Fastener Threading Specifications

The threads are obviously an important element of the threaded fastener. They give this sturdy, industrial product its unique ability to be installed, removed, and reinstalled as many times as we wish. They also affect fastener performance in a major way. Thread type, thread class, thread configuration, the way in which the threads are produced, and the fit between male and female threads can affect not only thread strength and therefore fastener tensile strength but also the resistance of the fastener to such things as self-loosening and fatigue. The amount of preload achieved for a given torque can be influenced by thread configuration and by whether the threads have been cut or rolled. All things considered, it's worthwhile to take a close look at threads.
Metric threads are identified by the code letters M and MJ. The basic geometries of metric and inch series threads are identical, but the way we define metric threads differs from the way we define the inch series threads.
The Whitworth thread, which had a 55° included angle instead of the now universal 60°, was for decades a British standard form.
Inc Series Thread Forms describe the basic Unified Thread form, identified by the code letters UN, UNR, UNJ. Altogether, there are eight constant-pitch thread series; 4, 6, 8, 12, 16, 20, 28, and 32 threads per inch. These are all standard for the UN and UNR forms, but only the constant- pitch series of 8, 12, and 16 threads per inch are standard for the UNJ form. UNF, UNRF, and UNJF all designate "fine-pitch" threads. UNEF, UNREF, and UNJEF designate "extra fine" threads. UNS, UNRS, or UNJS have pitch diameter combinations not found in any of the standard series above.

    1. METRIC
    2. UNIFIED
      1. UN
      2. UNJ
    3. ACME

Thread Class & Allowance

Unified Threads

Three basic fits or "classes" are defined for Unified threads. These are given the codes 1A, 2A, and 3A for male threads and 1B, 2B, and 3B for female threads. The pair 1A and 1B define the loosest fit; 3A and 3B define the tightest.

  • Class 1A/1B threads are used for rough work, for example, where some thread damage can be expected or conditions are very dirty.
  • Class 2A/2B threads are for general use.
  • Class 3A/3B fasteners are used for applications requiring an extra degree of precision.
  • Class 1A and 2A threads are assigned the same allowance in the Unified system, but the allowance in Class 2A threads may be used for plating.
  • Class 2A threads that may not use the plating allowance are designated as Class 2AG.
  • Classes 1A and 1B have more generous tolerances than Classes 2A and 2B.
  • Class 3A/3B threads are assigned a "zero allowance," so the fit can be line-to-line. A small tolerance on each thread makes assembly possible. Internal threads (Classes 1B, 2B, and 3B) have no allowances.

Metric Threads

The clearance between male and female metric threads is also determined by a basic allowance and by tolerances in the direction of less material. The number of tolerance and allowance options is greater with metric threads than with inch series threads, and different names are used to describe them. The design clearance between a thread and its basic profile, called the allowance in inch series threads, is called the tolerance position for metric threads and is identified by a letter symbol:

  • For Internal threads: G or H
  • For External threads: e,f,g,or h
  • H and h define zero allowance, no fundamental deviation from the basic profile.
  • G and g provide a small allowance that makes the thread ridges a little less fat and may be used to accommodate plating or perhaps to provide a looser fit.
  • For general-purpose M threads, the internal threads have been standardized with no allowance (H) and the external threads with a small allowance (g), similar to standard UN inch threads.
  • The MJ threads used in aerospace applications are designed with no allowance for either internal or external threads (H/h), similar to aerospace UNJ inch threads.
  • The larger external thread fits provided by positions f and e are not used except in special cases.