A2 Stainless Steel Cup Square Carriage Bolts (Coach Bolts) — M4 M5 M6 M8

Measure the total thickness of the materials being joined, then add enough length for a washer and nut — typically 6–12mm (about 1/4"–1/2") beyond the material. For example, joining two 20mm (≈3/4") boards would generally call for a bolt at least 50mm (≈2") long. When in doubt, size up — a slightly longer bolt can always be trimmed or accommodated with an extra washer.

A2 stainless steel carriage bolts are generally rated to property class 70 (ISO 3506), which typically provides a tensile strength of around 700 MPa. Actual load capacity depends on bolt diameter, length, and the materials being fastened. M8 bolts carry significantly more load than M4. Consult a structural engineer for load-critical applications.

Yes — A2 stainless steel offers good corrosion resistance for most outdoor environments including garden structures, decking, fencing, and pergolas. For highly aggressive marine or coastal salt-spray environments, A4 (316) stainless steel is generally recommended for longer service life. These bolts are not suitable for use with treated lumber containing copper-based preservatives without verifying compatibility.

These bolts use standard ISO metric coarse threads (DIN 603 / ISO 8677 equivalent). M4, M5, M6, and M8 are all standard metric sizes. They are compatible with DIN 934 metric hex nuts and DIN 125 flat washers in the corresponding size. Standard metric thread pitch applies: M4 = 0.7mm, M5 = 0.8mm, M6 = 1.0mm, M8 = 1.25mm.

No special tools are required. The cup square (domed) head is designed to be driven flush into wood, preventing rotation. You simply need a hammer or mallet to seat the square neck into the timber, then tighten the nut from the opposite side with a standard wrench or socket set. No wrench is needed on the bolt head side, making installation faster in tight spaces.

A carriage bolt (also called a coach bolt) has a smooth domed head and a square neck that bites into wood to prevent spinning during tightening — no wrench needed on the bolt head. A hex bolt has a hexagonal head that requires a wrench on both ends. Carriage bolts are preferred for wood-to-wood and wood-to-metal connections where a clean, flush, tamper-resistant head is desired on the visible face.

For light garden furniture and thin timber panels, M4 or M5 carriage bolts are typically sufficient. For decking boards and fence rails, M6 is the most common choice. For structural timber framing, post connections, and heavy-duty outdoor structures, M8 is generally preferred for its higher clamping force. Always verify with your local building code or a structural engineer for load-bearing applications.

Count the number of connection points in your project and add 10–15% extra for mistakes or future repairs. For a small repair or prototype, the 2-pack or 5-pack is ideal. For a deck or fence project, the 20, 50, or 100-pack offers the best per-unit value. All pack sizes ship the same bolt — only the quantity differs.