Thread classes are specified to designate the amount of tolerance allowance and installation fit desired. Thread classes are derived from formulas which the pitch diameter tolerances are based on increments of the major(nominal)diameter, the pitch, and the length of engagement of the thread. These formulas and the class identification only apply to unified threads series.
There are three classes of external(1A, 2A, and 3A)and internal threads(1B,2B,and 3B).The characteristics and typical application of each class are as follows:
Classes IA and 1B: These classes are the loosest fit, in that,the largest amount play or clearance in assembly.These are used for work where ease of assembly and disassembly is most important, such as stove bolts and other general rough bolts and nuts.
Classes 2A and 2B: These classes are specified for the typical fit grads of commercial products, such as machine screws and fasteners, where interchangeability and reasonable quality and fit predictability is required. These classes are the most commonly used for commercial and aerospace applications.
Classes 2AG: These application is specified to allow for rapid assembly, and to permit the application of a lubricant or for residual growth due to high-temperature expansion. In these applications, the G is specified to indicate, that when the thread is coated, the thread tolerance and size may not be exceeded by such coating.
Clashses 3A and 3B: These classes are specified for high quality commercial, precision applications and products, such as machine screws, were a close or snug fit is required.
Thread spcification examples；
3/8-16 UNC 2A; 8-32 UNC 2A; 1/4-20UNC 2B; 5/16-18UNC 2B;
8-36*2 UNF 2A Titanium Allied Head Screws;
5/16-18*1 UNC 2B Titanium Socket Head Screws;
5/16-24*3/4 UNF 2A Titanium Hex Flange Bolts;