ASME · ANSI / ASME B18.3.1M

ANSI/ASME B18.3.1M Standard: technical requirements for metric screws

Technical parameters of metric socket head cap screws according to ANSI/ASME B18.3.1M

This standard regulates the geometric and dimensional characteristics of metric series socket head cap screws. The document covers a nominal diameter range from 1.6 to 48 mm for standard versions and from 1.6 to 8 mm for screws with a reduced head. The EV Group China technical documentation includes detailed appendices: reference data for drilling and countersinking diameters (Appendix I), key head parameters (Appendix II), wear calculation formulas (Appendix III), comparative analysis of deviations according to ISO standards (Appendix IV), product codification system (Appendix V), and thread connection tolerances (Appendix VI).

The sizes specified in the standard are for reference only. To clarify the product nomenclature available for production and warehouse shipment, we recommend sending an inquiry to the specialists at EV Group China.

Compatibility with international ISO standards

Fasteners manufactured according to the ASME B18.3.1M specification are fully functionally interchangeable with screws according to the ISO 4762-1977 standard. Technical differences resulting from specific production processes and forming methods are detailed in Appendix IV.

Requirements for tolerance control

All linear dimensions are presented in millimeters (mm) and correspond to the state of the product before the application of anti-corrosion or decorative coatings, unless otherwise specified in the order.

Regulations for design changes

The selection of design options not strictly described in the standard remains with the manufacturer, subject to prior agreement with the customer. EV Group China disclaims liability for any impairment of performance characteristics resulting from unauthorized electroplating or other modifications performed outside our company's quality control.

Terminology and designation system

For correct identification of fastener parameters, it is recommended to use the terminology of ANSI B18.12. The designation of a metric screw in the specification must include:

  • ASME/ANSI standard number;
  • Nominal thread diameter;
  • Thread pitch (using the "x" symbol);
  • Nominal shank length (using the "X" symbol);
  • Name (the abbreviation SHCS is permitted for socket head cap screws);
  • Material grade and property class: for alloy steels — grade 12.9 according to ASTM A 574M; for stainless alloys — requirements of ASTM F 837M;
  • Drive type (default — internal hexagon).