ASME B18.2.4.5M-2008 — Metric Hex Jam Nuts

28 10 月, 2026
ASME Standards

🔩 ASME B18.2.4.5M-2008 provides complete general and dimensional data for metric hex jam nuts — the “thin nuts” of the metric hex nut family. A jam nut is much thinner than the standard hex nut of the same nominal size and is typically used together with a full-height nut: the jam nut is tightened against the main nut in the opposite direction, and the mutual wedging of the two threads locks the assembly against vibration loosening. The Standard revises ANSI B18.2.4.5M-1979 (R2003), was approved by ANSI on November 11, 2008, and issued on March 31, 2009.

💡 Understand “jamming” correctly: a jam nut is not meant to be used alone. It forms a two-nut system with a full nut, resisting loosening by back-locking; it is also used to lock on short thread ends where space is limited.

1. Scope 📌

This Standard covers the complete general and dimensional data for metric hex jam nuts recognized as an American National Standard. Principal coverage includes:

Dimensions: dimensional data for each nominal diameter in Table 1.
Width across flats and thickness: the thin height, roughly half that of a standard nut.
Tops and bearing surface, corner fill, true position of the hexagon to the tapped hole, and countersink.
Threads: series, tolerance class, and gaging requirements.
Material and mechanical properties, finish, identification symbols, inspection and quality assurance, designation, and workmanship.

⚠️ The thin section of a jam nut cannot independently carry the full thread-shear load of a heavily loaded joint; in structural duty it must serve only as a supplementary locking element, never as the primary load-carrying nut.

2. Coordination With ISO Standards 🌐

This Standard is coordinated with the ISO metric thin-hex nut standards (e.g., ISO 4035). Dimensions are stated in millimeters, letter symbols follow ISO usage, and the tolerance and acceptance systems are aligned so that products to either standard are functionally interchangeable.

Feature	Requirement	vs. Standard Nut
Thickness	~60% of standard nut height	Much thinner, lighter
Width across flats	Standard hex series (mm)	Same as standard nut
Thread tolerance	6H primarily	Same system
Top	Flat with chamfered corners	Same requirement
Main use	Two-nut back-locking / limited space	Auxiliary locking
Mechanical properties	Per applicable material standard	Same verification

3. Geometry and Dimensional Requirements 🔧

Geometric requirements for the jam nut include:

Width across flats: measured between opposite wrenching flats, perpendicular to the axis, and identical to the standard nut of the same size for common wrench compatibility.
Thickness: measured parallel to the axis from the top of the nut to the plane of the bearing surface — a thin dimension.
Tops and bearing surface: flat top with chamfer; a flat bearing face ensures good contact during back-locking.
Corner fill and countersink: smooth wrench engagement.
True position: position tolerance of the hexagon to the tapped hole ensures centering.

🚨 The order of a two-nut joint matters: tighten the main nut to design torque first, then apply the jam nut’s locking torque. Reversing the sequence or overtightening the jam nut can slacken the main nut or damage the threads.

4. Materials, Marking, and Acceptance 📏

Material, mechanical-property, and marking requirements follow the metric hex nut system:

Steel: hardness and proof load per property class; carbon and alloy steel nuts carry the manufacturer’s mark.
Stainless and nonferrous: property classes and marking per the applicable material standards (e.g., ASTM F 836M, F 467M).
Threads: metric coarse/fine series, tolerance class 6H, per ASME B1.13M, accepted by thread gages.
Acceptance: dimensions per Table 1, mechanical properties by lot sampling; government items ordered via the appendix numbering system.

5. Engineering Practice Essentials 🛠️

🔍 Selection workflow: identify the joint needing anti-loosening and the available thread length → select a same-size full nut plus jam nut → verify thread and across-flats → order by material and finish.
🔧 Back-lock assembly: after the main nut reaches torque, hold it with an open-end or box wrench and tighten the jam nut against it so that it seats fully on the main nut face.
📊 Applications: short thread ends on shafts, bearing locking, frequently adjusted mechanisms (limit stops), and auxiliary locking in tight spaces.
📋 Alternatives: for maintenance-free service, consider nylon-insert lock nuts, thread adhesives, or lock washers instead of a two-nut set to reduce part count.

6. Frequently Asked Questions ❓

❓ Q: Can a jam nut be used alone?
A: Yes, but only for light duty (adjustment stops, guard fastening). In load-carrying joints it must always accompany a full nut, since its thin section lacks the thread length to carry full load.

❓ Q: Why does two-nut back-locking resist loosening?
A: The jam nut, tightened against the main nut, loads each nut’s thread on a different flank of the bolt thread, creating a wedging effect that raises thread friction and resists vibration loosening.

❓ Q: What is the difference between a jam nut and a nylon-insert lock nut?
A: A jam nut locks by two-nut wedging (friction/wedge mechanism); a nylon-insert nut grips the thread with a nylon collar for friction locking, can be used alone, and its locking force should be checked after repeated cycles.

🔍 Q: Are jam nuts the same size as standard nuts?
A: The width across flats is the same (common wrenches), but the thickness is much smaller (~60%). Do not substitute by standard-nut thickness when purchasing.