Screw size and fractional threads are specified as one of three classes of fit (1B, 2B, 3B), while metric threads use classifications such as 4H and 6H. Class 2B for fractional threads and 6H for metric threads are the most common general threading applications. H limits for fractional threads are in increments of 0.0005 inches, while metric thread limits equal 0.013mm (0.00051 inches), making them extremely similar.

What are the three classes of fit for UNC/UNF threads?

Screw size and fractional threads are typically specified as one of three classes of fit:

• Class 1B: For low precision, or threads typically used in areas where dirt and grime are a constant factor.
• Class 2B: For general threading applications (by far the most common).
• Class 3B: For precision threads generally found in medical, aerospace, and applicable automotive applications.

These classes of fit do overlap even though they are progressive in accuracy.

What are H limits in UNC/UNF threads?

H limits are increments of 0.0005 inches that further break down the accuracy of threads within each class of fit. Every size/pitch tap has a specific or given basic pitch diameter that is the basis for the H limits and the class of fit for that size. The H limits help you understand what area of the class of fit you are actually working within.

How do pitch diameter limits work in UNC/UNF threads?

The class of fit gives you minimum and maximum pitch diameter limits that need to be maintained during manufacturing. These are typically your thread gauge limits. By seeing the H limits illustrated, you are better able to understand what area of the class of fit you are actually working within.

What are the metric thread classes of fit?

Metric threads have their own classifications for class fit and thread limits. When discussing UNC/UNF thread limits, we work with 3B, 2B, and 1B classes of fit and have H limits which help define accuracy within each class.

A metric 6H class of fit is quite similar to the fractional 2B class of fit, as is the metric 4H to the fractional 3B. The 6H class of fit is the standard offering in metric sizes. Special classes of fit can be produced for any given application upon request.

What are the tolerance increments for metric threads?

Within the class of fit, metric thread limits equal 0.013mm (0.013mm = 0.00051 inches), which is extremely similar to that of fractional H limit tolerance. The class of fit gives you maximum and minimum pitch diameter limits that need to be maintained during manufacturing.

What are D limits in metric threads?

D limits help control what area of the class of fit you are working within for metric threads. The D limits function similarly to H limits in fractional threads, providing incremental control within the specified class of fit.

Source and updates

Source: taylortool.com engineering data. Reviewed September 2026.