, while the letter identifies the thread location.

• 1A: Loose-fit external thread
• 1B: Loose-fit internal thread
• 2A: Standard external thread
• 2B: Standard internal thread
• 3A: Close-fit external thread
• 3B: Close-fit internal thread

For general commercial equipment, Classes 2A and 2B are the most common.

Example: 1/4-20 UNC-2A

The designation 1/4-20 UNC-2A means:

• Nominal diameter: 1/4 inch
• Basic major diameter: 0.2500 inch
• TPI: 20
• Pitch: 0.0500 inch
• Series: Unified National Coarse
• Class: 2
• Thread type: External

A mating nut or tapped hole would typically be specified as 1/4-20 UNC-2B.

Example: 3/8-24 UNF-2B

The designation 3/8-24 UNF-2B means:

• Nominal diameter: 3/8 inch
• Basic major diameter: 0.3750 inch
• TPI: 24
• Pitch: approximately 0.0417 inch
• Series: Unified National Fine
• Class: 2
• Thread type: Internal

Example: 1/2-28 UNEF-3A

The designation 1/2-28 UNEF-3A means:

• Nominal diameter: 1/2 inch
• TPI: 28
• Pitch: approximately 0.0357 inch
• Series: Unified National Extra Fine
• Class: 3
• Thread type: External

This callout identifies a precision external thread with a relatively small pitch.

Reading a Constant-Pitch UN Designation

A thread such as 2-8 UN-2A has:

• A 2-inch nominal diameter
• Eight threads per inch
• A Unified constant-pitch thread form
• Class 2 fit
• An external thread

The designation differs from 1-8 UNC-2A, even though both have eight threads per inch. The nominal diameters are different, and one belongs to a coarse series while the other uses a general constant-pitch UN designation.

Left-Hand Thread Designations

Standard threads are right-hand threads unless otherwise stated. A right-hand fastener tightens when turned clockwise.

A left-hand thread must be identified by the letters LH:

1/2-20 UNF-2A-LH

This designation indicates:

• 1/2-inch nominal diameter
• 20 TPI
• UNF series
• Class 2 external thread
• Left-hand thread direction

Left-hand threads are used where normal rotation could loosen a right-hand thread or where opposing adjustment is required.

Multiple-Start Thread Designations

Most standard fasteners use single-start threads. If a thread has two or more starts, the designation must provide enough information to identify both the pitch and lead.

For example, a two-start thread advances twice the pitch during one complete revolution:

$$L=2P$$

Multiple-start UN threads are specialized and should be fully defined on the engineering drawing to prevent confusion.

Thread Depth or Engagement Length

A drawing may add a depth after an internal thread designation:

3/8-16 UNC-2B × 0.75 DEEP

This means the component requires a 3/8-16 UNC Class 2B internal thread with a specified threaded depth of 0.75 inch.

For a blind hole, the drawing may need to distinguish between:

• Full thread depth
• Drilled hole depth
• Usable thread length
• Tap runout allowance

The drilled hole must normally extend beyond the required full thread depth to provide space for the tap point, chips, and incomplete threads.

Common Thread Callout Errors

Frequent specification mistakes include:

• Omitting the TPI
• Using only the nominal diameter
• Confusing UNC with UNF
• Omitting the internal or external designation
• Specifying an unavailable diameter-and-pitch combination
• Failing to identify a left-hand thread
• Treating a basic dimension as a production tolerance
• Failing to account for plating or coating
• Confusing UN threads with similarly sized metric or BSP threads

A complete and unambiguous designation should include at least the nominal size, TPI, series, class, and internal or external identification.

7. How to Read a UN Thread Designation

A UN thread designation provides the information needed to identify and manufacture a thread correctly. A standard callout normally includes the nominal diameter, threads per inch, thread series, tolerance class, and whether the thread is external or internal.

