It all has to do with psi or more simply hardness. Grade 8 is the hardest, Grade 5 is medium, Grade 2 is the softest.

Grade 8 is common in heavy manufacturing like large trucks and tractors. Grade 5 is very common in manufacture of automobiles. Grade 2 is found mostly in household fasteners like carriage bolts, machine screws, and various non hardened nuts and bolts. Most of the construction grade fasteners are Grade 2.
Grade 8 will have these markings on the head:

Grade 5 bolts have 3 lines on the head of the bolt. Grade 8 have a minimum of 6 lines on the head. Grade 2 have no lines at all.
Grade 5 will have these markings on the head:

Grade 2 will have no marking on the head:

Metric nuts and bolts have a similar rating system:

Metric 8.8 is equivalent to a grade 5. Metric 10.9, 12.9, are equivalent to grade 8.


It is always best to match graded components when clamping together. In other words use grade 5 nuts with grade 5 bolts etc. This will allow all components to work with the same level of hardness, thus insuring a better connection.

Most mechanics will opt for grade 5 unless the manufacturer specifically requires grade 8 for warranty or safety issues. Often using a bolt or nut that is overly hardened can be more harmful than beneficial. So it is important to understand the stress factors placed upon the fastener as they relate to the base materials that they are being applied to.

Sometimes, like in life itself, being a little more pliable than stiffly rigid can get you through the inevitable bumps in the road.