IS 4218 specifies the basic profile, general plan, dimensions, and tolerances for ISO metric screw threads. It is the fundamental standard applied to the threading of fasteners, including structural foundation bolts, to ensure strict geometric compatibility and interchangeability.
Lays down requirements for foundation bolts of nominal sizes 12 to 100 mm.
Quick Reference Values
Thread angle60 degrees
Fundamental triangle height (H)0.86603 P
Depth of external thread0.6134 P
Radius of root0.1443 P
Key Formulas
H = 0.86603 P — Height of fundamental triangle
d2 = d - 0.6495 P — Pitch diameter of external thread
Practical Notes
Galvanized foundation bolts often require overtapping of the corresponding nuts to accommodate the thickness of the zinc coating.
Always distinguish between coarse and fine pitch. Coarse pitch is standard for most structural and foundation bolt applications.