A typical designation is:

1/2-13 UNC-2A

The corresponding internal thread is normally designated:

1/2-13 UNC-2B

Nominal Thread Diameter

The first part of the designation indicates the nominal major diameter. For threads 1/4 inch and larger, the diameter is usually expressed as a fraction or whole number.

Examples include:

• 1/4-20 UNC
• 3/8-24 UNF
• 1/2-28 UNEF
• 1-8 UNC
• 1 1/2-12 UN

For machine screws smaller than 1/4 inch, numbered sizes are used:

• #4-40 UNC
• #6-32 UNC
• #8-36 UNF
• #10-32 UNF

The nominal diameter of a numbered screw can be estimated using:

$$D=0.060+0.013N$$

Where $N$ is the screw number. For example, a #10 screw has a nominal diameter of:

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

Threads per Inch

The second value indicates the number of threads per inch. It defines how closely the threads are spaced.

For example:

• 3/8-16 UNC has 16 TPI.
• 3/8-24 UNF has 24 TPI.
• 3/8-32 UNEF has 32 TPI.

Thread pitch can be calculated from TPI:

P=TPI1​

For a 3/8-24 UNF thread:

P=241​=0.0417 in

Although the three examples have the same nominal diameter, they cannot mate because their thread pitches are different.

Thread Series

The letters following the TPI identify the Unified thread series:

• UNC: Unified National Coarse
• UNF: Unified National Fine
• UNEF: Unified National Extra Fine
• UNS: Unified National Special
• UN: General Unified or constant-pitch thread series

For example:

• 5/16-18 UNC: Coarse thread
• 5/16-24 UNF: Fine thread
• 5/16-32 UNEF: Extra-fine thread
• 5/16-28 UNS: Special diameter-and-pitch combination

A thread series should never be identified from the diameter alone. The nominal diameter and TPI must both be measured.

Thread Tolerance Class

The number following the thread series identifies the tolerance class:

• Class 1: Loose fit
• Class 2: Standard commercial fit
• Class 3: Close precision fit

Class 2 is the most frequently used choice for general industrial fasteners. Class 3 is used when closer control and more accurate positioning are required.

External and Internal Thread Identification

The final letter indicates whether the thread is external or internal:

• A: External thread
• B: Internal thread

External threads are found on bolts, screws, studs, and externally threaded shafts. Internal threads are found in nuts, tapped holes, and internally threaded components.

A typical mating pair is:

• External: 3/8-16 UNC-2A
• Internal: 3/8-16 UNC-2B

The external and internal components must have the same nominal diameter, TPI, thread series, and compatible tolerance classes.

Example: 1/4-20 UNC-2A

This designation means:

The mating internal thread would normally be specified as 1/4-20 UNC-2B.

Example: 1/2-20 UNF-2B

This designation means:

• Nominal diameter: 1/2 inch
• Basic major diameter: 0.5000 inch
• Threads per inch: 20
• Pitch: 0.0500 inch
• Thread series: UNF
• Tolerance class: Class 2
• Thread type: Internal

Notice that a 1/2-20 UNF thread has the same pitch as a 1/4-20 UNC thread. However, they cannot mate because their nominal diameters are different.

Example: 1/2-28 UNEF-3A

This designation identifies:

• A 1/2-inch nominal diameter
• 28 threads per inch
• Unified National Extra Fine series
• Class 3 precision fit
• An external thread

The corresponding internal component would generally use the designation 1/2-28 UNEF-3B.

Reading a Constant-Pitch UN Thread

Large-diameter threads may use a constant-pitch series. For example:

2-8 UN-2A

This designation indicates:

• 2-inch nominal diameter
• Eight threads per inch
• Unified constant-pitch thread series
• Class 2 external thread

Other constant-pitch series include 4-UN, 6-UN, 8-UN, 12-UN, 16-UN, 20-UN, 28-UN, and 32-UN.

Left-Hand Thread Designation

UN threads are assumed to be right-hand unless otherwise specified. A left-hand thread is identified by the suffix LH.

