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No. Metric and inch threads are NOT interchangeable, even when the diameters appear similar. The key differences:

• Pitch: Metric uses mm pitch; UNC/UNF uses TPI — they never match exactly
• Thread angle: Both metric and unified use 60°, but BSW uses 55°
• Root/crest: Different profiles even at the same angle

Forcing the wrong thread will strip the threads and damage both components. Always verify with a thread gauge.

Step-by-step identification:

1. Measure major diameter with calipers (mm or inches)
2. Use a thread pitch gauge to determine pitch (mm) or TPI (threads per inch)
3. Check the thread angle: 60° = metric or unified; 55° = Whitworth/BSP
4. Cross-reference in the table above to confirm the standard

If the pitch is a clean mm value (0.5, 0.7, 1.0, 1.25, 1.5, 2.0, 2.5, 3.0), it's almost certainly metric.

Pitch is the distance between adjacent thread crests, measured in millimeters. Used for metric threads.

TPI (Threads Per Inch) is the number of complete thread crests in one inch. Used for imperial threads.

Conversion: Pitch (mm) = 25.4 / TPI

Examples: 20 TPI = 1.27 mm pitch, 18 TPI = 1.411 mm pitch, 16 TPI = 1.5875 mm pitch.

BSW (British Standard Whitworth) was the world's first national standard thread, created by Sir Joseph Whitworth in 1841.

Key features:

• 55° thread angle (vs 60° for metric/unified)
• Rounded roots and crests
• Diameter in inches, pitch in TPI

BSW has been largely replaced by metric and unified standards but is still found in legacy machinery, plumbing (BSP is related), and some British equipment.

UNC (Unified National Coarse) — the standard/default thread:

• Fewer threads per inch → faster assembly
• More forgiving for tapped holes in soft materials
• Less likely to cross-thread

UNF (Unified National Fine) — the fine-pitch variant:

• More threads per inch → higher tensile strength
• Better vibration resistance (self-locking effect)
• Finer adjustment capability
• Requires more precise tapping

Rule of thumb: Use UNC for general assembly, UNF for precision and vibration-prone applications.