ASTM A194 2H.
ASTM A194 2H / 2HM & ASTM A194M 2H /2HM.
The ASTM A194 specification covers carbon, alloy, and stainless steel nuts intended for use in high-pressure and/or high-temperature service. Unless otherwise specified, the American National Standard Heavy Hex Series (ANSI B 18.2.2) shall be used.
Nuts up to and including 1-inch nominal size shall be UNC Series Class 2B fit.
Nuts over 1-inch nominal size shall be either UNC Series Class 2B fit or 8 UN Series Class 2B fit.
High-strength ASTM A194 grade 2H nuts are common in the marketplace and are often substituted for ASTM A563 grade DH nuts due to the limited availability of DH nuts in certain diameters and finishes.