For example:

1/2-20 UNF-2A-LH

This identifies a 1/2-inch, 20-TPI, Class 2 external UNF left-hand thread.

Left-hand threads are used where normal equipment rotation could loosen a right-hand connection or where two opposing threads provide directional adjustment.

Thread Depth on Engineering Drawings

A blind tapped hole may be designated as:

3/8-16 UNC-2B × 0.75 DEEP

This specifies a Class 2B internal thread with a required thread depth of 0.75 inch.

The drawing should distinguish between:

• Full thread depth
• Usable thread length
• Drilled hole depth
• Tap runout
• Bottom clearance

The drilled depth is normally greater than the required full thread depth because the end of a standard tap cannot produce complete threads all the way to the bottom of a blind hole.

Common UN Thread Callout Mistakes

Common mistakes include:

• Omitting the TPI
• Omitting the thread class
• Failing to identify an internal or external thread
• Confusing UNC and UNF
• Specifying an unavailable thread combination
• Failing to identify a left-hand thread
• Ignoring plating or coating thickness
• Confusing nominal diameter with measured outside diameter

A complete thread callout prevents manufacturing errors and ensures that mating components are interchangeable.

8. UN Threads vs. Metric and Other Inch Thread Standards

UN threads can appear similar to metric, BSP, Whitworth, and other inch-based thread forms. However, threads with similar outside diameters are not necessarily compatible. Differences in pitch, thread angle, crest shape, root geometry, and tolerance system can prevent correct assembly or create an unsafe connection.

UN Threads vs. Metric Threads

UN and ISO metric threads both use a symmetrical 60-degree thread profile. Their main difference is the measurement system used to define diameter and pitch.

UN threads use:

• Nominal diameter in inches
• Threads per inch
• UNC, UNF, UNEF, or UNS series
• Classes such as 2A and 2B

Metric threads use:

• Nominal diameter in millimetres
• Pitch measured directly in millimetres
• Coarse or fine pitch series
• Tolerance classes such as 6g and 6H

For example:

• 1/2-13 UNC-2A: 1/2-inch diameter with 13 TPI
• M12 × 1.75-6g: 12 mm diameter with a 1.75 mm pitch

A 1/2-inch thread has a basic major diameter of 12.700 mm, while an M12 thread has a basic major diameter of 12.000 mm. Although they may look similar, they are not interchangeable.

Because both standards use a 60-degree profile, some combinations may begin to engage. This does not make them compatible. Forcing them together can damage the threads and significantly reduce joint strength.

UN Threads vs. British Standard Whitworth Threads

British Standard Whitworth threads use a 55-degree thread angle and rounded crests and roots. UN threads use a 60-degree profile with different truncation and root geometry.

UNC and BSW threads sometimes have similar nominal sizes and TPI values. For example, 1/4-20 UNC and 1/4-20 BSW share the same nominal size and TPI. However, their thread angles and profiles are different.

They may appear to assemble in loose or worn components, but they should not be treated as interchangeable in a properly engineered connection.

UN Threads vs. BSP Threads

BSP stands for British Standard Pipe. Unlike standard UN fastener threads, BSP threads are intended primarily for pipe and fluid connections.

The two main BSP types are:

• BSPP: British Standard Pipe Parallel
• BSPT: British Standard Pipe Taper

BSP threads use a 55-degree Whitworth thread profile. UN threads use a 60-degree profile.

Another major difference is nominal sizing. A BSP thread designation refers to the nominal bore of the associated pipe rather than directly representing the measured outside diameter.

For example, a 1/2-inch BSP male thread has an outside diameter of approximately 0.825 inch, not 0.500 inch.

A UN bolt thread should not be used as a replacement for a BSP connection, even if the measured diameter appears similar.

UN Threads vs. NPT Threads

NPT stands for National Pipe Taper. Both NPT and UN threads use a 60-degree thread angle, but they serve different purposes and have different geometries.

NPT threads are tapered at a rate of 1 in 16 on diameter. Standard UN threads are parallel. NPT sizes are also based on nominal pipe size rather than actual outside diameter.

