Bolt Head Markings

Why head markings matter

Bolt head markings indicate material and strength. Using a low-grade bolt in a high-stress application can cause failure; using a high-grade bolt where it is not needed adds cost. Matching the marking to the specification ensures safe, correct fastening.

Metric property classes

Metric bolts use a two-number class (e.g. 8.8, 10.9). First number ≈ 1/100 of tensile strength in MPa; second relates to yield ratio.

SAE grade symbols

Grade 2: often no radial lines. Grade 5: three radial lines. Grade 8: six radial lines. Higher grade means higher strength and typically alloy or heat treatment.

FAQ

What do the lines on a bolt head mean?

On SAE bolts, radial lines indicate grade: Grade 2 has no lines, Grade 5 has three, Grade 8 has six. More lines generally mean higher tensile strength.

What does 8.8 mean on a metric bolt?

8.8 is the metric property class. The first number is 1/100 of the tensile strength in MPa; the second is 10x the yield ratio. 8.8 means roughly 800 MPa tensile and 80% yield ratio.

Are metric and SAE head markings the same?

No. Metric bolts use numbers (e.g. 8.8, 10.9). SAE bolts use lines or no marking for Grade 2, three lines for Grade 5, six for Grade 8.