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

7. UNF vs. UNC Threads

UNF and UNC are the two most commonly used thread series within the Unified Thread Standard. Both use inch-based nominal diameters and a 60-degree thread profile, but they differ in pitch, thread depth, strength characteristics, and suitability for particular operating conditions.

• UNF: Unified National Fine
• UNC: Unified National Coarse

For the same nominal diameter, a UNF thread has more threads per inch and a smaller pitch than a UNC thread. For example, a 1/2-inch fastener may use either:

• 1/2-20 UNF: 20 threads per inch
• 1/2-13 UNC: 13 threads per inch

Although their nominal diameters are the same, these two threads are not interchangeable.

Tensile strength

Because a UNF thread is shallower, the fastener retains a larger core area than a comparable UNC fastener. This normally gives the UNF fastener a larger tensile stress area.

For example, when two bolts have the same nominal diameter, material, and strength grade, the UNF bolt may carry a higher tensile load before the threaded section fractures. However, the total strength of a bolted joint also depends on:

• Fastener grade
• Nut or tapped-hole material
• Length of thread engagement
• Installation preload
• Surface condition
• Operating temperature
• Fatigue loading
• Corrosion

A larger tensile stress area does not automatically make UNF the best choice for every joint.

Thread stripping resistance

UNC threads are deeper and have a coarser profile. They may provide better stripping resistance in low-strength materials such as cast iron, aluminium, plastics, or soft alloys, particularly when the available engagement length is limited.

UNF threads have more engaged threads over the same axial distance, but each thread is shallower. The actual stripping capacity must therefore be evaluated using the strength of both the external and internal thread materials.

Thin-walled components

UNF threads are often preferred for thin-walled parts because their smaller pitch allows more complete threads to engage within a short axial distance. Their shallower thread depth also removes less material from the wall.

This can be useful in:

• Thin nuts
• Tube and fitting components
• Sheet-metal assemblies
• Instrument housings
• Compact machine parts

The wall must still be sufficiently thick to resist thread stripping, distortion, and cracking.

Adjustment accuracy

A UNF fastener moves a shorter axial distance during one complete rotation. A 1/2-20 UNF screw moves:

201​=0.050 in per revolution

A 1/2-13 UNC screw moves:

131​≈0.0769 in per revolution

The smaller movement per revolution makes UNF more suitable for fine adjustment and accurate positioning.

Assembly and field service

UNC threads are usually easier and faster to assemble. Their deeper, more widely spaced threads are also more tolerant of:

• Dirt and contamination
• Minor surface damage
• Corrosion
• Rough handling
• Small angular misalignment

UNF threads require more rotations for the same engagement length and may be more easily damaged during field installation. Careful alignment is important to prevent cross-threading.

Vibration and loosening

The smaller helix angle of a fine thread may provide greater resistance to rotational loosening under some conditions. UNF threads also permit more accurate preload adjustment because each turn produces less axial movement.

However, vibration resistance depends on the complete joint design. Neither UNF nor UNC should be considered inherently vibration-proof. A suitable locking method may still be required.

When should UNF be selected?

UNF may be preferred when:

• A large tensile stress area is required
• Accurate adjustment is important
• The component has a thin wall
• The application uses short engagement with several threads
• The joint is protected from dirt and impact
• An industry specification requires a fine thread

UNC may be more suitable for general-purpose equipment, soft materials, dirty environments, rapid assembly, or components exposed to frequent handling.

8. How to Identify and Measure a UNF Thread

A UNF thread cannot be identified from its outside diameter alone. Both the nominal diameter and threads per inch must be determined. For critical applications, the thread class and dimensional limits must also be verified.

Step 1: Determine whether the thread is internal or external

An external thread is formed on the outside of a bolt, screw, stud, or shaft. An internal thread is formed inside a nut, fitting, housing, or tapped hole.

The measuring method may differ:

• External threads can usually be measured directly with a caliper.
• Internal threads may require plug gauges, internal measuring equipment, or comparison with a known fastener.

Step 2: Measure the major diameter

For an external thread, use a caliper or micrometer to measure across the thread crests. Position the measuring tool perpendicular to the fastener axis and avoid applying excessive pressure.

The measured diameter may be slightly smaller than the nominal size because of manufacturing tolerances. Approximate examples include:

A measurement close to 0.500 inch indicates a nominal 1/2-inch thread, but it does not reveal whether the thread is 1/2-20 UNF, 1/2-13 UNC, or another series.

Step 3: Determine the threads per inch

A thread pitch gauge is the most convenient tool for determining TPI. Place different gauge leaves against the thread until one fits the profile without visible gaps.

For example:

• 1/4 inch with 28 TPI indicates 1/4-28 UNF.
• 3/8 inch with 24 TPI indicates 3/8-24 UNF.
• 1/2 inch with 20 TPI indicates 1/2-20 UNF.

If a pitch gauge is unavailable, count the number of thread peaks over a known axial distance. For small fasteners or fine threads, count over 1/2 inch and multiply the result by two. This manual method is less accurate than using a proper gauge.

Step 4: Compare the measurements with a chart

Match the approximate major diameter and measured TPI with a UNF thread chart. Both values must correspond to a recognised thread designation.

For example, suppose an external thread measures approximately 0.373 inch and the gauge shows 24 TPI. The most likely size is 3/8-24 UNF.

Step 5: Check the thread profile

UNF threads have a 60-degree included angle. Other thread systems can have similar diameters but different pitches or profiles.

