The Unified system's allowance and tolerance designation controlling thread fit.
A is external, B is internal; 1 is loose, 2 is standard, 3 is tight. The common pair is 2A bolt with 2B nut, which assembles freely and is used for almost everything.
Class 3 has no allowance — the parts are made to nominal and assemble only if both are near the middle of their tolerance zones. It is specified for close-tolerance work such as aerospace fasteners and instrumentation, and it costs more because both parts must be made accurately.
Specifying class 3 "for quality". It buys tighter fit, not better quality, and it makes assembly harder and the parts more expensive.
This site uses Google Analytics to count visits and see which pages are useful. It sets cookies only if you agree. See the privacy policy.