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Automotive
Modern cars typically use Metric bolts (M6, M8, M10). Vintage US cars often use SAE. Fine threads are common on high-stress parts.
- •Suspension & Brakes
- •Engine components
- •Lug Nuts (M12x1.5, 5x114.3)
The ultimate reference guide for identifying, measuring, and selecting bolts. Compare Metric vs Standard, find wrench sizes, and check thread pitches instantly.
Metric ISO • Metric
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System: Metric (ISO) • Type: Hex Head • A4
| Size | Pitch (Coarse) | Pitch (Fine) | Head Width | Wrench Size |
|---|---|---|---|---|
| M3 | 0.5 mm | 0.35 mm | 5.5mm | 5.5mm |
| M4 | 0.7 mm | 0.5 mm | 7mm | 7mm |
| M5 | 0.8 mm | 0.5 mm | 8mm | 8mm |
| M6 | 1.0 mm | 0.75 mm | 10mm | 10mm |
| M8 | 1.25 mm | 1.0 mm | 13mm | 13mm |
| M10 | 1.5 mm | 1.25 mm | 17mm | 17mm |
| M12 | 1.75 mm | 1.5 mm | 19mm | 19mm |
| M14 | 2.0 mm | 1.5 mm | 22mm | 22mm |
| M16 | 2.0 mm | 1.5 mm | 24mm | 24mm |
| M18 | 2.5 mm | 1.5 mm | 27mm | 27mm |
| M20 | 2.5 mm | 1.5 mm | 30mm | 30mm |
| M22 | 2.5 mm | 1.5 mm | 32mm | 32mm |
| M24 | 3.0 mm | 2.0 mm | 36mm | 36mm |
| M27 | 3.0 mm | 2.0 mm | 41mm | 41mm |
| M30 | 3.5 mm | 2.0 mm | 46mm | 46mm |
| M33 | 3.5 mm | 2.0 mm | 50mm | 50mm |
| M36 | 4.0 mm | 3.0 mm | 55mm | 55mm |
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Quickly find the right tool for your bolt.
| Bolt Diameter | Metric Wrench | SAE Wrench |
|---|---|---|
| M6 | 10mm | - |
| M8 | 13mm | 1/2" |
| M10 | 17mm (16mm ISO) | 5/8" |
| M12 | 19mm | 3/4" |
| 1/4" | - | 7/16" |
| 5/16" | - | 1/2" |
| 3/8" | - | 9/16" |
| 1/2" | - | 3/4" |
Avoid common mistakes by measuring from the right reference points.
Measure the outer diameter of the threads (Major Diameter). Do not measure the head. Use calipers for best accuracy.
For Hex bolts, measure from under the head to the tip. For Flat/Countersunk heads, measure the entire bolt length including the head.
Metric pitch is the distance between peaks in mm (e.g., 1.5mm). Imperial pitch is threads per inch (TPI). Count threads in 1 inch.
Understanding head markings is critical for safety.
Standard high tensile strength.
Common in automotive suspension and structural parts.
High strength alloy steel.
Used for engine head bolts and critical stress points.
3 Radial Lines.
Standard automotive and general hardware use.
6 Radial Lines.
High strength for suspension and steering. Never downgrade!
⚠️ Warning: Never replace a Grade 8 or Metric 10.9 bolt with a lower grade fastener!
Coarse vs Fine threads: knowing the difference prevents stripped threads and stuck bolts.
Standard for most applications. They assemble faster and are less sensitive to dirt or damage.
Higher strength and better vibration resistance. Used in automotive suspension and precision machinery.
Where to use different bolt types and systems.
Modern cars typically use Metric bolts (M6, M8, M10). Vintage US cars often use SAE. Fine threads are common on high-stress parts.
Large diameter UNC bolts are standard for framing. Carriage bolts offer a smooth finish for wood-to-wood connections.
Socket Cap screws (Allen head) are used in tight spaces. High-strength grades (Class 12.9 or Grade 8) are essential.
Complete your workshop reference collection.
| Number | Fraction | Decimal (in) | Millimeters |
|---|---|---|---|
| #80 | — | 0.0135 | 0.34 |
| #50 | — | 0.0700 | 1.78 |
| #52 | 1/16" | 0.0625 | 1.59 |
| #41 | 3/32" | 0.0938 | 2.38 |
| #22 | 5/32" | 0.1562 | 3.97 |
| #11 | 3/16" | 0.1875 | 4.76 |
| #7 | 13/64" | 0.2031 | 5.16 |
| — | — | 0.2570 | 6.53 |
| — | 3/8" | 0.3750 | 9.53 |
| — | 1/2" | 0.5000 | 12.70 |
Numbered, lettered, metric, and fractional drill bit reference.
View chart| Millimeters | Decimal (in) | Fraction (in) |
|---|---|---|
| 0.5 | 0.020 | 0 |
| 1 | 0.039 | 1/16 |
| 6.35 | 0.250 | 1/4 |
| 12.7 | 0.500 | 1/2 |
| 19 | 0.748 | 3/4 |
| 25.4 | 1.000 | 1 |
| 38.1 | 1.500 | 1 1/2 |
| 50.8 | 2.000 | 2 |
| 76.2 | 3.000 | 3 |
| 101.6 | 4.000 | 4 |
Convert millimeters to inches with decimals and fractions for woodworking and engineering.
View chart| Fraction | Decimal | Percent |
|---|---|---|
| 1/64 | 0.0156 | 1.56% |
| 1/32 | 0.0313 | 3.13% |
| 1/16 | 0.0625 | 6.25% |
| 1/8 | 0.1250 | 12.5% |
| 3/16 | 0.1875 | 18.8% |
| 1/4 | 0.2500 | 25% |
| 5/16 | 0.3125 | 31.3% |
| 3/8 | 0.3750 | 37.5% |
| 1/2 | 0.5000 | 50% |
| 3/4 | 0.7500 | 75% |
Fractions from 1/64 to whole numbers for math education and precision cutting.
View chart| Inches (fraction) | Inches (decimal) | Millimeters | Centimeters |
|---|---|---|---|
| 1/4" | 0.250 | 6.35 | 0.64 |
| 1/2" | 0.500 | 12.70 | 1.27 |
| 1" | 1.000 | 25.40 | 2.54 |
| 1 1/2" | 1.500 | 38.10 | 3.81 |
| 2" | 2.000 | 50.80 | 5.08 |
| 2 1/2" | 2.500 | 63.50 | 6.35 |
| 3" | 3.000 | 76.20 | 7.62 |
| 4" | 4.000 | 101.60 | 10.16 |
| 5" | 5.000 | 127.00 | 12.70 |
| 6" | 6.000 | 152.40 | 15.24 |
Imperial to metric conversions for hardware sizing and fabrication.
View chartExplore more: All Charts · Categories · Use Cases
Common questions about bolt sizing and selection.