Thread Tolerance and Fit Grade Guide for Small Screws

Thread Tolerance and Fit Grade Guide for Small Screws

Thread tolerance directly affects the running-in smoothness, clamping force and loosening resistance of small screws. Improper selection can cause assembly jamming, thread stripping or unstable torque. This article explains common tolerance classes and fit selection points.

Common Tolerance Classes

For metric external threads, 6g and 6h are common, while 6H is typical for internal threads. 6g is the most widely used class, balancing running fit and plating thickness. For thicker platings or special requirements, classes such as 5g6g or 6e can be selected.

Fit Selection Principles

Standard 6g/6H fits suit most assembly applications. For vibration environments, use thread locker or anti-loosening structures rather than tightening thread tolerance. For precision instruments or frequent disassembly, focus on pitch diameter consistency to avoid seizure.

Inspection Methods

Thread plug and ring gauges (GO/NO-GO) are the fastest on-site checks. For critical batches, thread profile measuring instruments can verify pitch diameter and flank angle. Suppliers should provide full inspection or batch inspection reports.

Jiali Hardware strictly controls small screw thread tolerances to ISO/GB standards, providing GO/NO-GO full inspection and test reports for stable batch assembly.