Why this matters
A fitting without a clean mark is a fitting without traceability. MSS SP-25, "Standard Marking System for Valves, Fittings, Flanges, and Unions," is the de-facto global standard referenced by ASME B16.9, B16.5, B16.11 and most project specifications. Its current edition is ANSI/MSS SP-25-2018. Buyers who reject material at inspection because of a missing or wrong mark always have the standard on their side. This guide explains the marking standards MSS SP-25 expects, what to verify, and what to write into the PO.
Referencing the marking standards MSS SP-25 puts every party on the same page about what each character on the fitting means.
Field-by-field marking content
MSS SP-25 specifies five core marking fields for fittings and flanges. Optional fields are required when called out by the product standard.
1. Manufacturer's identification. Name, registered trademark, or facility identifier. The mark must trace back to the manufacturing site, not just a brand owner.
2. Material designation. ASTM grade and any UNS number (e.g., "A234 WPB", "A105", "A182 F316L / S31603"). Some grades require additional condition (e.g., "WPHY 65" for line-pipe fittings).
3. Pressure class / rating.
