Metric Carriage Bolts have a square shoulder under the head designed to fit into a square-bored hole, preventing rotation by holding the bolt in place while the nut is tightened.
Available in Class 4.6/8.8 Zinc Plated Steel and A2 Stainless Steel (best for corrosion resistance in damp environments).
Need a washer? BelMetric stocks DIN 440 Square Hole Washers specifically designed to work with the square neck of a carriage bolts.
