🔩 Thread Identifier Tool

Thread Standards Explained

Metric Threads (ISO)

Designated as M followed by diameter and pitch: M6 × 1.0

• Most common worldwide
• Measured in millimeters
• Coarse and fine pitch available
• Example: M8 × 1.25 (8mm diameter, 1.25mm pitch)

UNC (Unified National Coarse)

Designated as diameter-TPI: 1/4-20

• Standard for USA/Canada
• Coarse thread - easier to start, faster assembly
• Better for dirty or damaged threads
• Example: 1/4-20 (1/4" diameter, 20 threads per inch)

UNF (Unified National Fine)

Designated as diameter-TPI: 1/4-28

• Fine thread version of UNC
• More threads per inch = stronger
• Better for precision applications
• Less likely to vibrate loose
• Example: 1/4-28 (1/4" diameter, 28 threads per inch)

BSW (British Standard Whitworth)

Old British standard, still found on classic cars

• 55° thread angle (vs 60° for metric/unified)
• Measured in inches with specific TPI
• Common on pre-1960s British vehicles
• Being replaced by metric in modern applications

Common Metric Threads

Thread	Pitch (mm)	Tap Drill	Common Use
M3 × 0.5	0.5	2.5mm	Electronics, small machinery
M4 × 0.7	0.7	3.3mm	Small fasteners
M5 × 0.8	0.8	4.2mm	General machinery
M6 × 1.0	1.0	5.0mm	Automotive, machinery
M8 × 1.25	1.25	6.8mm	Automotive, structural
M10 × 1.5	1.5	8.5mm	Heavy machinery
M12 × 1.75	1.75	10.2mm	Structural, automotive

Common UNC Threads

Thread	TPI	Tap Drill	Common Use
1/4-20	20	#7 (0.201")	Most common small bolt
5/16-18	18	F (0.257")	General purpose
3/8-16	16	5/16 (0.313")	Structural, machinery
7/16-14	14	U (0.368")	Heavy duty
1/2-13	13	27/64 (0.422")	Automotive, structural

Common UNF Threads

Thread	TPI	Tap Drill	Common Use
1/4-28	28	#3 (0.213")	Precision equipment
5/16-24	24	I (0.272")	Automotive fine thread
3/8-24	24	Q (0.332")	Precision machinery
7/16-20	20	25/64 (0.391")	Heavy machinery fine
1/2-20	20	29/64 (0.453")	Automotive, aircraft

How to Identify Threads

• Thread Pitch Gauge: Most accurate method - match threads to gauge
• Caliper Measurement: Measure diameter and count threads per inch/mm
• Test Fit: Try known nuts/bolts (safest way for damaged threads)
• Thread Angle: 60° for metric/unified, 55° for Whitworth

UNC vs UNF - When to Use Each

• Use UNC when: General assembly, may need frequent removal, dirty/damaged threads
• Use UNF when: High vibration, precision needed, limited thread depth, automotive applications

Thread Compatibility

• Never mix: Metric and imperial threads don't interchange
• UNC and UNF: Same diameter but different TPI - not compatible
• Close but not exact: M6 and 1/4" are similar but NOT the same
• Always verify: Use thread gauge or test fit before forcing