ASME B18.2.4.1M-2002 — Metric Hex Nuts, Style 1

28 10 月, 2026
ASME Standards

🔩 ASME B18.2.4.1M-2002 provides complete general and dimensional data for metric hex nuts, Style 1, the most widely used regular-height hex nut in the ASME B18 metric fastener series. Revised from ANSI B18.2.4.1M-1979 (R1995), the Standard was approved by the American National Standards Institute on January 24, 2002, and issued on April 1, 2002. Style 1 hex nuts mate with metric hex bolts and screws across industries from general machinery to automotive and heavy equipment.

💡 “Style 1” is the basic member of the metric hex nut family: it has a regular height and suits most applications. For the same nominal diameter, Style 2 (heavy) and jam (thin) nuts are also defined — select by load and space constraints.

1. Scope 📌

This Standard covers the complete general and dimensional data for metric hex nuts, Style 1, recognized as an American National Standard. Its principal content includes:

Dimensions: dimensional data in Table 1 for each nominal diameter.
Width across flats: measured perpendicular to the nut axis between two opposite wrenching flats.
Thickness: the overall distance measured parallel to the axis from the top of the nut to the plane of the bearing surface.
Geometric requirements on tops and bearing surfaces, corner fill, true position of the hexagon to the tapped hole, and countersink.
Threads: thread series, tolerance class, thread gaging, and overtapped threads.
Material and mechanical properties for steel, corrosion-resistant steels, and nonferrous metals.
Finish, identification symbols, inspection and quality assurance, designation, options, and workmanship.

⚠️ As with all B18 product standards, the dimensional data do not imply that every size is a stock production item — consult manufacturers for stock lists; government purchases must also meet the standard-item requirements of the applicable appendix.

2. Coordination With ISO Standards 🌐

This Standard is coordinated with the corresponding ISO metric hex nut standards (ISO 4032 covers Style 1 nuts). Dimensions are stated in millimeters, letter symbols follow ISO usage, and the tolerance and acceptance systems are aligned so that nuts made to this Standard are functionally interchangeable with bolts made to ISO standards.

Feature	Requirement	Remark
Thread series	Metric coarse/fine threads	Per ASME B1.13M
Tolerance class	6H primarily (6G optional)	Internal thread tolerance
Width across flats	Standard hex series (mm)	Common wrench compatibility
Thickness	Regular height (Style 1)	Matches bolt strength grade
Material	Carbon/alloy steel, stainless, nonferrous	Grouped by property class
Thread gaging	Per ASME B1.2/B1.3	Ensures assemblability

3. Dimensions and Geometry 🔧

The Standard sets clear geometric requirements for Style 1 hex nuts:

Width across flats: measured between opposite wrenching flats, perpendicular to the axis.
Thickness: measured parallel to the axis from the top of the nut to the plane of the bearing circle, excluding raised identification marks.
Top: flat with chamfered corners; chamfer-circle diameter equals maximum across flats within a tolerance of minus 15%.
Corner fill and countersink: ensure smooth wrench engagement without sharp burrs.
True position of hexagon to tapped hole: ensures centering in the wrench and prevents eccentric loading.
Overtapped threads: permitted only within the specified limits to preserve joint strength.

🚨 Nut thickness directly controls joint strength: an undersized nut can shear the threads. Always match the nut’s property class to the bolt’s strength grade — never pair a low-grade nut with a high-grade bolt.

4. Materials, Mechanical Properties, and Marking 📏

Mechanical properties are specified for three material categories:

Steel: hardness and proof load per metric property classes (e.g., 8, 10); carbon and alloy steel nuts carry the manufacturer’s mark.
Corrosion-resistant steels: property classes per the corresponding metric material standards (e.g., ISO 3506), marked with property class and manufacturer.
Nonferrous metals: grades and properties per standards such as ASTM F 467M.

For identification, the Standard requires source marking; carbon and alloy steel nuts are marked by property class, and stainless and nonferrous nuts per the applicable material standard, ensuring traceability.

5. Engineering Practice Essentials 🛠️

🔍 Selection workflow: determine bolt size and strength grade → select a Style 1 hex nut of the same grade → verify width across flats against wrench size → confirm the finish (zinc, black oxide, Dacromet, etc.).
🔧 Assembly torque: apply preload per the joint design; account for thread lubrication effects on the friction coefficient.
📊 Inspection and acceptance: verify dimensions per Table 1 and threads with thread gages; run hardness and proof-load tests per batch.
📋 Anti-loosening: use Style 1 nuts for general joints; under vibration or elevated temperature, switch to flange nuts, nylon-insert lock nuts, or add washers and witness marks.

6. Frequently Asked Questions ❓

❓ Q: How does a Style 1 nut differ from other metric hex nuts?
A: The metric hex nut family is divided by geometry and height into Style 1 (regular), Style 2 (heavy), and jam (thin) nuts. Style 1 has the standard height and the broadest application; Style 2 serves higher loads and pairs with high-strength bolts (e.g., 10.9).

❓ Q: What does the 6H thread tolerance class mean?
A: 6H is the common tolerance band for metric internal threads: H denotes a zero fundamental (lower) deviation and 6 the tolerance grade. 6H gives good assembly and adequate thread strength and is the default choice.

❓ Q: Must stainless steel nuts be marked with a property class?
A: Yes. The Standard requires stainless nuts to be marked with property class and manufacturer per the applicable material standard (e.g., ASTM F 836M / ISO 3506-1), distinguishing A2-70, A4-80, and other grades.

🔍 Q: Can nuts be zinc plated?
A: Yes, but plating changes thread fit and friction; use over-tapped or post-plating processed threads and follow the agreed finished-dimension tolerances in the Standard.