There are three established Classes of Thread, designated in the Unified series by adding “A” for screws and “B” for nuts (or other internal threads) to show definite limits and tolerances.
Class lB Thread is that in which a 1 A screw can be run in readily for quick and easy assembly. The hole is classified as 1 B. The fit is 1 B Thread, and rarely used in modern metalworking.

Class 2B Thread consists of a 2A screw in a 2B hole. This 2B Thread has wide application, accommodates plating, finishes, and coating to a limited extent and, therefore, has fair tolerance allowances.
Class 3B Thread means a 3A Screw in a 3B Nut or threaded hole for applications where tolerance limits are close.

In the tables that follow, tap selections are shown for the Class of Thread desired and, under the Class of Thread heading, applicable GH Numbers are listed. 'G” designates Ground Thread. “H” means that pitch diameter is on the high side of basic. These two letters are followed by a numeral showing the tolerance of pitch diameter oversize as follows:
H1 = Basic to Plus 0.0005'
H2 = Basic Plus 0.0005' to Plus 0.0010'
H3 = Basic Plus 0.0010' to Plus 0.0015'
H4 = Basic Plus 0.0015' to Plus 0.0020'
H5 = Basic Plus 0.0020' to Plus 0.0025'
H6 = Basic Plus 0.0025' to Plus 0.0030'
H7 = Basic Plus 0.0030' to Plus 0.0035'

The diagram below, exaggerated for clarity, illustrates these several selectives in Pitch Diameter tolerance--including 'L' (undersize tolerance), although no 'L' taps are shown in this book. Pitch Diameter varies with the number of threads per inch because the number of threads of Pitch of screw determines the height of thread. Since Basic Pitch Diameter is measured from points half the height of the fully formed thread, a hole drilled to provide theoretical 50% thread engagement would be of the same diameter as the pitch diameter of the tap.

All measurements must have a controlling point or base from which to start. In the case of a screw thread, this control point is called the BASIC or theoretically correct size, which is calculated on the basis of a full form thread. Thus, on a given screw thread, we have the Basic Major Diameter, the Basic Pitch Diameter and Basic Minor Diameter.
While it is impossible in practice to form screw threads to their precise theoretical or BASIC Sizes, it is possible and practical to establish limits which the deviation must not exceed. These are called the “Maximum” and “Minimum” Limits. If the product is no smaller than the “Minimum Limit” and no larger than the “Maximum Limit,” then it is within the size limits required. This difference between the Maximum and Minimum Limits is the TOLERANCE.
In actual practice the Basic Size is not necessarily between the Maximum and Minimum Limits. In most cases, the Basic Size is one of the Limits. In general, tolerances for internal threads will be above Basic and for external threads, below Basic. See drawing below.

For graphic representation, the Basic Pitch Diameter is commonly designated by a line with variations from it indicated by shorter lines spaced to represent a numerical scale, as shown on the left half of the drawing below.
On an actual screw thread, the Basic Dimensions would follow the contour of the theoretically perfect thread, as on the right half of the drawing below.
To find the basic pitch diameter or basic minor diameter of any screw thread, subtract the constant for the number of threads per inch from the basic major diameter.

Engineers frequently receive a request for a Class 3B (or other class) tap. Many times, too, the customer will ask for a tap to produce a “Class 3B Fit”. Ordering taps by these specifications is incorrect, and often impractical. The following information is presented to clarify the difference between the terms Class of Thread, Tap Limits and Product Limits to make ordering taps easier and aid the customer in obtaining the tap best suited for his requirements.
Class of Thread refers simply to the tolerances that control the closeness of fit between two threaded mating parts. This term should be used only in reference to a threaded assembly, as, for example, a screw and nut.
Product Limits refers to the limits and tolerances of the internal or female thread. The° of tolerance is expressed by the terms Class 2, 2B, 3 and 3B. Product limits refer to the various limits and tolerances applying to nuts or internal threads and are identified by the terms Class 2, 2B, 3 or 3B.
Tap Limits refers to the various sizes of taps manufactured to permit selection of a tap which will produce an internal thread within the desired product limit. Tap limits are designated as L-i, H-1 H-2, H-3, etc.
In the chart on this page we have illustrated the difference between tap and product limit, using a 1/4-20 tapped hole as the example.
Although ground thread taps are produced to precision tolerances under closely controlled manufacturing processes and are guaranteed for accuracy of individual elements, there is always the possibility of the presence of unknown factors which can be detrimental to good tap performance. The tap manufacturer, therefore, is not able to guarantee the size of the tapped hole.

To summarize, the following points should be remembered:
1. A tap cannot produce a Class of Thread. It can produce a tapped hole within specific product limits.
2. Since it is used only in tapping a hole, or producing an internal thread, a tap has no control over the fitting properties of the mating external thread.
3. To produce what is commonly referred to as a “Class of Thread,” both the external and internal threads must be within their respective product limits. Only when both members of a threaded assembly fall within the desired class limits can the proper fit be assured.
4. The acceptability of any class of threaded hole is determined only by an accurate “Go” or “Hi” thread plug gage of the corresponding class. Similarly, the acceptability of a male part with an external thread is also determined by a corresponding “Go” or “Lo” thread ring gage.
5. When ordering taps, specify either the tap limit or the class of thread that identifies the limit of the tapped hole.
6. In special applications, supply as much information as possible. The more information our engineers have to work with, the better they can recommend a tap to do the job properly with least expense.
7. Remember, it is not always the most expensive tap which will do the job best. Our files contain many examples of special jobs which were solved with inexpensive standard taps.