Fully and Partially Threaded Metric Bolts: Which is Right for Your Job?
It’s easy to get confused when deciding whether to use a fully or partially threaded metric bolt for a particular assembly. Often, people simply use whatever bolt is available—assuming that if it fits, it’s probably close enough. However, it’s important to bear in mind that the size of a bolt hole is just one small part of the overall engineering that goes into a completed assembly.
How a bolt grips and how loads are distributed by the bolt, are critical factors in the strength of a union. Bolts grip and distribute differently, depending on variables such as thread pitch, head size and, of course, whether the bolt is fully or partially threaded. To ensure proper grip and resistance to breakage, it’s important to use a bolt that meets the original engineering specs.
Full vs. Partial Threads
To understand how these bolts grip differently, it’s important to understand the basic differences in their designs.
- Fully Threaded Metric Bolts: These bolts have threads running the full length of the bolt, from the underside of the head to the tip.
- Partially Threaded Metric Bolts: Starting at the underside of the head, these bolts have an area that is free of threads. Called the “grip length,” this area is the full thickness of the bolt shank, and will vary in length according to the recommended application of the bolt.
Fully Threaded Applications
Fully threaded bolts are typically found in assemblies where grip strength is more critical than alignment or shear strength. Think of body panels on a vehicle: the body panels themselves have mounting holes that are larger than the associated holes in the vehicle frame. This allows the body panel to be positioned for the best appearance.
A fully threaded fastener is then inserted through the mounting hole, into the fully threaded hole in the frame. Since the bolt is fully threaded, mounting tension is exerted along its full length, generating extreme holding pressure at the point where the bolt head meets the body panel. In this assembly, the bolt strongly resists loosening, and shearing isn’t a concern, since the body panel would fail before the bolt.
Partially Threaded Applications
For critical alignment and shear resistance, partially threaded bolts are the way to go. These may be installed in threaded holes, or in bolt-through applications, using a washer and nut on the threaded end. This type of assembly is often found in things like alternators, water pumps, and motor mounts.
One example is an alternator mounting bracket:
- A metal bracket is placed against an engine block, and a partially threaded bolt is then tightened into a threaded hole in the block.
- The grip length on the bolt forces the bracket into perfect alignment with the mounting hole in the block.
- As the bolt is tightened, the threads pull the grip length through the bracket, creating extreme holding force where the underside of the bolt head meets the face of the bracket.
In this installation, a strong holding force is achieved, along with extreme shear resistance. Where the bracket meets the mounting surface, the full diameter of the shank is present in the mounting hole. Since there are no threads along this grip length, there are no weak spots in the shank. In order to shear, the bolt would have to fail at its thickest, strongest point.
The Right Metric Bolt in the Right Place
It is easy to find a bolt that will fit in a hole. But, is it the right metric bolt for the application? Size is just one consideration. To ensure a secure assembly, you need to account for all of the factors that went into engineering that bolt hole.