Unified National Fine

U = Unified, N = National, F = Fine, identifying the fine pitch series within ASME B1.1’s Unified thread system.

Fine pitch thread

UNF uses more threads per inch and a smaller pitch than UNC at the same nominal diameter.

Unified thread family

UNC (coarse), UNF (fine), and UNEF (extra fine) are three related series sharing the same 60 degree thread form but differing in thread spacing.

Simple example

1/2-20 UNF describes a 0.500 inch nominal diameter, fine pitch thread with 20 threads per inch.

1/4-28 UNF ⭐

One of the most commonly encountered UNF sizes, used across automotive and general mechanical assembly.

5/16-24 UNF

Common in automotive and moderate load fine-thread applications.

3/8-24 UNF ⭐

Frequently used in automotive and precision mechanical assembly.

7/16-20 UNF

Used in moderate to heavier fine-thread fastening applications.

1/2-20 UNF ⭐⭐⭐

One of the most recognized UNF sizes, widely referenced in automotive and mechanical engineering contexts.

5/8-18 UNF

Common in heavier automotive and industrial fine-thread applications.

3/4-16 UNF

Used in heavier mechanical and industrial fine-thread connections.

7/8-14 and 1-12 UNF

Used in larger mechanical and industrial applications requiring fine thread engagement.

Major diameter

The largest diameter of the thread, measured at the crest for external threads. This equals the nominal size stated in the designation, such as 0.5000 inches for 1/2-20 UNF.

Pitch diameter ⭐⭐⭐

An imaginary cylinder diameter where thread width equals groove width. This is the primary reference dimension used for thread measurement, inspection, and gaging, and directly relates to thread class and tolerance.

Minor diameter (external)

The smallest diameter of the external thread, measured at the root. Relates directly to tensile stress area and fastener strength calculations.

Minor diameter (internal)

The smallest diameter of the internal (nut) thread, measured at the crest of the internal thread rather than the root, since internal and external threads are not dimensionally identical at every point along the profile.

Potential advantages

• More threads per inch, offering finer adjustment
• Smaller pitch, which can suit certain engagement geometries
• Slightly larger tensile stress area at the same nominal diameter
• Useful where wall thickness or clearance is limited

Potential limitations

• More susceptible to cross-threading during assembly
• More sensitive to thread damage than coarser threads
• Less forgiving in dirty, rough, or corroded conditions
• Can assemble more slowly due to higher thread count

Thin materials

Fine threads may be selected where available radial or axial space is limited, since the shallower thread depth can suit thin-walled parts.

Vibration-sensitive applications

Fine threads are sometimes selected in vibration-sensitive contexts as part of a broader engineering rationale, but UNF alone does not guarantee vibration resistance; proper torque, locking features, and joint design remain essential.

Thread engagement

Required engagement length depends on the tapped material’s strength relative to the fastener, not on TPI alone; a fine thread does not automatically reduce the needed engagement length.

UNF thread strength

Tensile stress area, combined with material grade, determines actual tensile capacity. UNF’s slightly larger tensile stress area at the same nominal diameter is one input, not the deciding factor for overall joint strength.

UNF does not equal bolt grade

A Grade 2, Grade 5, and Grade 8 bolt can all use identical 1/2-20 UNF threads. Thread series and material grade are independent properties. See the Bolt Grade Chart.

UNF and bolt torque

Correct tightening torque depends on bolt grade, lubrication, coating, friction, and required preload, not the UNF designation alone. See the Bolt Torque Chart.

Automotive

Common in engine components, brackets, and precision hardware; always follow the vehicle manufacturer’s service specification for safety-critical fasteners rather than a general assumption.

Aerospace

Fine threads are used in specific aerospace fastening applications governed by the applicable engineering specification.

Machinery and industrial equipment

Used across general machinery and industrial assemblies, particularly where finer adjustment or thin-section engagement is beneficial.

Precision assemblies

Selected where finer thread adjustment or tighter tolerance is required by the design.

Construction equipment ⭐

UNF fasteners appear in construction machinery, specialty brackets, and mechanical assemblies; keep structural fastening requirements (governed by their own specification) separate from generic UNF sizing decisions.

General fastening

UNF remains a viable choice for many general applications where its characteristics suit the specific joint design.

1/4-28 UNF ⭐

Major diameter 0.250 in, 28 TPI, pitch 0.0357 in (0.907 mm). Compare with 1/4-20 UNC (20 TPI) at the identical diameter.

3/8-24 UNF ⭐

Major diameter 0.375 in, 24 TPI, pitch 0.0417 in (1.058 mm). Compare with 3/8-16 UNC (16 TPI) at the identical diameter.

1/2-20 UNF ⭐⭐⭐

Major diameter 0.500 in, 20 TPI, pitch 0.0500 in (1.270 mm). Compare directly with 1/2-13 UNC (13 TPI); same diameter, different pitch, different designation.

5/8-18 UNF

Major diameter 0.625 in, 18 TPI, pitch 0.0556 in (1.411 mm). Compare with 5/8-11 UNC (11 TPI) at the identical diameter.

3/4-16 UNF

Major diameter 0.750 in, 16 TPI, pitch 0.0625 in (1.588 mm). Compare with 3/4-10 UNC (10 TPI) at the identical diameter.

#10-32 UNF

Major diameter 0.190 in, 32 TPI, pitch 0.0312 in (0.794 mm). One of the most common small UNF sizes in machine screw applications.