Editing Unified Thread Standard (section)

==Tolerance classes==
A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the [[Engineering fit|fits]] used with assembled parts.

*Class 1 threads are loose fit, intended for ease of assembly or use in a dirty environment.
*Class 2 threads are free fit, and the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.
*Class 3 threads are medium fit, still quite common and used for closer tolerances on high quality work.
*Class 4 threads previously designated a close fit for even tighter tolerances, but this classification is now obsolete.
*Class 5 fit is an [[Interfering thread nut|interference thread]], requiring the use of a wrench for turning. These can be seen in applications like spring shackles on an automobile.

The letter suffix "A" or "B" denotes whether the threads are external or internal, respectively. Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.<ref name="AFT Fasteners">{{cite web |author= AFT Fasteners & Industrial Supply |title= Thread Terminology |url=https://www.aftfasteners.com/content/Technical%20Guides/Reference_Thread%20Terminology.pdf |website= Fastener Reference Guides |access-date= 2024-12-24 }}</ref>

Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above. There are several methods that are used to measure the pitch diameter. The most common method used in production is by way of a [[go/no go gauge|go/no-go gauge]].