Possible sources of confusion include:

• UNC threads
• UNEF threads
• UNS special threads
• Metric threads
• British Standard Whitworth threads
• BSPP and BSPT pipe threads
• NPT pipe threads

UNF threads are straight parallel fastening threads. They should not be confused with tapered pipe threads used to create pressure-tight connections.

Step 6: Verify the thread using gauges

For production inspection or safety-critical applications, use calibrated gauges:

• Thread ring gauges for external threads
• Thread plug gauges for internal threads
• GO gauges to verify the maximum-material functional condition
• NO-GO gauges to check that the thread does not exceed the permitted limit

A GO gauge should assemble as specified by the inspection procedure. A NO-GO gauge should not pass beyond the permitted engagement. The acceptance criteria should follow the relevant standard and quality-control procedure.

Common identification mistakes

Frequent mistakes include:

• Measuring only the diameter
• Confusing TPI with metric pitch
• Assuming all fine threads are UNF
• Forcing a UNC bolt into a UNF tapped hole
• Identifying a pipe thread as a fastener thread
• Ignoring wear, coatings, burrs, or corrosion
• Using a ruler for very fine thread pitches
• Assuming a mating component proves the thread class

If the thread is damaged or heavily coated, clean and inspect it before measuring. Never force two components together to determine whether their threads match.

9. UNF Standards, Applications, and Selection Guide

UNF threads are primarily defined by ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form). This standard establishes the basic thread form, standard series, thread classes, allowances, tolerances, and dimensional requirements for unified inch screw threads.

Engineering drawings may also reference standards covering fastener dimensions, mechanical properties, coatings, inspection methods, or industry-specific requirements. The thread designation must therefore be evaluated together with all notes shown on the drawing.

UNF thread designation on engineering drawings

A typical designation is:

5/8-18 UNF-2A

This specifies:

• Nominal diameter: 5/8 inch
• Threads per inch: 18
• Thread series: UNF
• Class: 2A external thread

The mating internal thread may be specified as:

5/8-18 UNF-2B

Additional drawing requirements may include:

• Thread length
• Minimum full-thread depth
• Chamfer dimensions
• Surface coating
• Heat treatment
• Material specification
• Inspection requirements
• Left-hand thread designation
• Torque or preload requirements

When the thread is left-handed, the designation normally includes LH. If LH is not stated, a right-hand thread is generally assumed.

Automotive applications

UNF fasteners are widely used in automotive systems where compact construction, accurate clamping, and vibration resistance are important.

Applications may include:

• Engine components
• Transmission assemblies
• Brake systems
• Suspension components
• Steering systems
• Drivetrain equipment
• High-strength chassis fasteners

A thread should never be selected or substituted based solely on physical fit. Fastener grade, preload, fatigue performance, and manufacturer specifications must also be considered.

Aerospace applications

The aerospace industry frequently uses fine threads for high-strength fasteners and closely controlled assemblies. UNF or aerospace-specific unified threads may be selected because of their larger tensile stress area and precise fit.

Aerospace applications often require:

• Tighter thread classes
• Controlled surface finishes
• Special materials
• Traceable manufacturing processes
• Calibrated thread inspection
• Approved locking methods

General commercial fasteners should not be substituted for aerospace fasteners even when their nominal thread designations appear identical.

Hydraulic and pneumatic applications

UNF threads may appear on hydraulic and pneumatic components, including adapters, ports, fittings, and adjustable components. However, the UNF thread itself does not necessarily provide the pressure seal.

Many fluid connections use a separate sealing element, such as:

• O-ring
• Metal sealing cone
• Flared tube
• Bonded seal
• Gasket
• Sealing washer

For example, an O-ring boss connection may use a straight unified thread to retain the fitting while the O-ring creates the pressure seal. Applying thread sealant to such a connection does not replace the intended sealing mechanism.

The complete port or fitting standard must be identified before selecting or installing a component.

Precision machinery and instrumentation

UNF threads are suitable for mechanisms that require small axial movement for each fastener rotation. Examples include:

• Adjustment screws
• Instrument mounting hardware
• Positioning devices
• Calibration mechanisms
• Machine-tool assemblies
• Optical equipment
• Measurement fixtures

Their fine pitch allows more controlled positioning than a comparable coarse thread.

Factors to consider when selecting a UNF thread

The selection process should evaluate the complete operating environment.

Load and fastener strength

Determine the expected tensile, shear, bending, and fatigue loads. Select the fastener diameter, material, and strength grade with an appropriate safety factor.

Internal-thread material

Soft materials may strip before the bolt reaches its tensile capacity. A greater engagement length, thread insert, larger diameter, or coarse thread may be required.

Vibration and fatigue

A properly controlled preload is essential for fatigue resistance. The design may also require a locking feature to prevent rotational loosening.

Component thickness

Fine threads can provide more engaged threads in a thin component. However, the available material must still support the required load without stripping or distortion.

Corrosion and temperature

Corrosion can reduce thread dimensions and make disassembly difficult. Elevated temperatures can affect material strength, coating performance, lubricant behavior, and tightening torque.

Assembly conditions

UNF threads may be less tolerant of dirt, impact damage, and angular misalignment. UNC threads may be more practical for rough field assembly.

Industry and drawing requirements

The specified thread series should not be changed without engineering approval. Components with similar dimensions may have different materials, pressure ratings, mechanical properties, or qualification requirements.

A UNF thread size chart is useful for preliminary identification and selection, but final manufacturing dimensions should be taken from the applicable standard, approved drawing, or controlled engineering specification.