🔩 Bolt Grade Identifier

Understanding Bolt Grades

Bolt grades indicate the strength and material properties. Higher grades can handle more stress before failing.

SAE/Imperial Bolt Grades

Grade	Head Markings	Material	Tensile Strength	Typical Use
Grade 2	No marks	Low carbon steel	60,000 psi	General purpose, non-critical
Grade 5	3 radial lines	Medium carbon steel	120,000 psi	Automotive, machinery
Grade 8	6 radial lines	Alloy steel	150,000 psi	High stress, critical
Grade 8.8	8.8 on head	Alloy steel	830 MPa	Heavy machinery

Metric Bolt Classes

Class	Head Markings	Tensile Strength	Yield Strength	Typical Use
4.6	4.6 on head	400 MPa	240 MPa	Low strength, light duty
4.8	4.8 on head	420 MPa	340 MPa	General purpose
8.8	8.8 on head	800 MPa	640 MPa	Most common, automotive
10.9	10.9 on head	1040 MPa	940 MPa	High strength applications
12.9	12.9 on head	1220 MPa	1100 MPa	Maximum strength, critical

How to Read Bolt Markings

• SAE Grade 2: No markings on head - lowest strength
• SAE Grade 5: Three radial lines (like clock at 12, 4, 8)
• SAE Grade 8: Six radial lines evenly spaced around head
• Metric Class 8.8: Numbers "8.8" stamped on bolt head
• Metric Class 10.9: Numbers "10.9" stamped on head
• Metric Class 12.9: Numbers "12.9" stamped on head

Torque Specifications (Dry, Lubricated is 15-25% less)

Size	Grade 5 (Nm)	Grade 8 (Nm)	Class 8.8 (Nm)	Class 10.9 (Nm)
M6 / 1/4"	9-11	13-15	10-12	14-17
M8 / 5/16"	22-27	31-36	25-30	35-41
M10 / 3/8"	43-54	61-75	49-59	69-83
M12 / 7/16"	75-92	108-130	85-102	120-145
M14 / 1/2"	117-142	169-203	135-162	190-230
M16 / 5/8"	183-220	264-318	210-250	295-355
M20 / 3/4"	366-427	528-610	410-490	580-700

Material Properties Explained

• Tensile Strength: Maximum pulling force before breaking
• Yield Strength: Force where permanent deformation begins
• Proof Load: Stress that can be applied without permanent deformation
• Hardness: Resistance to surface deformation or penetration

When to Use Each Grade

• Grade 2 / Class 4.6: Non-structural, furniture, light brackets
• Grade 5 / Class 8.8: Automotive, machinery, general structural
• Grade 8 / Class 10.9: High-stress, critical structural, racing
• Class 12.9: Extreme applications, aerospace, specialized equipment

Important Safety Notes

• Never substitute lower grade bolt for higher grade specification
• Always use correct torque - over-torque can break even Grade 8 bolts
• Inspect bolts for cracks, especially after over-torquing
• Replace stretched bolts - they won't regain full strength
• Use thread locker for vibration-prone applications
• Match bolt grade to nut grade for proper strength

Torque Application Tips

• Use calibrated torque wrench for critical applications
• Apply torque in stages - typically 50%, 75%, then 100%
• Lubricated bolts require 15-25% less torque
• Follow proper tightening sequence (star pattern for multi-bolt)
• Check torque after initial use and retighten if needed
• Clean threads before torquing - dirt affects readings

Common Mistakes

• Using impact wrench on critical bolts - can over-torque
• Reusing one-time-use (TTY) bolts - designed to stretch once
• Mixing metric and imperial - threads don't match properly
• Not using washers where specified - affects load distribution
• Over-torquing in soft materials (aluminum) - strips threads