Unified screw-thread fit classes: Which class of fit is generally considered high quality and used for the bulk of interchangeable screw thread work in standard practice?

Introduction / Context:
Thread classes define tolerance and allowance, balancing ease of assembly with accuracy and strength. Selecting the right class supports manufacturability and reliable interchangeability across suppliers.

Given Data / Assumptions:

• We refer to Unified thread classes (for example, 1A/1B, 2A/2B, 3A/3B).
• The focus is on typical, interchangeable production hardware.
• We seek the widely used “workhorse” class.

Concept / Approach:

Class 2 fits (2A external, 2B internal) are standard for general-purpose, high-quality, interchangeable threads. Class 1 is loose for easy assembly; Class 3 is precision/tight tolerance for specialized applications. Class 4 is not used in modern Unified practice.

Step-by-Step Solution:

Verification / Alternative check:

Standards and handbooks list 2A/2B as normal production fits found in commercial nuts, bolts, and tapped holes.

Why Other Options Are Wrong:

Class 1: too loose for most quality interchangeable applications.

Class 3: too tight for general use; increased cost and assembly difficulty.

Class 4: obsolete/not standard in current Unified practice.

Common Pitfalls:

Specifying Class 3 without considering manufacturing variability, which can cause galling or assembly issues.

Final Answer:

Class 2