For hardware structural components exported to North America, “ANSI standards” are among the most common specifications found on engineering drawings. ANSI (American National Standards Institute) does not formulate standards itself;
instead, it approves and publishes standards developed by organizations such as ASME. In the fastener sector, the core standards are the ASME B18 series, complemented by the ASME B1.1 Unified Inch Screw Thread system.
For companies targeting the US market, ANSI standards signify the “Imperial system”: dimensions are measured in inches, and threads are defined by threads per inch (TPI)—differing completely from the metric system.
This article outlines the commonly used ANSI/ASME standards.
I. Thread Basics: ASME B1.1 Unified Inch Screw Threads
ASME B1.1 specifies the thread profiles and dimensions for Unified Inch Screw Threads (UN series). It categorizes threads into Coarse (UNC), Fine (UNF), and Extra-Fine (UNEF) series, and defines tolerance classes such as 2A/2B and 3A/3B.
Example designation: “1/4-20 UNC-2A” indicates a 1/4-inch diameter, 20 threads per inch, coarse thread pitch, and an external thread with Class 2A tolerance.
Understanding this designation is a prerequisite for interpreting any ANSI fastener drawing.
II.
Bolts and Screws: ASME B18.2.1
ASME B18.2.1 covers Imperial-series square-head, hex, and heavy hex bolts, as well as hex-head screws, flange screws, and wood screws. It specifies head dimensions, width across flats, shank diameter, thread length, and tolerances—serving as the “dimensional dictionary” for Imperial hex bolts.
Heavy structural bolts are also covered by ASME B18.2.6 (which serves as the dimensional standard for A325/A490 structural bolts);
compliance with this standard is mandatory for construction projects under US and Canadian building codes.
III. Nuts, Hex Socket Products, and Machine Screws
- ASME B18.2.2: Inch-series general-purpose hex nuts, square nuts, flange nuts, and machine screw nuts, including heavy hex nut series;
- ASME B18.3: Hex socket head cap screws (Allen bolts) and other hex-socket-driven products—featuring high strength and compact heads, suitable for precision assemblies with space constraints;
- ASME B18.6.3: Inch-series machine screws, self-tapping screws, and metallic drive screws;
- ASME B18.31.3: Inch-series threaded rods (thread configurations such as UNC, UNF, 8UN, etc.);
- ASME B18.24: Part Identification Number (PIN) coding system for B18 fastener products, used for procurement and inventory management.
IV.
Four Key Differences Between the ANSI System and the Metric System
- Dimensional Units: ANSI uses inches (1 inch = 25.4 mm), while the metric system uses millimeters;
- Thread Standards: ANSI uses UNC/UNF thread profiles and TPI (threads per inch) designations, whereas the metric system uses ISO metric threads (M × pitch);
- Strength Designation: ANSI typically uses ASTM material specifications and grades (e.g., Grade 5, Grade 8), while the metric system uses property classes (e.g., Class 8.8, 10.9);
- Non-interchangeability: Inch and metric fasteners cannot be mixed—thread profiles, wrenching sizes (across-flats dimensions), and strength systems differ completely; mixing them will result in assembly failure.
V. Practical Recommendations and Trends
When handling orders from the U.S., the following recommendations apply: First, ensure that standard specifications on drawings are complete (e.g., “ASME B18.2.1 1/2-13 UNC-2A Grade 5”), including the specific version and tolerance class. Second, establish a reference database comparing Imperial and Metric specifications to facilitate quick identification during quoting and incoming material inspection.
Third, equip your facility with Imperial-standard inspection tools—such as thread gauges and calipers—and avoid using Metric tools to measure Imperial parts. Regarding trends, ASME standards are continuously updated (such as the new B18.6.3-2024 version), and ANSI is promoting the standardization of fastener coding (B18.24 PIN codes);
by embedding “standard identification” into your supply chain management processes, you can minimize dimensional errors when fulfilling North American orders.