For example, a 1/2-14 NPT male thread has an outside diameter much larger than 0.500 inch. It must not be confused with a 1/2-inch UN fastener thread.

UN Threads vs. SAE Straight Threads

The term “SAE thread” can be ambiguous. In some industries, it informally refers to UNF fastener threads. In hydraulic systems, however, an SAE straight thread commonly refers to an SAE O-ring boss, or SAE ORB, connection.

An SAE ORB fitting normally uses a straight UN or UNF thread, but the thread itself does not create the pressure seal. Sealing is provided by an elastomeric O-ring compressed against a machined port surface.

For example, an SAE ORB connection may use a 3/4-16 UNF thread. A normal 3/4-16 bolt could share the nominal thread geometry, but it does not necessarily have the required fitting shoulder, O-ring groove, port dimensions, material, or pressure rating.

Thread compatibility alone does not establish functional interchangeability.

UN Threads vs. JIC Threads

JIC fittings typically use straight UNF threads and a 37-degree flare sealing surface. The threads draw the mating flare surfaces together but do not directly provide the fluid seal.

A JIC fitting is identified by several combined features:

• Straight UNF thread
• 37-degree flare seat
• Tube or hose dash size
• Correct male and female sealing surfaces

A component with a matching UNF thread but no 37-degree sealing seat is not a functional JIC connection.

UN Threads vs. UNJ Threads

UNJ threads are based on the Unified thread system but use a controlled larger root radius on the external thread. This design reduces stress concentration and improves fatigue resistance.

UNJ threads are widely used in aerospace and other high-fatigue applications.

A UNJ external thread may require a larger minor diameter and a specially controlled internal thread form to provide clearance for its root radius. UN and UNJ components should not be assumed interchangeable without checking the applicable specifications.

UN Threads vs. ACME Threads

ACME threads are power-transmission threads rather than general-purpose fastening threads. They use a 29-degree included angle and have broad, flat crests and roots.

ACME threads are better suited to transmitting axial motion and high loads in lead screws. They are not interchangeable with UN threads.

Comparison of Common Thread Standards

Why Similar Threads Must Not Be Forced Together

Two threads may appear to fit during the first few turns but still be incompatible. Forcing mismatched threads together can cause:

• Cross-threading
• Flank deformation
• Reduced engagement
• Inaccurate preload
• Leakage in fluid systems
• Thread stripping
• Fatigue failure
• Difficulty during future disassembly

Correct identification requires checking at least:

• Outside diameter
• Threads per inch or metric pitch
• Thread angle
• Parallel or tapered geometry
• Thread series
• Sealing surface
• Tolerance class

A thread gauge and caliper can provide an initial identification, but critical threads should be verified using the applicable dimensional standard, plug gauge, ring gauge, or manufacturer’s specification.

9. How to Identify and Measure a UN Thread

Correctly identifying a UN thread requires checking several features, including the outside diameter, threads per inch, thread angle, thread direction, and whether the thread is parallel or tapered. Measuring only the outside diameter is not sufficient because UN, metric, BSP, and pipe threads may have similar dimensions.

Clean and Inspect the Thread

Before measuring, remove dirt, oil, corrosion, sealant, and metal particles from the thread. Inspect it for:

• Damaged or flattened crests
• Worn thread flanks
• Cross-threading
• Burrs and deformation
• Corrosion
• Coating buildup
• Incomplete threads

Measurements taken over damaged or contaminated areas may produce incorrect results.

Identify an Internal or External Thread

First, determine whether the component has an external or internal thread:

• External thread: Found on bolts, screws, studs, shafts, and male fittings
• Internal thread: Found inside nuts, tapped holes, and female components

In UN designations, external threads use the letter A, while internal threads use the letter B.

For example:

• 1/2-13 UNC-2A: External thread
• 1/2-13 UNC-2B: Internal thread

Measure the Major Diameter

Use a caliper or micrometer to measure across the crests of an external thread. Take several measurements along the threaded length to check for wear, taper, and out-of-roundness.

