When determining the appropriate fit between two threaded components, possessing an understanding of both internal and external thread classes is essential. That’s why the team at Kozak has provided this detailed breakdown of the different thread classes, as well as their application specifications, for your review.

An Overview of Imperial Thread Class

Thread class is used to designate the amount of tolerance allowance and installation fit desired on a fastener. Imperial thread class is derived from formulas in which the pitch diameter tolerances are based on increments of the major, or nominal, diameter, the pitch, and the length of engagement of the thread.

What Are Internal and External Threads?

Internal threads refer to the threads that are located inside of a hole, such as those on a nut. On the other hand, external threads are located on the outside of the part, like those that are found on bolts and screws.

How Do Internal and External Thread Classes Interact?

When determining how each thread class interacts with another, it’s important to know the classifications of each. There are three different classes of external and internal threads, with external classes being labeled as 1A, 2A, and 3A, and internal classes being labeled as 1B, 2B, and 3B. Internal and external threads of the same class are designed to fit together, so when combining each thread of the same specification a secure connection can be made.

Mismatching the thread class, such as combining a 1A external thread with a 3B internal thread, could result in a decrease in performance and, in some cases, application failure.

How to Measure Screw Threads Before Choosing a Thread Class

Having accurate measurements for the components you are working with is essential, as incorrect measurements can lead to damaged materials, weak connections, or misaligned parts. If you’re unsure about how to measure screw thread size, you’ll be happy to learn that the process involves just a few simple steps.

• Measure the diameter – The first thing you need to do is measure the diameter of the screw. This should include the entire width of the screw and will usually be measured at its widest point across the thread.
• Identify the thread pitch – Next, you’ll want to determine the thread pitch. This can be determined by either using a thread pitch gauge or manually counting the threads. If you are using a thread pitch gauge, you’ll line up the screw threads with the notches on the gauge to find the exact match. If you are manually counting, you’ll want to count the number of threads over a specific length, usually one inch for standard screws or a few millimeters for metric screws.
• Measure the length – Lastly, you’ll measure the overall length of the screw. For screws with flat heads, the measurement starts from the top of the head and extends to the tip of the screw. For screws with different heads, you’ll measure from the top to the bottom of the pitch.

By following these steps and identifying the proper size for your thread class, you can ensure a secure fit and optimal performance in your application.

Characteristics and Applications of Each Thread Class

Let’s examine the main differences between each imperial thread class, as well as when the classes are most commonly utilized.

Classes 1A and 1B

This imperial thread class is considered the loosest fit of all three classes, meaning that it has the largest amount of clearance or play during the assembly process. With that in mind, the 1A and 1B thread class is best for applications where simple assembly and disassembly are essential.

Common applications that require a 1A and 1B thread class include stove bolts and other general types of rough bolts and nuts.

Classes 2A and 2B

Known as the most common fit grade for commercial products, the 2A and 2B thread class is perfect for applications where interchangeability, reasonable quality, and fit predictability are required. That’s why this class is mainly used in general applications, which include the production and application of bolts, nuts, screws, and other similar fasteners. 

Classes 3A and 3B

The 3A and 3B imperial thread class is specified for high-quality commercial and precision applications and products where a close or snug fit is crucial. This can include applications suited for close tolerance fasteners, such as aerospace bolts and nuts, connecting rod bolts, socket cap and set screws, as well as other high-strength fasteners.

Where Coatings Come into Play with Thread Class

Coatings can be applied to the surface of bolts, screws, nuts or other fasteners to help improve performance. They are used to protect the underlying metal from rust or corrosion. Some common coatings include:

These coatings play an important role when it comes to selecting a thread class, especially for applications where precision is essential. You can’t just randomly choose a coating and hope everything fits together. You need to pick a thread class that accounts for the coating thickness. With that being said, some things to keep in mind include:

• Follow standards such as ASTM F1941 on how to properly handle coatings
• Each coating has a different thickness
• Be sure to test that the coated bolts still fit properly with their thread class
• Sometimes a looser thread class is necessary when using a thick coating, so the bolt can still be screwed in properly

Contact Kozak Micro Adjusters for More Information

Want to know more about which imperial thread class will best accommodate your project needs? If so, feel free to reach out to our team for more information or to get started, as we look forward to assisting you!