In Part 1 of our Threads Series, we provided some terminology and explained some of the nomenclatures of unified inch series threads. Now, in Part 2, we’re going to be talking about the difference between 2A and 3A threads.

2A and 3A: What’s the Difference?

The terms “2A” and “3A” refer to classes of fit for external unified inch series threads on screws, bolts, and studs. Internal threads, such as those found in nuts or tapped holes, have thread fit classes termed “2B” and “3B”. The term “Thread Fit” is defined as the measure of the looseness or tightness between mating threads when an externally threaded fastener is assembled into an internally threaded hole or nut. 2A/2B and 3A/3B mating parts are designed to fit together to allow free-running assembly with no interference. 3A/3B is a tighter fit than 2A/2B.

Thread fits are developed using allowances and tolerances. An allowance is an intentional clearance between mating threads. Allowances are applied to external threads. The major, pitch, and minor diameter maximums are less than the basic size by the amount of the allowance. Tolerances are specified amounts by which dimensions are permitted to vary for manufacturing convenience. Tolerance is the difference between the maximum and minimum permitted limits for a given dimension. Class 2A fit has allowance and tolerance. Class 3A fit, however, has only tolerance, creating a tighter fit. The two diagrams below should nicely illustrate allowance and tolerance.

Diagrams of 2A Thread and 3A Thread showing Tolerance and Allowance


Here’s an Example

Let’s look at a specific example. The difference between class 2A and 3A external threads is shown for 5/8-18 UNF parts. Note that for class 2A, both the major and pitch diameter maximums are below their respective basic values by the 0.0014 inch allowance. The allowance is 30% of the class 2A tolerance. For class 3A, the major and pitch diameter maximums are at the basic size. Also, the pitch diameter tolerance for class 3A is 0.0035 inches, which is smaller than the class 2A tolerance of 0.0047 inches. See figures 4 and 5.

Diagrams of 5/8-18 2A and 3A ThreadsA Side-by-Side Comparison

Now that we’ve examined an example of external threads, we are going to look at an entire mated thread system. Keeping the 5/8-18 example, let’s look at a side-by-side comparison of the complete clearance. The complete clearance consists of the allowance (if any) for the external thread and tolerances for the external and internal mating threads. See Figure 6 below for a side-by-side comparison of 5/8-18 UNF 2A/2B and 3A/3B pitch diameter. The 3A/3B thread fit has no allowance and smaller tolerances than the class 2A/2B thread fit, resulting in a tighter fit.

Side by Side comparison of 2A/2B and 3A/3B threadsIn addition to the 2A/2B and 3A/3B thread fits, there is one other thread fit for Unified Inch series threads. It is class 1A/1B. This is the loosest fit. However, our company does not manufacture 1A/1B parts, so we will not be talking about them. We have tried to provide some basic facts about Unified Inch series threads here. For more information, we recommend referring to the ANSI/ASME B1.1 standard. Our Threads Series continues in Part 3, where we provide a brief overview of metric series threads.

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