Head Marking
It is important to recognize the head mark on every fasteners so as to ascertain the correct grade of fasteners to be
used. The following are the standard head markings for both metric and imperial fasteners
Metric Steel Bolts
Standards ISO 898 Part 1
Identification Mark
Product Specifications Grade 4.8 Grade 5.8 Grade 8.8
<= M16
(> M16)
Grade 10.9 Grade 12.9
Socket Head Cap Screws
Material Low/Medium Carbon Steel Low/Medium Carbon Steel Medium Carbon Steel
Heat Treated
Alloy Steel
Heat Treated
Alloy Steel
Heat Treated
Min. Tensile
Strength (MPa)
420 520 800
(830)
1040 1220
Min. Yield
Strength (MPa)
340 420 640
(660)
940 1100
Proof Load
(MPa)
310 380 580
(600)
830 970
Hardness
(Min-Max)
HV130-220 HV 160-220 HRC 22-32
(HRC 23-34)
HRC 32-39 HRC 39-44
Compatible Nut Class 4 Class 5 Class 8 Class 10 -
Metric Steel Bolts
Standards BS 4190
Product
Specifications
Grade 4.6 Grade 4.8 Grade 6.8 Grade 8.8 Grade 10.9
Material Low/Medium Carbon Steel Low/Medium Carbon Steel Low/Medium Carbon Steel
Heat Treated
Medium Carbon Steel
Heat Treated
Alloy Steel
Heat Treated
Min. Tensile
Strength (N/mm2)
400 400 600 800 1000
Min. Yield
Strength (N/mm2)
240 320 480 640 900
Hardness
(Min-Max)
HV 120-220 HV 160-220 HV 190-250 HRC 22-32 HRC 32-39
Compatible Nut Class 4 Class 4 Class 6 Class 8 Class 10

Note : When a thick protective coating is applied to a bolt of grade 8.8 or 10.9, which requires the nut thread to be overtapped, the next higher grade of nut should be used.

Inch Steel Bolts
Standards SAE J429
Identification
Mark
Product
Specifications
Grade 2 Grade 5 Grade 8
Nominal Size
Range
1/4 - 3/4
(>3/4 - 1-½)
1/4 - 1
(>1 - 1-½)
1/4 - 1-½
Material Low/Medium Carbon Steel Medium Carbon Steel
Heat Treated
Medium Carbon Steel
Heat Treated
Min. Tensile
Strength (psi)
74000
(60000)
120000
(105000)
150000
Min. Yield
Strength (psi)
57000
(36000)
92000
(81000)
130000
Proof Load
(psi)
55000
(33000)
85000
(74000)
120000
Core Hardness
(Min-Max)
HRB 80-100
(HRB 70-100)
HRC 25-34
(HRC 19-30)
HRC 33-39
Compatible Nut Grade 2 Grade 5 Grade 8

For detailed information, please refer to complete governing standards.