Detailed Information On Bolt Specifications

Whether for production or application, bolt specifications are governed by strict standards. Therefore, understanding bolt specifications is essential for purchasers. Below is a detailed introduction to bolt specifications.

I. Metric Bolts

Thread Code Metric bolts are designated by the major diameter of the thread. For example, M3 means the thread major diameter is 3.00 mm, and M4 means the thread major diameter is 4.00 mm.

Thread Size × Pitch Pitch is marked in metric bolt designations in the format of size × pitch, such as M3×0.5, M4×0.7, M5×0.8, M6×1. The pitch is generally omitted for coarse threads, while it must be marked for fine threads.

Length Code The unit of metric bolt length is millimeters (mm). For bolts with protruding heads such as hexagon head and socket cap head bolts, the length refers to the shank only, excluding head height. For countersunk (flat head) bolts, the marked overall length includes the head height.

Drive Code Common drive types: slotted, Phillips, combination slotted/Phillips, hexagon socket, one-way security drive (can only be tightened, not removed), etc.

Head Style Common head styles: countersunk head (flush with the workpiece after installation), oval countersunk head, round head, pan head, large pan head, hexagon head, etc.

Finish Code Common surface finishes: zinc plating, nickel plating, tin plating, zinc plating with iridescent passivation, zinc plating with green passivation, anti-oxidation treatment, no surface treatment.

II. US Standard Bolts

Thread Code Unified inch threads are marked by gauge number or inch major diameter, e.g. #2‑56, #4‑40, #6‑32, #8‑32, #10‑24. The number after the gauge indicates threads per inch.

Length Code The original length unit for US standard bolts is inch. The conversion formula to millimeters: (Length ÷ 32) × 25.40 = Length in mm.

Head Style, Drive Code and Finish Code The specification system for head styles, drive types and surface finishes of US standard bolts is similar to that of metric bolts and shall be selected in accordance with relevant US fastener standards.

III. British Standard Whitworth Bolts

Whitworth threads are specified by threads per inch, with a thread angle of 55°. Metric threads define pitch as the distance between adjacent thread crests in millimeters, with a thread angle of 60°. When measuring thread parameters, you can press the thread crests onto white chalk for clearer counting and measurement.

Summary

A good knowledge of bolt specifications helps avoid wrong size and specification selection during purchase. The above covers basic knowledge of bolt specifications, which should be further practiced and learned combined with standards in actual purchasing and assembly work.