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ASME B18.3.5M-1986 (2002) — Hexagon Socket Flat Countersunk Head Cap Screws (Metric Series)
🔧 ASME/ANSI B18.3.5M-1986 is the dedicated standard for metric-series hexagon socket flat countersunk head cap screws, covering complete general and dimensional requirements for nominal sizes from 3 mm to 20 mm. First approved in 1983 and revised in 1986 (adding corrosion-resistant steel and incorporating the companion ASTM specifications by reference), it was reaffirmed in 2002. Flat countersunk head screws seat flush in a countersunk hole with the head level with the surface — ideal where a smooth, obstruction-free face is required, such as machine ways, covers, and instrument mounting plates. The Standard explicitly cautions that wrenchability is limited by the socket size and key engagement, and that this product is not recommended where maximum fatigue resistance is required.
💡 “Flush seating” is the core value of countersunk screws: once installed, the head is level with the base surface, so it does not obstruct sliding parts, snag operators, or interfere with finishing and sealing.
1. Scope and Design Limitations 📌
Clause 1 covers metric-series hexagon socket flat countersunk head cap screws from 3 mm to 20 mm; all dimensions are in millimeters (mm) and apply before plating unless stated otherwise; terminology follows ANSI B18.12. Clause 1.1.3 requires that screws purchased for government use conform to this Standard and to Appendix II. Two limitations are stated: (1) wrenchability is limited by socket size and key engagement; (2) because of the head configuration, the product is not recommended where maximum fatigue resistance is required. Installation requires machining a conical countersink in the base so the head seats flush.
⚠️ Under tension, the head cone creates a wedging effect and stress concentration, giving lower fatigue performance under cyclic load than a cylindrical head. For joints under high alternating or shock loading, avoid countersunk screws.
2. Size Series and Typical Applications 🔩
Table 1A of the Standard gives head dimensions, body diameter, and socket sizes; Table 1B gives body and grip lengths; Table 2 gives hexagon socket dimensions; Table 3A gives socket gage dimensions. Common sizes and typical applications are summarized below.
| Nominal Size (dia × pitch) | Head Cone Angle | Typical Applications |
|---|---|---|
| M3 × 0.5 | ≈ 90° | Instrument panels, flush sheet-metal mounting |
| M4 × 0.7 | ≈ 90° | Electronics, panel assembly |
| M5 × 0.8 | ≈ 90° | General machinery, covers |
| M6 × 1.0 | ≈ 90° | Machine-way guards, fixtures |
| M8 × 1.25 | ≈ 90° | Power equipment, structural panels |
| M10 × 1.5 | ≈ 90° | Heavy-equipment covers |
| M12 × 1.75 – M20 × 2.5 | ≈ 90° | Large-structure flush joints |
💡 Unlike the 82° inch head, the metric countersunk head cone angle is approximately 90°, and the two geometries are not interchangeable: mixing inch countersinks with metric screws leaves the head standing proud or sinking too deep.
3. Interchangeability with BS 4168 🔬
Clause 1.2 states that flat countersunk head cap screws made to this Standard are functionally interchangeable with BS 4168-1982 and may be substituted for those parts; no ISO standard exists for the product at this time, and this design differs from and is not interchangeable with many similar metric parts. The principal difference is the thread fit — 4g6g per long-standing U.S. practice versus 5g6g in the BS document. Appendices also give formulas for dimensions (Appendix I), the government part-numbering system and preferred sizes (Appendix II), metric thread dimensions (Appendix III), and a wall thickness gage (Appendix IV).
🚨 No ISO standard plus non-interchangeability with many similar metric parts means: order strictly to ASME B18.3.5M or BS 4168 sizes and tolerances — do not assume a generic “metric hex socket screw” description will suffice.
4. Materials and Mechanical Properties 🔩
Clause 3 sets material, processing, and mechanical requirements; the 1986 revision added corrosion-resistant steel products and incorporates the companion ASTM specifications (e.g., ASTM A 574M-type) for mechanical, chemical, and testing requirements. Alloy steel products are heat treated to the specified hardness; thread, hardness, and decarburization requirements follow the referenced specifications. Surface finishes follow the purchase agreement and dimensions apply before plating. Because of the head design, countersunk screws are not assessed by the class 12.9 tensile values of cap screws — size the application by actual head-load and fatigue requirements.
⚠️ Clamp-load capacity of countersunk screws is limited by the head structure; and countersink quality directly affects load capacity — an oversized or mismatched countersink leaves the head unsupported or stressed. Inspect countersunk holes, e.g., with the wall thickness gage (Appendix IV), before assembly.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection: choose countersunk where a flush surface is required; switch to cylindrical-head screws or other solutions under cyclic or shock loading.
- 📐 Countersinking: machine the conical hole with a countersink matching the head cone angle (≈ 90°) and control depth so the head seats flush.
- 🔧 Tightening: countersunk screws have limited wrenchability — control torque to avoid rounding the socket, and use a proper bit and guide sleeve where needed.
- 📋 Acceptance: verify dimensions per Tables 1A/1B, head flushness, and countersink quality; check thread fit and length tolerances.
6. Frequently Asked Questions ❓
❓ Q: Why are countersunk screws unsuitable for high fatigue?
A: Under tension the head cone produces a wedging effect and stress concentration; under alternating load, fatigue cracks initiate readily at the cone-to-shank transition, so the Standard does not recommend the product for maximum-fatigue service.
A: Under tension the head cone produces a wedging effect and stress concentration; under alternating load, fatigue cracks initiate readily at the cone-to-shank transition, so the Standard does not recommend the product for maximum-fatigue service.
🔍 Q: Are metric and inch (82°) countersunk screws interchangeable?
A: No. The metric head cone is about 90° while the inch head is 82°, so the countersink angles do not match — heads stand proud or sink too deep. Thread systems also differ; keep the two systems separate.
A: No. The metric head cone is about 90° while the inch head is 82°, so the countersink angles do not match — heads stand proud or sink too deep. Thread systems also differ; keep the two systems separate.
❓ Q: How does this Standard relate to BS 4168?
A: Per Clause 1.2, screws are functionally interchangeable with BS 4168-1982 and may substitute for those parts; the main difference is thread fit (4g6g vs 5g6g). No ISO standard exists.
A: Per Clause 1.2, screws are functionally interchangeable with BS 4168-1982 and may substitute for those parts; the main difference is thread fit (4g6g vs 5g6g). No ISO standard exists.
❓ Q: How do I ensure flush seating when installing?
A: Use a countersink tool matching the head cone angle, control the depth, and after assembly confirm the head does not protrude with a straightedge or flatness check (use the wall thickness gage to check countersink wall thickness where applicable).
A: Use a countersink tool matching the head cone angle, control the depth, and after assembly confirm the head does not protrude with a straightedge or flatness check (use the wall thickness gage to check countersink wall thickness where applicable).
