Note: The ASTM document describing Anchor Bolt manufacturing standards is F-1554. The current version is F 1554-07a.
Regarding thread class, quoting directly from ASTM 1554-07a, "Class 2A shall be furnished when the class is not specified."
Typically the prints we see spell out all relevant criteria for bolt manufacturing but rarely mention threading class. As shown above F1554-07a takes care of that but what does 2A mean?
In bolt threading the externally threaded item (the bolt) is the A class. The internally threaded nut is the B class. Class indicates tolerance. Class 1 is the loosest fit. It is generally designed for applications where a fast installation is desired and/or the working environment would make tighter fits impractical. An example of the would be for bolts securing concrete forms. Often All Threaded Rod is manufactured to a 1A tolerance.
Class 2 fit is the general construction industry standard. It's anchor bolt tolerance is 32% tighter than the 1A fit. This allows for a more secure fit yet still provides enough leeway to accomodate field conditions such as damage or grit in the threads. Properly done, an anchor bolt threaded to a 2A fit can have the usual rust and light contaminents on the threads and still have the nut thread by hand until it bottoms out.
The Class 3A is used for machine bolts or other applications where a tighter fit is required. Usually this means the working environment will not create problems in the thread fit, and installation time is not an issue. Tolerance is reduced by 25% compared to the 2A fit.
The chart to the left (fig. 1) displays how the desired fit is achieved. As the Class goes up the tolerance decreases. Most of the decrease comes from raising the lower tolerance. With the matching nut the tighter fit is achieved by lowering the upper tolerance.
For addition information regarding F1554 Anchor Bolts;