The measured value should be compared with the nearest nominal UN size.

An actual external thread may measure slightly smaller than its nominal diameter because of manufacturing allowance and tolerance. For example, a 1/2-inch external thread may measure slightly below 0.500 inch.

Internal thread diameters are more difficult to measure directly. Plug gauges, internal micrometers, bore gauges, or known mating components are normally used.

Determine the Threads per Inch

Use an inch thread pitch gauge to determine the TPI. Place different gauge leaves against the thread until the teeth fit evenly into the thread grooves.

The correct gauge should:

• Contact the thread flanks evenly
• Fit without rocking
• Show no visible gaps
• Match several consecutive threads

If a thread pitch gauge is unavailable, count the number of thread intervals over a known axial distance.

For example, if eight pitches occupy 1/2 inch:

TPI=0.58​=16

The thread therefore has 16 TPI.

For short threaded components, measure several pitches and calculate:

TPI=Ln​

Where:

• $n$ = number of thread intervals
• $L$ = measured axial length in inches

Measuring across several pitches provides a more accurate result than measuring only one.

Compare Diameter and TPI

Once the major diameter and TPI have been measured, compare both values with a UN thread size chart.

Both diameter and TPI must match. A 1/2-13 UNC bolt cannot mate correctly with a 1/2-20 UNF nut, even though their nominal diameters are identical.

Verify the Thread Angle

UN threads have a symmetrical 60-degree included angle. A thread profile gauge, optical comparator, or toolmaker’s microscope may be used to verify the angle.

This check helps distinguish UN threads from Whitworth-based threads, including BSP, BSW, and BSF, which use a 55-degree profile.

Threads with similar diameters and pitches should not be considered interchangeable unless their profiles also match.

Check for Parallel or Tapered Geometry

Standard UN fastening threads are parallel. Their diameter remains nearly constant over the threaded length.

Measure an external thread near both ends. A progressively changing diameter may indicate a tapered pipe thread such as NPT or BSPT rather than a standard UN thread.

This distinction is important because NPT threads also use a 60-degree profile but are tapered and sized according to nominal pipe dimensions.

Determine the Thread Direction

Most UN threads are right-hand threads. They tighten when turned clockwise and loosen when turned counterclockwise.

Left-hand threads tighten counterclockwise and should include the suffix LH:

1/2-20 UNF-2A-LH

Viewed from the side, a right-hand thread helix rises toward the right, while a left-hand thread helix rises toward the left.

Measure the Pitch Diameter

Pitch diameter is the most important dimension controlling the fit between internal and external threads. It cannot normally be measured accurately with a standard caliper.

Common pitch-diameter measurement methods include:

• Thread micrometer
• Three-wire method
• Optical comparator
• Coordinate measuring machine
• GO and NO-GO gauges

A thread micrometer uses a V-shaped anvil and conical spindle designed to contact the thread flanks. The correct measuring anvils must be selected for the applicable TPI range.

Three-Wire Measurement Method

The three-wire method is a highly accurate way to measure the pitch diameter of an external thread. Three precision wires are placed in the thread grooves, and a micrometer measures over the wires.

For a 60-degree thread, the approximate best wire size is:

$$W=0.57735P$$

Where:

• $W$ = best wire diameter
• $P$ = thread pitch

The measured value over the wires is then converted into pitch diameter using the applicable formula and correction factors.

This method is commonly used for precision machining and calibration work.

Inspect with GO and NO-GO Gauges

Thread gauges provide a practical way to verify whether a thread meets its specified dimensional class.

For internal threads, use:

• GO plug gauge
• NO-GO plug gauge

For external threads, use:

• GO ring gauge
• NO-GO ring gauge

The GO gauge should engage the complete functional thread length without excessive force. The NO-GO gauge should not engage beyond the limit permitted by the applicable inspection standard.

