You can ignore the head style when shopping for airframe bolts. Most airplanes use thousands or even hundreds of thousands of bolts in their construction. Along with rivets, airframe bolts are one of the most common. They are designed to hold structures together. Airframe bolts, however, are available in several different head styles, including the following.
Hex
Hex head airframe bolts are characterized by a hexagonal head. They feature a solid head with six equally spaced sides.
You can tighten or loosen hex head airframe bolts using a standard wrench or socket. The wrench or socket will fit over the hexagonal-shaped head. Turning the tool will then tighten or loosen the bolt.
Hex head bolts are commonly used in fastening applications involving high tensile loads, such as wing attachments and landing gear components. With their hexagonal-shaped head, they can resist forces that try to pull them apart.
Internal Wrenching
Airframe bolts with an internal wrenching head feature a hole at the top-center of the head. Also known as hex socket airframe bolts, they are typically installed using a hex key. The hole is typically six-sided. Hex keys, of course, are also six-sided. Therefore, placing a hex key inside of the hole and turning it clockwise will drive the internal wrenching bolt forward.
Internal wrenching bolts are commonly used in fastening applications where space is limited. They still require a tool to install, but hex keys are typically easier to use in small spaces than traditional wrenches and sockets.
Countersunk
There are also airframe bolts with a countersunk head. As the name suggests, they feature a countersunk design that allows them to sit flush with the surface in which they are installed. This countersunk design eliminates protrusions that could otherwise disrupt airflow or interfere with adjacent components.
Most countersunk bolts have a recessed head — similar to internal wrenching bolts. You can install them by placing the appropriate tool inside of the recess and turning the tool. The main difference between internal wrenching bolts and countersunk bolts is that the former protrudes out of the surface, whereas the latter sits flush with the surface.
In Conclusion
While you can find airframe bolts in a wide range of head styles, some of the most common are hex, internal wrenching and countersunk. Hex head bolts have a hexagonal head with six sides. Internal wrenching bolts have a recess in the top-center of the head. Countersunk bolts have an angled head that, when driven into an object, allows them to sit flush with the object’s surface.
