Fully & Partially Threaded Metric Bolts: Which One to Choose?

Choosing the right type of metric bolt between a fully threaded and partially threaded metric bolt is quite a tricky task. It entirely depends on the application or assembly for which the bolt is required. Most people just consider the size of the hole while selecting a bolt. However, the hole size is only one of the factors, and one cannot decide the type of bolt just by knowing the size of the bolt hole. This post will discuss in detail the two different types of metric bolts – fully threaded and partially threaded. We will also discuss their applications.

What are Fully Threaded Metric Bolts?

As the name implies, fully threaded metric bolts have threads that run the full length from bottom to top. These types of bolts are used for those assemblies, where grip strength is more important compared to shear strength and alignment.

What are the Applications of Fully Threaded Metric Bolts?

Fully threaded metric bolts are widely used in the automobile industry. Check out the body panels of a vehicle. They have mounting holes of larger size as compared to that of the holes associated. The main advantage of using these bolts is that the body panel can be positioned in a way to provide best appearance. The vehicle frame has a fully threaded hole specially designed for these metric bolts. Since these bolts have threads running from bottom to top of the bolt’s length, the mounting pressure is distributed throughout the entire length of the bolt. This further helps in producing a good holding pressure at the point where the underside of the head of the bolt meets the panel. Another advantage of these types of bolts is that they resist loosening.

What are Partially Threaded Metric Bolts?

Unlike the fully threaded metric bolts, these fasteners have partial threads. The threads start from the bottom of their length. The threads do not run throughout the length, but the bolt has an area without threads. This threadless area is referred to as the grip length. The length of this area varies depending upon the application for which the bolt is used.

Where are Partially Threaded Metric Bolts Used?

These types of metric fasteners are of great help when alignment and shear strength are more important compared to the grip strength. You can use nuts and washers on the threaded end of these bolts. Partially threaded metric bolts can be used in bolt-through applications or threaded holes. These types of bolts are widely used in applications such as motor mounts, water pumps, alternators, etc.

The example below explains how the partially threaded metric bolts are used in alternators:

• An engine block with a threaded hole is taken. A metal bracket is placed on the block. The bracket is tightened on the engine block with the help of a partially threaded metric fastener.
• The grip length of the bolt allows the bracket to be fixed with perfect alignment with the mounting hole in the engine block.
• On tightening the fastener, the grip length is pulled towards the metal bracket. This creates a good holding force at the point where the face of the bracket meets the underside of the bolt’s head.

So, as a final point, you should select the type of bolts based on the application. There are a number of firms that can design both fully, as well as partially threaded metric bolts for your applications. Norwood Screw Machine is one of the leading manufacturers of different types of screws, bolts, bushings, and nuts, and has a huge experience of more than five decades in the field.