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Thread Standards & Specifications

Comprehensive Guide to Metric, Imperial & Special Threads

ISO Metric Threads (ISO 68-1)

Thread Designation: M [nominal diameter] × [pitch]

Example: M12 × 1.75 means 12mm nominal diameter with 1.75mm thread pitch

Coarse Pitch Metric Threads

Thread Size Pitch (mm) Major Diameter (mm) Minor Diameter (mm) Stress Area (mm²)
M6 × 1 1.0 6.000 4.773 20.1
M8 × 1.25 1.25 8.000 6.466 36.6
M10 × 1.5 1.5 10.000 8.160 58.0
M12 × 1.75 1.75 12.000 9.853 84.3
M16 × 2 2.0 16.000 13.546 157
M20 × 2.5 2.5 20.000 16.933 245
M24 × 3 3.0 24.000 20.319 353
M30 × 3.5 3.5 30.000 25.706 561
M36 × 4 4.0 36.000 31.093 817

Fine Pitch Metric Threads

Thread Size Pitch (mm) Major Diameter (mm) Minor Diameter (mm) Applications
M8 × 1 1.0 8.000 6.773 Thin wall applications, precision instruments
M10 × 1.25 1.25 10.000 8.466 Fine adjustment, automotive
M12 × 1.5 1.5 12.000 10.160 Spark plugs, thin sections
M16 × 1.5 1.5 16.000 14.160 Hydraulic fittings, precision equipment
M20 × 1.5 1.5 20.000 18.160 Aerospace, high vibration environments

Unified Threads (ANSI/ASME B1.1)

UNC - Unified Coarse Threads

Thread Size TPI (Threads Per Inch) Major Diameter (in) Minor Diameter (in) Stress Area (in²)
1/4"-20 UNC 20 0.2500 0.1850 0.0318
5/16"-18 UNC 18 0.3125 0.2403 0.0524
3/8"-16 UNC 16 0.3750 0.2983 0.0775
1/2"-13 UNC 13 0.5000 0.4056 0.1419
5/8"-11 UNC 11 0.6250 0.5135 0.226
3/4"-10 UNC 10 0.7500 0.6201 0.334
1"-8 UNC 8 1.0000 0.8376 0.606

UNF - Unified Fine Threads

Thread Size TPI (Threads Per Inch) Major Diameter (in) Minor Diameter (in) Applications
1/4"-28 UNF 28 0.2500 0.2036 Automotive, fine adjustment
5/16"-24 UNF 24 0.3125 0.2614 Thin sections, precision work
3/8"-24 UNF 24 0.3750 0.3239 Hydraulic fittings
1/2"-20 UNF 20 0.5000 0.4459 Aerospace, high stress
5/8"-18 UNF 18 0.6250 0.5528 Heavy machinery
3/4"-16 UNF 16 0.7500 0.6733 Industrial equipment

Pipe Threads

NPT - National Pipe Thread (Tapered)

Thread Size TPI Outside Diameter (in) Hand-Tight Engagement Wrench Makeup Turns
1/8" NPT 27 0.405 4 turns 2-3 turns
1/4" NPT 18 0.540 4 turns 2-3 turns
3/8" NPT 18 0.675 4 turns 2-3 turns
1/2" NPT 14 0.840 5 turns 2-3 turns
3/4" NPT 14 1.050 5 turns 2-3 turns
1" NPT 11.5 1.315 5 turns 3-4 turns

BSP - British Standard Pipe (Parallel)

Thread Size TPI Major Diameter (mm) Minor Diameter (mm) Usage
1/8" BSP 28 9.728 8.566 Small fittings, instruments
1/4" BSP 19 13.157 11.445 General plumbing
3/8" BSP 19 16.662 14.950 Pneumatic systems
1/2" BSP 14 20.955 18.631 Industrial piping
3/4" BSP 14 26.441 24.117 Process piping

Thread Tolerance Classes

ISO Metric Threads:

  • 6g - Fine tolerance for external threads (bolts)
  • 6H - Medium tolerance for internal threads (nuts)
  • 6g/6H - Standard combination for general purpose fasteners

Unified Threads:

  • Class 2A - External threads (bolts) - normal fit
  • Class 2B - Internal threads (nuts) - normal fit
  • Class 3A/3B - Tighter tolerance for precision applications