Quick reference for identifying metric bolt and screw thread pitches. Helpful for finding replacement fasteners and matching hardware.
Format: M[Diameter] × [Pitch] (e.g., M8 × 1.25)
Diameter: Major diameter in millimeters
Pitch: Distance between threads in millimeters
If pitch is omitted, coarse thread is assumed. Use a thread pitch gauge or calipers to identify unknown threads.
| Size | Pitch (mm) | Tap Drill (mm) | Common Use |
|---|---|---|---|
| M3 | 0.5 | 2.5 | Electronics, small hardware |
| M4 | 0.7 | 3.3 | Electronics, light fixtures |
| M5 | 0.8 | 4.2 | Bicycles, furniture |
| M6 | 1.0 | 5.0 | Very common - automotive, machinery |
| M8 | 1.25 | 6.8 | Very common - automotive, structural |
| M10 | 1.5 | 8.5 | Automotive, construction |
| M12 | 1.75 | 10.2 | Heavy machinery, structural |
| M14 | 2.0 | 12.0 | Automotive (wheel studs) |
| M16 | 2.0 | 14.0 | Heavy structural |
| M20 | 2.5 | 17.5 | Large structural bolts |
| Size | Pitch (mm) | Tap Drill (mm) | Advantage |
|---|---|---|---|
| M6 × 0.75 | 0.75 | 5.25 | Tighter adjustment |
| M8 × 1.0 | 1.0 | 7.0 | More threads, stronger in thin material |
| M10 × 1.0 | 1.0 | 9.0 | Fine adjustment, vibration resistant |
| M10 × 1.25 | 1.25 | 8.75 | Common automotive |
| M12 × 1.25 | 1.25 | 10.75 | Higher strength, less stripping |
| M12 × 1.5 | 1.5 | 10.5 | Common automotive |
| M14 × 1.5 | 1.5 | 12.5 | Spark plugs, wheel studs |
Fine threads: better for thin materials, precision adjustment, and vibration resistance
| Metric | Close Imperial | Notes |
|---|---|---|
| M3 | #4-40 | Similar, not interchangeable |
| M4 | #8-32 | M4 slightly smaller |
| M5 | #10-24 | Similar diameter |
| M6 | 1/4"-20 | 1/4" is 6.35mm - close but different |
| M8 | 5/16"-18 | 5/16" is 7.94mm |
| M10 | 3/8"-16 | 3/8" is 9.53mm |
| M12 | 1/2"-13 | 1/2" is 12.7mm |
⚠️ Never force metric into imperial or vice versa - threads will strip!
Method 1: Thread Pitch Gauge
Method 2: Calipers + Counting
Method 3: Known Nut
Method 4: Calipers for Diameter
M6 × 1.0: Furniture hardware, machinery guards, license plates
M8 × 1.25: Automotive (body panels, interior trim), bicycles
M10 × 1.25: Automotive suspension, engine mounts
M10 × 1.5: General machinery, structural brackets
M12 × 1.25: Heavy automotive (caliper bolts, strut mounts)
M12 × 1.75: Construction equipment, structural steel
M14 × 1.5: Spark plugs, wheel studs (many vehicles)
M14 × 2.0: Drain plugs, heavy equipment
| Bolt Size | Hex Head | Socket Head |
|---|---|---|
| M4 | 7mm | 3mm |
| M5 | 8mm | 4mm |
| M6 | 10mm | 5mm |
| M8 | 13mm | 6mm |
| M10 | 17mm | 8mm |
| M12 | 19mm | 10mm |
| M14 | 22mm | 12mm |
| M16 | 24mm | 14mm |
Metric bolts are marked with numbers indicating tensile strength:
First number × 100 = tensile strength in MPa. Match replacement bolts to original grade.
Metric fasteners use a different system than imperial - thread pitch is measured in millimeters between threads rather than threads per inch. Mixing metric and imperial threads strips both parts instantly, so accurate identification is essential. When a bolt size omits the pitch (e.g., "M8"), coarse thread is assumed, but many applications use fine thread for better vibration resistance.
This guide provides complete thread pitch tables for metric coarse and fine threads, tap drill sizes for threading operations, metric-to-imperial near-equivalents (and why you can't substitute them), and methods for identifying unknown threads. You'll also find wrench sizes and strength grades for metric fasteners.
Whether you're working on a European car, Japanese motorcycle, or any equipment with metric fasteners, this reference helps you identify threads correctly, find the right tap drill sizes, and select appropriate wrenches and replacement fasteners.
Cross-threading metric and imperial fasteners damages parts beyond repair. This guide helps you identify threads correctly and find the right specifications for any metric fastener application.