ASME B18.2.7.1M-2002 — Metric 12-Spline Flange Screws

28 10 月, 2026
ASME Standards

🔩 ASME B18.2.7.1M-2002 provides complete general and dimensional data for metric 12-spline flange screws. This screw was developed from the ANSI Special Study Committee’s 1971 investigation of optimum head designs for metric fasteners. Compared with a standard hexagon head, the 12-spline flanged screw offers: lower nominal bearing pressures, flange protection of assembled surfaces against abrasion from wrenching tools, lower stress concentration factors in head-to-shank fillets, and significant raw-material savings (about 30% in smaller sizes and 10% in intermediate sizes). The Standard was approved by ANSI on January 24, 2002.

💡 This Standard began as IFI-511 (1976), was withdrawn in 1986 because of limited demand, and was later redeveloped by the B18 Committee as a formal American National Standard — a classic case of a superior design winning a second life.

1. Scope and Erratum 📌

This Standard covers the complete general and dimensional data for metric 12-spline flange screws recognized as an American National Standard, including:

Dimensions and spline: tooth form and gaging requirements for the 12-spline.
Top of head, head height, spline height, position of head, flange, bearing surface, and fillet.
Body diameter, length, point, straightness, and the thread-length series.
Threads, material and mechanical properties, finish, identification, options, workmanship, and inspection/quality assurance.

Additionally, ASME issued an Erratum in December 2002: the angle in the right figure of Table 4 was revised, with the complete table shown on the overleaf. Users should apply the corrected content.

⚠️ The December 2002 erratum corrects an angular dimension in the Table 4 illustration. Design, manufacturing, and inspection personnel must work from the errata-included version to avoid mis-machining from the incorrect drawing.

2. Technical Advantages of the 12-Spline Head 🔧

Per the original design studies, the 12-spline flange head offers these advantages over a standard hex head:

Lower nominal bearing pressures: the flange enlarges the bearing area, lowering unit pressure and reducing the risk of bearing-surface indentation.
Flange protection: the flange shields the assembled surface from abrasion by wrenching tools.
Lower stress concentration: the head-to-shank fillet has a lower stress concentration factor, improving fatigue performance.
Material savings: about 30% in small sizes and 10% in intermediate sizes.

Feature	Key point	Remark
Wrenching configuration	12-spline	Table 1: spline gaging configurations
Position/runout	Tolerances in Table 2	Head-to-body concentricity
Underhead fillet	Head-to-shank transition	Reduces stress concentration
Body type	Full / reduced body (Type R)	Reduced body per Table 4
Threads	Metric coarse, etc.	Per ASME B1.13M
Grip gaging length	L_G,max / L_S,min	Table 8: threads into grip

3. Thread-Length System 📏

The Standard refines the thread-length requirements (Section 17 and associated tables):

Maximum grip gaging length (L_G,max): ensures that, within this grip, the thread lies wholly within the gripped material.
Minimum body length (L_S,min): ensures adequate unthreaded body within the grip.
Nominal thread length (B) and transition thread length (X): define the geometry between the threaded and unthreaded portions.
Together these parameters guarantee that the loaded thread is complete, the body does not interfere, and joint strength and assemblability are preserved.

🚨 A grip exceeding L_G,max can cause thread-end interference or insufficient loaded thread; a grip below L_S,min leaves too little unthreaded body. Always verify grip and body length against Table 8 during design.

4. Materials, Mechanical Properties, and Marking 🔧

Steel: mechanical properties per metric property classes (e.g., 8.8, 10.9, 12.9), specifying tensile strength, hardness, and proof load.
Corrosion-resistant steels: property classes per the corresponding metric material standards (e.g., ISO 3506-1).
Nonferrous metals: per the applicable material standards.
Marking: property-class marking plus the manufacturer’s source symbol for traceability.

5. Engineering Practice Essentials 🛠️

🔍 Selection workflow: establish joint load and torque → select nominal diameter and length → choose full or reduced body → set property class and material → specify dedicated 12-spline tooling.
🔧 Assembly torque: the 12-spline head accepts high torque, but follow the recommended torque-preload relationship to avoid over-tightening and bolt stretch.
📊 Tool matching: always use a matching 12-spline socket or spline wrench; never force standard hex tools onto the head.
📋 Inspection: dimensions per Table 1, spline per gages, threads with thread gages, and material certificates per the referenced standards.

6. Frequently Asked Questions ❓

❓ Q: What is the difference between 12-spline and 12-point?
A: A 12-spline uses spline teeth for the drive, while a 12-point is a double-hex tooth form. The driving principles differ and the tools are not interchangeable — distinguish them when selecting.

❓ Q: Why does the 2002 edition carry an erratum?
A: After issuance, ASME found an incorrect angular dimension in the right figure of Table 4 and issued the December 2002 erratum; always work from the corrected content.

❓ Q: When should reduced-body (Type R) screws be selected?
A: When weight reduction or a longer threaded section is required; strength must be calculated on the minimum body diameter, and body-to-grip matching must be checked.

🔍 Q: How does this Standard differ from B18.2.5M’s 12-point flange head screws?
A: Both are flange head screws, but the drive differs — B18.2.7.1M uses a 12-spline and B18.2.5M a 12-point. The spline transmits torque more uniformly; the 12-point has more common tooling.