The marks on the head of a bolt indicate its hardness and strength grade.
- No marks signify a lower-strength bolt, typically made of low-carbon steel.
- Three marks indicate a medium-strength bolt, often classified as Grade 5.
Six marks represent a high-strength bolt, such as a Grade 8 alloy steel bolt, known for its improved tensile strength and resistance to shearing.
For stainless steel bolts, head markings typically specify the type of stainless steel, such as 304 or 316, rather than strength grade.