Hex keys, also called Allen keys or Allen wrenches, are used for internal-drive hexagonal bolts on engine covers, brake calipers, and aftermarket parts. The L-shape gives you two options: use the long side for deep reach, or the short side for high leverage. Many hex keys feature a ball end on the long arm, allowing you to turn the fastener at an angle of up to 25 degrees. Always select the correct metric or SAE size, as using a close-enough size will easily strip the soft internal socket.
Mechanic Glossary
Hex Key
An L-shaped tool with a six-sided hexagonal tip used to turn hex screws and bolts.
Frequently Asked Questions
No. While a 5mm key might feel close to a 3/16-inch bolt, it will have play. Applying force will strip the internal socket, requiring an extractor to remove the bolt.
The tip is rounded into a ball shape, allowing the key to slide into the bolt head at an angle. This is perfect for bolts that are blocked by other parts.
You can drive a slightly larger Torx key into the stripped hex socket with a hammer, or use a screw extractor to grab and turn the bolt out.