Simply testing a thread with a normal bolt or nut does not prove that it meets the specified tolerance class.

UN Thread Identification Checklist

Before identifying a thread as UN, verify:

• Nominal major diameter
• Threads per inch
• 60-degree thread angle
• Parallel geometry
• Internal or external thread
• Right-hand or left-hand direction
• UNC, UNF, UNEF, UN, or UNS series
• Required tolerance class
• Coating or plating condition
• Sealing surface if the thread belongs to a fluid fitting

10. Selecting the Correct UN Thread Size

Selecting a UN thread requires more than choosing a bolt diameter. The thread series, pitch, tolerance class, material strength, engagement length, loading conditions, and operating environment all affect joint performance.

Select the Nominal Diameter

The nominal diameter must be large enough to support the applied tensile, shear, bending, and fatigue loads.

A larger thread diameter generally provides:

• Greater tensile capacity
• Higher shear strength
• More thread contact area
• Greater bending resistance
• Higher potential clamp load

However, larger fasteners also require more space, material, installation torque, and larger tools. Safety-critical fasteners should be selected using engineering calculations rather than diameter alone.

Choose Between UNC and UNF

UNC and UNF threads offer different advantages.

UNC threads are commonly selected for:

• General machinery
• Structural assemblies
• Rapid installation
• Soft internally threaded materials
• Cast components
• Dirty working environments
• Frequent assembly and disassembly

UNF threads are commonly selected for:

• Precise adjustment
• Automotive and aerospace equipment
• Thin-wall components
• Short engagement in strong materials
• Higher tensile stress area
• Smaller axial movement per revolution

Neither thread series is always stronger. The correct choice depends on the anticipated failure mode and the materials used in the joint.

Use UNEF for Specialized Applications

UNEF threads provide a very small pitch and precise axial adjustment. They are used for:

• Thin retaining nuts
• Precision instruments
• Aerospace components
• Electrical connectors
• Thin-wall fittings
• Specialized hydraulic equipment

Because UNEF threads are shallow and closely spaced, they are more sensitive to dirt, galling, and mechanical damage. They also require more turns to assemble.

Consider the Tensile Stress Area

The tensile stress area represents the effective cross-sectional area of a threaded fastener subjected to axial tension. It can be approximated by:

At​=4π​(D−TPI0.9743​)2

Where:

• At​ = tensile stress area
• $D$ = nominal diameter in inches
• $TPI$ = threads per inch

For the same nominal diameter, fine threads generally provide a larger tensile stress area because their thread depth is smaller.

For example:

However, the larger tensile area of UNF does not automatically make the complete joint stronger. Internal-thread stripping, fatigue, preload loss, material strength, and engagement length must also be evaluated.

Determine the Required Thread Engagement

Thread engagement is the axial length over which the internal and external threads are in contact. Insufficient engagement can cause thread stripping before the fastener reaches its intended tensile load.

The required engagement depends on:

• External thread strength
• Internal thread material
• Thread diameter
• Thread pitch
• Applied load
• Operating temperature
• Manufacturing tolerances
• Number of assembly cycles

Steel-to-steel joints may require less engagement than a hardened steel bolt installed in aluminium or plastic.

Engagement of approximately one nominal diameter is sometimes used as a preliminary starting point for steel components, but it is not a universal design requirement. Critical joints require a thread-stripping calculation.

Consider the Internally Threaded Material

The internal thread is often the weakest part of a joint, particularly when a high-strength steel bolt is installed into aluminium, cast iron, brass, or plastic.

General considerations include:

• Steel: Commonly suitable for UNC or UNF threads
• Aluminium: Often benefits from UNC threads and greater engagement
• Cast iron: Coarse threads help provide deeper engagement
• Brass: Requires controlled torque to avoid stripping
• Plastic: Usually requires coarse threads, long engagement, or inserts
• Thin sheet: May require a nut, threaded insert, or formed feature

Threaded inserts can improve durability when a soft material must withstand repeated assembly.

