Bolt grade is a strength rating, and on an imperial bolt you read it off the head. Grade 5 has three radial lines, Grade 8 has six, and a plain unmarked head is Grade 2, the weakest of the three. Count the lines and you know roughly what you are holding.
Structural steelwork uses a different family again, and mixing the two up is a genuine hazard rather than a technicality.
Bolt Grade Chart
| Grade | Head marking | Tensile strength | Typical material | Where it belongs |
|---|---|---|---|---|
| Grade 2 | No lines | 74,000 psi (up to 3/4 in) | Low carbon steel | Non-structural, light assembly |
| Grade 5 | 3 radial lines | 120,000 psi (up to 1 in) | Medium carbon, quenched and tempered | Automotive, machinery, general work |
| Grade 8 | 6 radial lines | 150,000 psi | Medium carbon alloy, quenched and tempered | High-stress machinery, suspension |
| A325 (F3125 Gr. A325) | A325 or the grade name | 120,000 psi | Medium carbon, heat treated | Structural steel connections |
| A490 (F3125 Gr. A490) | A490 or the grade name | 150,000 psi | Alloy steel, heat treated | Structural, where A325 is not enough |
Tensile strength falls off in the largest diameters for some grades, which is why the table qualifies Grade 2 and Grade 5 by size. Always check the specification for the actual diameter being used rather than assuming the headline number.
Why Grade 8 Is Not Simply Better
A stronger bolt is also a harder, less forgiving one. Strength in steel is bought with hardness, and hardness costs ductility: the higher the grade, the less the bolt stretches before it breaks.
That matters in a connection because a ductile bolt yields, redistributes load to its neighbours in the group, and gives visible warning. A very hard bolt holds full load right up to the point it snaps. Higher grades are also more sensitive to hydrogen embrittlement, which is why plating and galvanizing rules get stricter as the grade rises.
So substituting Grade 8 where Grade 5 was specified is not a free upgrade. In a structural joint it can change the failure mode from one that bends to one that lets go.
Structural Bolts Are a Separate System
Steel buildings do not use Grade 5 and Grade 8 hardware. They use the structural family, historically A325 and A490 and now consolidated under ASTM F3125, and the differences run deeper than the strength number.
| Grade 5 / Grade 8 | Structural (A325 / A490) | |
|---|---|---|
| Thread length | Long, often full length | Short, to keep threads out of the shear plane |
| Head | Standard hex | Heavy hex, larger across the flats |
| Installed by | Torque to a value | Snug-tight or pretensioned per a defined method |
| Inspection | Rarely formal | Defined inspection regime |
| Governed by | SAE J429 | RCSC Specification plus ASTM F3125 |
The thread length point is the one that surprises people. A structural bolt has a deliberately short thread so the plain shank can sit in the shear plane, because a threaded section has less area there and the connection is designed knowing which case applies.
Installation is the other real difference. A pretensioned structural joint is tightened to a defined minimum tension so the plates clamp together and transfer load by friction. That is not a torque value pulled off a chart; it is a procedure. The Research Council on Structural Connections publishes the 2025 Specification for Structural Joints Using High-Strength Bolts (checked September 2026), which defines the installation methods, pretension values and inspection requirements.
Snug-Tight, Pretensioned and Slip-Critical
Three terms that get used loosely and mean quite different things:
Snug-tight is the plies brought into firm contact, roughly the full effort of an ironworker with an ordinary spud wrench. Most ordinary shear connections are snug-tight, and that is a legitimate specified condition rather than a shortcut.
Pretensioned means the bolt is tightened to a defined minimum tension, typically around 70 percent of its tensile strength, using turn-of-nut, a tension indicator or a calibrated wrench.
Slip-critical is pretensioned plus a controlled faying surface condition, so the joint is designed not to slip at all. Used where movement would be unacceptable, such as under fatigue loading or where the connection is holding alignment.
Which one applies is on the drawings. Installing a snug-tight joint where the design called for slip-critical produces a connection that looks identical and behaves differently.
Galvanizing and Bolt Grade
Exposed bolts need corrosion protection, and hot-dip galvanizing is the usual answer. The American Galvanizers Association describes the coating as providing barrier and cathodic protection plus a zinc patina, typically two to five times longer between maintenance than paint (checked September 2026).
There is a catch specific to bolts. Galvanizing changes the friction between the nut and the thread, so the relationship between torque and the tension actually achieved changes with it. Galvanized structural assemblies are supplied and tested as a matched lot of bolt, nut and washer for this reason, and mixing a galvanized nut from one source onto a bolt from another undoes that.
The highest grades are also restricted: A490 is not hot-dip galvanized, because of the embrittlement risk at that hardness. Our article on whether galvanized steel rusts covers how the coating behaves over time.
Frequently Asked Questions
How do I tell a Grade 5 bolt from a Grade 8?
Count the radial lines on the head: three for Grade 5, six for Grade 8, none for Grade 2. The lines are short marks running outward from the center, and they are cast in rather than printed.
Can I use a Grade 8 bolt instead of A325?
Not as a substitution. They differ in thread length, head size, ductility and the installation and inspection regime the connection design assumes. Structural connections specify structural bolts for reasons beyond the strength number.
What is the metric equivalent of Grade 8?
Roughly property class 10.9, which is marked with the number on the head rather than with lines. Grade 5 corresponds approximately to class 8.8. These are near equivalents, not drop-in substitutes.
Does a stronger bolt mean I can use fewer of them?
Sometimes, but the connection may then fail elsewhere: bearing on the plate, tear-out at the edge, or block shear through the bolt group. Those checks depend on the plate, not the bolt, which is why connection design considers the whole joint, starting from the member properties in the wide flange beam sizes chart.
Should structural bolts be reused?
Pretensioned assemblies should not be, because tightening them stretches the bolt into its inelastic range and a second installation will not reach the same tension. Snug-tight bolts are treated differently, and the specification covers both cases.
The Short Version
Three lines is Grade 5, six is Grade 8, none is Grade 2, and higher is harder rather than simply better. Structural steel uses its own family with short threads, heavy hex heads and a defined installation and inspection procedure, which is not interchangeable with hardware-store grades. Which bolt, which grade and which tightening method are all specified by the engineer.
Reference information only, not engineering advice. Disclaimer.