An 8mm hex key fits fasteners with an internal 8mm hex-shaped recess—often called socket head or cap screws. You’ll see this size on automotive parts, machinery, furniture, and some bike components.

• Use with hand tools: A standard L‑shaped hex key or T‑handle in 8mm works for most jobs.
• Use with ratchets: An 8mm hex bit socket lets you drive the same fastener using a 1/4", 3/8", or 1/2" ratchet, as shown on this page.
• Tips: Fully seat the key, keep the recess clean, and apply steady pressure. For power tools, choose impact‑rated hex bit sockets where appropriate.

The closest SAE size to 8mm is 5/16 inch. In decimals, 8mm is about 0.315", while 5/16" is 0.3125"—a small but real difference.

• Fit considerations: On a clean, undamaged fastener, 5/16" may feel close. However, that slight gap can increase the chance of rounding, especially under higher torque.
• Best practice: When possible, use a true 8mm socket or hex bit for a snug fit.
• Context: Many socket sets include both metric and SAE sizes, so it’s convenient to match the fastener precisely and reduce wear.

Results can vary based on tool tolerances and fastener condition.

It depends on the fastener type you’re working with:

• External hex nut/bolt: A metric 8mm box‑end or combination wrench is a good alternative. 5/16" is the closest SAE size, but use it cautiously to avoid rounding.
• Internal hex (socket head screw): Use an 8mm hex key or an 8mm hex bit socket with a ratchet.
• Nut drivers: An 8mm nut driver can be handy for lighter‑duty tasks and tight spaces.

If you’re unsure, measure the fastener across the flats to confirm size. Matching the metric size typically offers the most secure fit and can help reduce the risk of damage.

An 8mm socket is sized to fit fasteners that measure 8mm across the flats (about 0.315 inches). That measurement refers to the fastener, not the socket’s outer diameter.

• Outer size varies: Wall thickness, drive size (1/4", 3/8", 1/2"), and whether it’s shallow or deep all affect the socket’s outside diameter and length.
• Different styles: Thin‑wall sockets help in tight spaces; impact sockets are heavier‑duty; hex bit sockets include an 8mm bit for internal hex screws.

Choosing the right style and drive can help with clearance and torque needs while keeping a secure fit on the fastener.

8mm sockets are commonly offered in 1/4", 3/8", and 1/2" drives. The best choice depends on access and torque needs:

• 1/4" drive: Compact for tight spaces and lighter‑duty work on small fasteners.
• 3/8" drive: A versatile, everyday choice with a good balance of strength and size for general maintenance.
• 1/2" drive: Useful when higher torque is expected or when using impact tools; typically bulkier.

You can use adapters if needed, but they can add flex and affect clearance. If you’re using power or impact tools, select sockets rated for that use to help manage wear and reduce breakage risks.