The primary use of thread tolerance is in coated threads. The metric thread tolerance standard (ISO 965) identifies thread coating as a key reason for establishing thread tolerance. This article provides a detailed explanation of tolerance classes for both metric and unified thread standard (UTS) threads.7

Thread designations include an additional combination of letters and numbers following the standard designation to specify thread tolerance. For instance, a typical designation is M10 x 1, where the letter M signifies a metric thread, 10 represents the nominal diameter in millimeters, and x 1 indicates a thread pitch of 1 millimeter. 

The structure of the thread tolerance designation adheres to these guidelines:

• Letters: Uppercase letters denote internal threads in the metric thread tolerance system, while lowercase letters are used for external threads. These letters also reflect the tolerance position; for UTS threads, A indicates external threads and B indicates internal.

• Numbers: These indicate the degree of tolerance.

• Sequence: The tolerance for pitch diameter is listed before that of the major diameter.

Tolerance Letter Designation:

For metric threads, the letters AH are used within the tolerance system. Internal threads exclusively utilize G and H, whereas external threads use ah. As illustrated, H indicates a tolerance of zero deviation from the base profile, while G signifies a tolerance for deviations above the base profile. Additionally, h represents zero deviation for external threads, with ga denoting deviations below the theoretical thread profile, known as the base profile. External thread dimensions must be equal to or smaller than the base profile, whereas internal thread dimensions must always be greater than or equal to it.

Figure 2: Metric deviation of the internal thread tolerance positions H (zero deviation) and G (positive deviation).

Figure 3: Metric external thread tolerance deviation positions h (zero deviation) and ga (negative deviations).

Table 1: Examples of fundamental deviations for internal and external threads.

UTS threads

Only two letters are used to designate the UTS thread tolerance, A and B. The letter A stands for external thread and the letter B for internal thread.

Indication of the degree of tolerance

Metric thread

Metric threads use numbers 3-9 to indicate tolerance class. The higher the number, the looser the tolerance. There are four groups of tolerance classes:

• Internal thread diameter: 4, 5, 6, 7, 8
• External thread outside diameter: 4, 6, 8
• Diameter internal thread pitch: 4, 5, 6, 7, 8
• Diameter external thread pitch: 3, 4, 5, 6, 7, 8, 9

The following tables are examples of these four tolerance classes. See ISO 965 for the complete tables.

Table 2: Tolerances on external thread outside diameter

Table 3: Tolerances on the internal thread outside diameter

Table 4: Tolerances on the external thread pitch diameter

Table 5: Tolerances on the internal thread pitch diameter

UTS Threads

The Unified Thread Standard (UTS) includes five classes, akin to the ISO classes, but only three are commonly utilized:

• Class 1: Designed for loose thread tolerances, allowing easy use even in dirty conditions.
• Class 2: The most frequently utilized class, which optimizes strength.
• Class 3: Provides an average fit suitable for high-quality applications.

Classes 4 and 5 are less relevant today; Class 4 is theoretical and has become obsolete due to its overlap with Class 3, while Class 5 denotes interference threads that require a wrench for tightening and are intended for permanent or semi-permanent applications.

Sequence of Tolerances

Metric Thread

For tolerance designations related to thread fits (both external and internal), the internal designation precedes the external one, separated by a slash. For instance, in the designation 5H/5g6g, the pitch diameter is first, followed by two tolerance designations for either the external or internal thread. Here, 5g indicates that the pitch diameter class is 5g, while 6g indicates the tolerance class for the major diameter.

When a single tolerance designation applies to both external and internal threads, the pitch diameter and nominal diameter will be the same. For example, 5H indicates that the tolerance for the pitch diameter of the internal thread is 5H, which also applies to the minimum diameter.

UTS Threads

For UTS thread tolerance designations, the format is simpler, consisting of a number followed by a letter. Example designations include 1B or 2A.

Recommended Tolerance Classes

ISO 965 provides specific guidelines for tolerance classes based on thread fineness and engagement length. Factors influencing these recommendations include:

• Thread size: Fine, medium, and coarse.
• Thread engagement length: Short, normal, and long.

The classification of engagement length is determined by the outside diameters and pitches of both external and internal threads, with ISO 965 offering more details on these factors. For general applications, ISO 965 recommends using tolerance classes 6H and 6g, or 6H/6g for medium, normal threads.

The UTS wire standard suggests tolerance classes tailored to specific applications: Class 1 for loose requirement scenarios, Class 2 for general use, and Class 3 for high-precision applications.