Views: 0 Author: Site Editor Publish Time: 2025-08-07 Origin: Site
Hex head fasteners are among the most common types of bolts and screws, used in everything from automotive repairs to construction. But choosing the right wrench size is crucial—using the wrong tool can strip the bolt, damage the tool, or even cause injuries.
In this guide, we’ll cover:
✔ Standard hex head fastener sizes and their corresponding wrench sizes
✔ Types of wrenches best suited for hex bolts
✔ How to measure hex bolts for the correct wrench fit
✔ Pro tips to avoid rounding off bolt heads
Let’s dive in.
Hex head fasteners (also called hex bolts or hex cap screws) come in standardized sizes, typically measured in imperial (inches) or metric (millimeters). The wrench size needed depends on the bolt’s "across-flats" (AF) measurement—the distance between two parallel sides of the hex head.
| Bolt Size (Diameter) | Head Size (Across Flats, inches) | Standard Wrench Size |
|---|---|---|
| #6 | 1/4" | 1/4" (6mm) |
| #8 | 9/32" | 9/32" (7mm) |
| #10 | 5/16" | 5/16" (8mm) |
| 1/4" | 7/16" | 7/16" (11mm) |
| 5/16" | 1/2" | 1/2" (13mm) |
| 3/8" | 9/16" | 9/16" (14mm) |
| 1/2" | 3/4" | 3/4" (19mm) |
| 5/8" | 15/16" | 15/16" (24mm) |
| Bolt Size (mm) | Head Size (mm, Across Flats) | Standard Wrench Size |
|---|---|---|
| M4 | 7mm | 7mm |
| M5 | 8mm | 8mm |
| M6 | 10mm | 10mm |
| M8 | 13mm | 13mm |
| M10 | 17mm | 17mm |
| M12 | 19mm | 19mm |
| M14 | 22mm | 22mm |
| M16 | 24mm | 24mm |
Note: Some metric bolts may have slightly different head sizes (e.g., M8 bolts can sometimes use a 14mm wrench). Always check the manufacturer’s specifications.
Not all wrenches are created equal. The best choice depends on accessibility, torque requirements, and bolt condition.
Pros: Double-ended (open + box end), good for tight spaces.
Best for: General maintenance, automotive work.
Pros: Fits ratchets for speed, deep sockets reach recessed bolts.
Best for: Engine bays, machinery.
Pros: Fits multiple sizes, portable.
Cons: Higher risk of rounding bolts.
Note: Some "hex head" fasteners have internal hex drives—these require an Allen wrench, not a standard open-end wrench.
If you don’t know the bolt size, follow these steps:
Use a Caliper – Measure the distance between two flat sides (across flats).
Check a Wrench Size Chart – Match the measurement to standard sizes.
Test Fit – The wrench should fit snugly without excessive play.
Warning: If the wrench slips or feels loose, you’re using the wrong size and risk stripping the bolt.
Use a 6-point socket instead of 12-point (better grip, less rounding risk).
Tap the wrench lightly with a hammer if the bolt is stuck (helps break corrosion).
Avoid cheap wrenches—low-quality steel can deform and damage bolt heads.
Apply penetrating oil (like WD-40) before loosening rusted bolts.
A: Sometimes, but it’s risky. For example, a 19mm wrench is close to 3/4" (19.05mm), but even a slight mismatch can strip the bolt.
A: Don’t force it—use the correct size or try a flank-drive socket (reduces rounding).
A: These are socket head cap screws, not standard hex bolts. They have an internal hex drive.
Using the right wrench for hex head fasteners prevents damage and makes your work easier. Always:
✅ Match the wrench size to the bolt’s across-flats measurement
✅ Prefer 6-point sockets for high-torque applications
✅ Avoid adjustable wrenches on stubborn bolts
Panda Fastener manufactures premium hex head fasteners that strictly comply with international standards (ISO, DIN, ASTM). Global buyers are welcome to request quotes for high-quality, certified fastener solutions.
