Step bolts have a lower profile head than carriage bolts. The square neck under the head locks the bolt into place.DIMENSIONAL ATTRIBUTES Diameter (Nominal): 1/4" Thread Pitch: 20 Diameter (Dimension): 0.25" Diameter (Minimum): 0.237" Diameter (Maximum): 0.26" Length (Nominal): 2-3/4" Thread Length (Nominal): 0.75" Neck Width: 0.26" Neck Height: 0.156" Head Diameter (Minimum): 0.813" Head Diameter (Maximum): 0.844" Head Height, Overall (Minimum): 0.125" Head Height, Overall (Maximum): 0.145"PRODUCT ATTRIBUTES Category: Step Bolts System of Measurement: Inch Brand: Aspen Fasteners Head Style: Truss Head Head Style Detail: Square Neck Thread Length: Standard Thread Length Thread Direction: Right-Hand Thread Material: Steel Material Grade: Low Carbon Steel Coating: Zinc Plated Coating Color: Clear Origin: Origin Undefined Standards and Specifications: ASME B18.5