Grade is a strength specification
A bolt’s grade or property class defines its tensile and yield strength. Substituting a lower grade into a joint designed around a higher one reduces the clamping force the joint can develop, and in a bolted structural or pressure connection that is a safety matter rather than a cost saving.
| Class / Grade | Relative strength | Typical use |
|---|---|---|
| 4.6 / Grade 2 | Low | General non-structural assembly |
| 8.8 / Grade 5 | Medium-high | The common structural and machinery default |
| 10.9 / Grade 8 | High | Highly loaded machinery and structural connections |
| 12.9 | Very high | Precision machinery, high-preload joints |
| A2-70 stainless | Moderate | Corrosion resistance in general environments |
| A4-80 stainless | Moderate | Corrosion resistance including chloride exposure |
Stainless fasteners resist corrosion but are generally not as strong as high-grade carbon steel, and they are prone to thread galling on assembly. Where both strength and corrosion resistance are needed, a coated high-grade carbon steel fastener is often the better answer.
Threads and dimensions
Metric and imperial thread systems do not interchange, and within each system coarse and fine pitches are distinct. Specify the diameter, the pitch or thread count, the length, and whether the bolt is fully or partially threaded — a partially threaded bolt with too short a thread run cannot clamp a thick joint.
- Bolt length is measured under the head, except for countersunk heads which are measured overall
- Grip length must suit the total thickness of the parts being clamped, plus washers
- Fine threads develop slightly more preload and resist vibration loosening a little better; coarse threads tolerate damage and corrosion better
- Confirm the drive type and head style, since these affect the tooling needed on site
Coatings and corrosion
Coating is chosen against the environment and the required service life. Outdoor, humid and chemically exposed installations need meaningfully more protection than indoor dry assembly.
- Zinc plating gives light indoor protection only
- Hot-dip galvanising gives substantial outdoor protection but thickens the thread, so nuts must be tapped oversize to suit
- Dacromet and zinc-flake systems give high corrosion resistance with minimal thread build-up
- PTFE and similar coatings reduce galling on stainless assemblies and give consistent torque
Watch for galvanic corrosion. Joining dissimilar metals in a damp environment — a stainless fastener into aluminium, for instance — will corrode the less noble metal preferentially. Isolate the joint or match the materials.
Anchor bolts and structural hardware
Anchor bolts transfer load into concrete and are selected on the load, the concrete strength, edge distance and embedment depth. Cast-in anchors are placed before the pour; mechanical and chemical anchors are installed into hardened concrete afterwards, and chemical anchors generally develop higher capacity in cracked or edge-critical conditions.
What to include in a fastener enquiry
- Diameter, thread pitch and length, and full or partial threading
- Property class or grade, and the material
- Head style and drive type
- Coating or plating requirement
- Matching nuts and washers, with their grade
- Quantity, delivery city and required date