ASME B18.2.5M-2013 — Metric 12-Point Flange Head Screws

28 10 月, 2026
ASME Standards

🔩 ASME B18.2.5M-2013 provides complete general and dimensional data for metric 12-point flange head screws. Combining a 12-point wrenching head with an integral flange, these screws offer 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) compared with standard hex heads. Revising ASME B18.2.5M-2009 and approved as an American National Standard on March 13, 2013, the Standard serves high-load, high-torque mechanical joints.

💡 The 12-point design (two hexagons offset by 30°) gives twelve wrench engagement points, transmitting higher torque in the same space with less slippage; the flange replaces a washer and protects the surface — an ideal combination for compact, high-strength joints.

1. Scope (Revision Highlights) 📌

The 2013 revision includes:

Updated material references: for steel screws, the withdrawn ASTM F568M was replaced by ISO 898-1, and for stainless steel ASTM F738M was replaced by ISO 3506-1, aligning with the international metric screw system.
Quality assurance: changes conform with other B18 standards and improve the quality assurance of ASME B18.18.4 through B18.18.
Drawing and dimension corrections: the drawings associated with Tables 1 and 2, and the L_G (maximum grip gaging length) and L_S (minimum body length) dimensions for M36 in Table 9, were corrected.

⚠️ Because material references now point to ISO 898-1 / ISO 3506-1, confirm that suppliers issue material and property certificates to the new references rather than legacy ASTM grades.

2. Head Configuration and Key Dimensions 🔧

Structural requirements for the 12-point flange head include:

Top of head: full form, chamfered or rounded.
Head height and wrenching height: ensure full engagement of the 12-point spline and reliable torque transfer.
Gaging of the 12-point flange head: verify the 12-point profile per the gages in Table 2.
Flange and bearing surface: flange diameter sets the bearing area; the bearing face is flat and perpendicular to the axis.
Underhead fillet: Type F and Type U configurations (Tables 4 and 5); the radius affects fatigue strength.
Body diameter: full body or reduced body (Type R, Table 6).
Position of head: ensures coaxiality with body and thread.

Feature	Key point	Typical range / remark
Wrenching configuration	12-spline	High torque, slip resistant
Underhead fillet	Type F / Type U	Per Tables 4 and 5
Body type	Full body / reduced body (R)	Reduced for weight saving
Thread series	Metric coarse primarily	Per ASME B1.13M
Steel property class	Per ISO 898-1	e.g., 8.8 / 10.9 / 12.9
Stainless steel	Per ISO 3506-1	e.g., A2-70 / A4-80
Length tolerance	L tolerances in Table 7	By length band

3. Body, Threads, and Length Requirements 📏

Body diameter: a full body shall be a precise body diameter meeting the straightness/runout requirements of Table 2; the reduced body (Type R) diameter is given in Table 6.
Threads: series, tolerance class, and gaging per ASME B1.13M/B1.3M; thread length (Table 10) and transition thread length are per the Standard.
Grip gaging length (L_G) and body length (L_S): Table 9 gives maximum grip gaging length and minimum body length so that an adequate unthreaded body remains when all threads enter the grip.
Point: point radius per Table 8 to ease starting and alignment.
Straightness: long screws are checked by the gaging procedure specified in the Standard.

🚨 Strength calculations for reduced-body (Type R) screws must use the minimum body diameter — never treat them as full-body screws. Also verify that reduced-body length matches the grip, or the body may be overloaded or too little thread may enter the grip.

4. Materials and Mechanical Properties 🔧

Steel: mechanical properties per ISO 898-1 property classes 8.8, 10.9, 12.9, specifying tensile strength, hardness, and proof load.
Corrosion-resistant steels: property classes A2-70, A4-80, etc., per ISO 3506-1.
Marking: property-class marking plus the manufacturer’s source symbol for traceability.
Finish: zinc plating, Dacromet, black oxide, etc., as agreed; account for plating effects on thread accuracy.

5. Engineering Practice Essentials 🛠️

🔍 Selection workflow: determine joint load and torque → select nominal diameter and length → choose full or reduced body → set property class and material → specify finish and 12-point socket tooling.
🔧 Assembly torque: the 12-point head allows higher tightening torque, but the torque-preload relationship must be controlled per the recommended values to avoid over-tightening.
📊 Tool matching: always use a 12-point socket — never force a standard hex socket onto a 12-point head, which damages the spline.
📋 Inspection: dimensions per Table 1, 12-point head per the Table 2 gages, threads with gages, and materials verified per ISO 898-1/ISO 3506-1.

6. Frequently Asked Questions ❓

❓ Q: What are the advantages of 12-point flange head screws over hex head screws?
A: The 12-point spline has more engagement points, higher torque capacity, and less slippage; the flange lowers bearing pressure and protects surfaces; the underhead fillet reduces stress concentration for better fatigue performance; and up to 30% of material is saved in small sizes.

❓ Q: Why were material references changed to ISO 898-1?
A: The former ASTM F568M has been withdrawn, and ISO 898-1 is the current internationally recognized standard for metric steel screw mechanical properties; referencing it aligns with the global metric system and simplifies sourcing.

❓ Q: When are reduced-body (Type R) screws used?
A: Where weight reduction or a longer threaded section is needed; however, strength must be calculated on the minimum body diameter, not the full body.

🔍 Q: Can a hex socket drive a 12-point head?
A: No. A matching 12-point socket must be used; forcing a hex socket onto the head damages the spline and causes slippage and loss of torque control.