M8 Carriage Bolts A2-70 Stainless Steel | Round Head Square Neck Coach Screws

A general rule is to select a bolt length equal to the combined thickness of all materials being joined, plus enough thread to fully engage the nut — typically 1–2 thread pitches beyond the nut face. For example, connecting two 20mm boards generally calls for a 50mm (≈2") bolt when using a washer and nut. Available lengths run from 30mm (≈1-3/16") to 70mm (≈2-3/4").

A2-70 per ISO 3506 specifies a minimum tensile strength of 700 MPa (≈101,500 psi). For M8 bolts, this translates to a proof load generally in the range of 16–18 kN, making them suitable for structural wood joints, outdoor furniture, and light-to-medium load-bearing connections. For critical structural steel applications, consult an engineer for load calculations.

Yes. A2 austenitic stainless steel resists rust, oxidation, and staining in most outdoor conditions including rain, humidity, and mild coastal exposure. For direct saltwater immersion or highly aggressive marine environments, A4 (316) stainless is typically the recommended upgrade. These bolts are well-suited for decks, pergolas, fences, and garden structures in standard outdoor conditions.

These bolts use the M8 metric coarse thread (ISO 68-1) with a standard pitch of 1.25mm. They are compatible with standard M8 hex nuts, M8 nylon insert lock nuts (Nyloc), and M8 flat or spring washers. They are not compatible with UNC/UNF inch-standard hardware.

The square shoulder under the round dome head is designed to embed into wood or soft composite as the nut is tightened, preventing the bolt from spinning. This allows single-person installation — you tighten the nut from one side without needing to hold the bolt head with a second tool. It also leaves a smooth, snag-free dome on the visible face of the joint.

For small repairs or single-use projects, a 15-piece box is generally sufficient. For deck framing, pergola construction, or fence post connections, the 25- or 30-piece boxes offer better value per bolt. All packs ship in a labeled storage box to keep fasteners organized on the job site and easy to store for future use.

Yes. A2 stainless steel is generally compatible with ACQ (Alkaline Copper Quaternary) and CA (Copper Azole) pressure-treated lumber, which can rapidly corrode standard zinc-plated or hot-dip galvanized fasteners over time. Stainless steel fasteners are typically the recommended choice for treated wood in residential deck and fence construction per IRC guidelines.