Introduction
It is specifically used for "old drawings" that followed a 2-class system (1st class, 2nd class) rather than the newer f, m, and c classes introduced in standard MSF1-2021. Tolerance Table (Machine Cut) Ms81-2021 Standard Pdf
Tolerance Classes: The standard typically categorizes precision into three main classes: f (Fine): For high-precision components. Introduction It is specifically used for "old drawings"
Warning: As of 2026, several unofficial “free PDF” sites distribute corrupted, incomplete, or pre-release drafts of MS81-2021. Using these can lead to catastrophic equipment failures. Using these can lead to catastrophic equipment failures
The 2021 revision of the standard (implying an evolution from previous iterations) reflects the advancement of metallurgy and the changing demands of the global supply chain. It likely introduces more stringent testing for hydrogen embrittlement—a phenomenon where hydrogen atoms infiltrate the metal lattice, causing it to become brittle and crack unexpectedly. By codifying the detection and prevention of such microscopic flaws, the standard acts as a barrier against catastrophic failure. It is a shield that operates at the molecular level.
: Drawings that followed previous internal tolerance classes (such as 1st class or 2nd class) are directed to follow Annex MS81-2021 Classification : In general, Class 1 tolerances
Leakage Limits: It defines strict "allowable leakage" rates. For example, a 12-inch metal-seated valve is allowed up to 480 cc of water leakage per minute at 40 psi—anything more, and it fails the standard.