Carriage Bolts Specifications and Selection Guide

Carriage bolts, also known as coach bolts, feature a smooth round head and a square neck beneath the head that prevents rotation when the nut is tightened. This self-locking design makes them ideal for wood-to-metal and wood-to-wood connections where only one side is accessible for tool use. The most common standard is DIN 603, which defines dimensions for round head square neck bolts. Other relevant standards include ISO 8677 and ASME B18.5. Carriage bolts are manufactured in diameters from M5 to M24, with lengths ranging from 20mm to 300mm. The square neck width is slightly larger than the nominal bolt diameter, creating a interference fit in the pre-drilled hole. For B2B buyers sourcing carriage bolts for African and Middle Eastern markets, it is essential to specify the standard, material grade, and surface finish, as these directly affect both performance and pricing.

Carriage bolts are available in several material grades, each suited to different applications. Grade 4.6 (low carbon steel) is the most economical option and works well for light-duty wood connections. Grade 8.8 (medium carbon steel, quenched and tempered) offers significantly higher tensile strength and is recommended for structural applications. Grade 10.9 provides the highest strength for heavy-duty structural connections. Stainless steel (A2-70, A4-80) carriage bolts offer excellent corrosion resistance for outdoor and marine applications, making them popular in coastal regions of East Africa and Southeast Asia. Surface finishes include zinc plating (clear or yellow), black oxide, hot-dip galvanizing, and mechanical galvanizing. For bulk procurement, consider that hot-dip galvanized carriage bolts cost 15-25% more than zinc-plated options but provide significantly longer service life in humid or corrosive environments.

Selecting the correct carriage bolt size requires understanding both the grip length and the thread length. The grip length is the portion of the bolt shank that passes through the connected materials — it should equal the total thickness of the materials being joined. The thread length varies by standard and diameter. For DIN 603, threads are typically 60-70% of the bolt length for shorter bolts and a fixed length for longer bolts. When ordering, specify the nominal diameter, total bolt length, thread pitch (coarse or fine), grade, and finish. For wood applications, the hole should be drilled 1-2mm larger than the bolt diameter to allow for wood movement, while the square neck requires a snug fit to prevent rotation. In hot and humid climates, always use corrosion-resistant finishes or stainless steel. For projects in Zimbabwe and surrounding SADC countries, carriage bolts with SABS or ZABS certification may be required for government-funded construction projects. For more information, also see our guides on [hex bolts](/en/industry/hex-bolt-dimensions-chart), [bolt grades](/en/industry/bolt-grade-markings-guide), [nuts](/en/industry/nylon-nuts-bolts-guide).