Account for Vibration and Fatigue

Fine threads have a smaller helix angle and permit more precise preload adjustment, but thread pitch alone does not prevent loosening.

Vibration resistance depends mainly on:

• Correct preload
• Joint stiffness
• Prevention of joint slip
• Surface condition
• Installation accuracy
• Locking method
• Repeated external loading

Where loosening is possible, consider prevailing-torque nuts, thread-locking adhesive, lock wire, tab washers, or other approved locking methods.

For fatigue-loaded joints, avoid exposing the first engaged thread to excessive stress and ensure the fastener maintains adequate preload throughout operation.

Select the Thread Class

For most industrial applications, the normal selection is:

• 2A: External thread
• 2B: Internal thread

Use Class 1 when generous clearance and easy assembly are required. Use Class 3 when close fit and high positional accuracy provide a clear functional benefit.

An unnecessarily tight tolerance class increases:

• Machining cost
• Inspection requirements
• Rejection risk
• Sensitivity to contamination
• Sensitivity to coating thickness
• Risk of assembly interference

A tighter thread class does not automatically increase joint strength.

Consider Coating and Plating Thickness

Plating and coating change the effective dimensions of a thread. A coating increases the effective size of an external thread and decreases the available clearance in an internal thread.

Common finishes include:

• Zinc plating
• Nickel plating
• Cadmium plating
• Hot-dip galvanizing
• Paint
• Dry-film lubricant
• PTFE-based coating

The drawing should specify whether thread dimensions apply before or after coating. Thick coatings may require additional allowance or special tapping dimensions.

Prevent Galling

Stainless steel, nickel alloys, aluminium, and titanium threads may gall during assembly. Galling occurs when contacting surfaces adhere and tear under pressure.

The risk can be reduced by:

• Cleaning the threads
• Using a compatible lubricant
• Reducing installation speed
• Avoiding impact tools where inappropriate
• Preventing misalignment
• Selecting suitable material combinations
• Using approved surface coatings
• Controlling tightening torque

Lubrication reduces friction and can significantly increase fastener preload for the same applied torque. Dry and lubricated torque values should not be treated as equivalent.

Consider the Operating Environment

Thread selection should account for:

• Corrosive chemicals
• Marine exposure
• High or low temperature
• Thermal cycling
• Dirt and dust
• Repeated disassembly
• Vibration
• Pressure
• Radiation
• Cleanroom requirements

Material, coating, lubricant, and locking method must all remain suitable for the expected environment.

Practical Selection Guide

The final thread selection should comply with the applicable design standard and be verified by calculation for safety-critical joints.

Conclusion

UN threads are standardized inch-based screw threads used extensively in machinery, structural assemblies, automotive equipment, aerospace components, hydraulic systems, and general industrial applications. Their 60-degree profile and defined diameter-and-pitch combinations allow properly manufactured internal and external threads to be assembled interchangeably.

The principal Unified thread series are:

• UNC: Coarse threads for general use, rapid assembly, and softer materials
• UNF: Fine threads for precise adjustment and a larger tensile stress area
• UNEF: Extra-fine threads for thin-wall and precision applications
• UNS: Special diameter-and-pitch combinations
• UN: Constant-pitch series commonly used for larger diameters

A UN thread size chart helps users identify the nominal diameter, TPI, pitch, thread series, basic dimensions, tolerance class, and tap drill size. Correct identification requires checking both the major diameter and threads per inch. The thread angle, direction, taper, and fit class should also be verified.

Threads that appear similar should never be forced together. UN, metric, BSP, NPT, Whitworth, and other thread standards may differ in diameter, pitch, angle, taper, and sealing method.

For general commercial applications, Classes 2A and 2B are normally appropriate. Critical components should be manufactured and inspected using the dimensional limits specified by the applicable standard rather than relying only on basic chart values. Proper thread selection, adequate engagement, correct preload, compatible materials, and controlled installation are all necessary to produce a safe and reliable threaded joint.