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Automotive Repair
Modern vehicles use Metric (M6, M8, M10, M12). Vintage US cars require SAE. Always match the thread pitch to avoid stripping fasteners.
- •Engine & Transmission (Grade 10.9)
- •Suspension Bolts (M12x1.25)
- •Body work (M6-1.0)
The professional workshop reference for identifying Metric and Standard bolts. Compare M4 to M36 and 1/4" to 1" sizes. Includes critical data on wrench sizes, thread pitches, and head marking strength grades.
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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Instantly find the correct wrench or socket size for any Metric or SAE bolt head.
| Bolt Diameter | Metric Wrench | SAE Wrench |
|---|---|---|
| M6 | 10mm | - |
| M8 | 13mm | 1/2" (Close Fit) |
| M10 | 17mm (DIN) / 16mm (ISO) | 5/8" |
| M12 | 19mm (DIN) / 18mm (ISO) | 3/4" |
| 1/4" | 6.35mm | 7/16" |
| 5/16" | 7.94mm | 1/2" |
| 3/8" | 9.53mm | 9/16" |
| 1/2" | 12.7mm | 3/4" |
Understanding bolt markings is critical for automotive and structural safety. Never replace high-tensile bolts with lower grades.
"Standard" high-tensile steel. Equivalent to SAE Grade 5.
High strength alloy steel. Used for engine and suspension.
Identified by 3 radial lines on the bolt head.
Identified by 6 radial lines. Maximum strength.
In automotive and high-stress applications, never replace a Grade 8 or Metric 10.9 bolt with a lower grade. Doing so can lead to catastrophic failure under load. Always verify head markings before installation.
Proper identification starts with accurate measurements. Use this guide to avoid ordering the wrong size.
Measure the major diameter of the threads using digital calipers. Do not measure the width of the bolt head.
For Hex bolts, measure from under the head to the tip. For Flat/Countersunk bolts, measure the entire length including the head.
Metric pitch is the mm distance between threads (e.g., 1.5mm). Imperial pitch is TPI (Threads Per Inch).
Coarse vs Fine threads: knowing the difference prevents stripped threads and project delays.
Standard for most applications. Assemble faster and less sensitive to dirt or damage.
Higher strength and better vibration resistance. Used in performance automotive parts.
After you identify the bolt diameter and thread pitch, the next check is usually the tool or the hole. Use the Wrench Size chart to match common metric or SAE hex head sizes, and use the tap drill size reference before cutting internal threads.
Tapping a threaded hole after identifying the bolt size? Use the tap drill size reference to confirm the exact #36, #29, #7, or letter drill before cutting threads. Do not round to the nearest fraction until you know the thread standard, material, and required thread percentage.
For machine screws or wood screws rather than bolts, compare diameter and thread references in the Screw Size chart.
Which bolt system and strength grade should you use?
Modern vehicles use Metric (M6, M8, M10, M12). Vintage US cars require SAE. Always match the thread pitch to avoid stripping fasteners.
UNC Coarse threads are preferred for wood framing and deck building due to their speed and durability on site.
Socket Head Cap Screws (Allen) are common in tight industrial spaces. High-strength Grade 12.9 is often required.
Complete your workshop reference collection with these printable guides.
| 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 chart