ASME/ANSI B 18.5 Round Head Square Neck Bolts
ASME/ANSI B18.5 Round Head Square Neck Bolts are designed for fastening arrangements where the bolt needs to remain stationary while the nut is tightened from the opposite side. A square section directly beneath the rounded head engages with the mating material, helping prevent the bolt from turning during assembly. This allows the connection to be tightened from one side without requiring a wrench or other tool to hold the bolt head.
The rounded head provides a smooth external profile once the bolt is installed, while the square neck creates the mechanical engagement that distinguishes this fastener from conventional hexagon head bolts. Rather than relying on the friction between the bolt head and the workpiece alone, the square neck provides a positive anti-rotation feature within the joint. This makes the design particularly useful for through-bolted components where access to the bolt head is limited or where a clean exposed surface is preferred.
ASME/ANSI B18.5 bolts are intended for inch-series fastening systems and can be selected in different diameters, lengths, materials, grades, and finishes according to the requirements of the assembly. Carbon and alloy steel versions can be used for mechanically loaded connections, while stainless steel options are available for applications where corrosion resistance is more important. The correct combination of bolt diameter, length, neck dimensions, material, and mating nut should be considered to achieve the intended fastening performance.
Specification:
ASME/ANSI B18.5 Round Head Square Neck Bolts are specified by the relationship between the rounded head, square neck, threaded shank, and applicable inch-series thread dimensions. The square neck is formed directly below the head, while the threaded portion is prepared to accept a compatible inch-series nut. The combination of these features defines the basic configuration of the bolt and distinguishes it from standard hexagon head fasteners.
For purchasing and production, the key specifications include nominal diameter, thread series, bolt length, head diameter, head height, square neck dimensions, material, mechanical grade, and surface finish. Carbon steel and alloy steel can be specified for applications requiring different mechanical properties, while stainless steel options are available for corrosion-resistant assemblies. Surface treatments can also be selected according to the material and operating environment.
| Item | Specification |
|---|---|
| Standard | ASME/ANSI B18.5 |
| Product Type | Round Head Square Neck Bolt |
| Head Type | Round Head |
| Neck Type | Square Neck |
| Thread System | Inch |
| Thread Series | UNC/UNF |
| Thread Diameter | #10-1“ |
| Grade | A307, SAE GR.2 5 8 |
| Material | Carbon Steel/Alloy Steel/Stainless Steel |
| Surface Treatment | Zinc Plated/Black Oxidation/HDG/Dacromet/Bright Finish/Passivation |
| Mating Nut | Compatible Inch-Series Nut |
- RELATED BOLT LISTS:
- ASME B 18.31.5 HRA Hook Bolts
- ASTM A394 HDG Tower Bolts
- ASTM A354 Hex Bolts
- ASME B 18.2.1 Square Head Bolts
- ASME/ANSI B 18.5 Round Head Square Neck Bolts
- ASME/ANSI B18.5 T- Head Bolts
- ASME/ANSI B18.2.1 Hex Head Flange Bolts
- ASME/ANSI B18.5 80°Slotted Countersunk Head Bolts
- ASME/ANSI B18.2.1 Hex Cap Bolts
- A307 Hex Head Bolts
Dimension Chart of ASME/ANSI B 18.5 Round Head Square Neck Bolts:
The ASME/ANSI B18.5 dimension chart provides the measurements needed to select a Round Head Square Neck Bolt for a specific inch-series fastening connection. In addition to the nominal bolt diameter and length, the chart covers the dimensions of the rounded head and square neck, which determine how the bolt fits through the mating component and how the neck engages with the material during installation.
The nominal diameter (d) identifies the threaded portion of the bolt, while the length (l) determines the available threaded engagement after the bolt passes through the assembled components. The head diameter (dk) and head height (k) define the external profile of the rounded head. Below the head, the square neck width and neck length are important when checking the hole configuration and ensuring that the square section can properly engage with the mating material.
| Thread Diameter | #10 | 1/4 | 5/16 | 3/8 | 7/16 | 1/2 | 5/8 | 3/4 | 7/8 | 1 |
| P | 24|32 | 20|28|32 | 18|24|32 | 16|24|32 | 14|20|28 | 13|20|28 | 11|18|24 | 10|16|20 | 9|14|20 | 8|12|20 |
| ds max | 0.199 | 0.26 | 0.324 | 0.388 | 0.452 | 0.515 | 0.642 | 0.768 | 0.895 | 1.022 |
| ds min | 0.159 | 0.213 | 0.272 | 0.329 | 0.385 | 0.444 | 0.559 | 0.678 | 0.795 | 0.91 |
| dk max | 0.469 | 0.594 | 0.719 | 0.844 | 0.969 | 1.094 | 1.344 | 1.594 | 1.844 | 2.094 |
| dk min | 0.436 | 0.563 | 0.688 | 0.782 | 0.907 | 1.032 | 1.219 | 1.469 | 1.719 | 1.969 |
| k max | 0.114 | 0.145 | 0.176 | 0.208 | 0.239 | 0.27 | 0.344 | 0.406 | 0.459 | 0.531 |
| k min | 0.094 | 0.125 | 0.156 | 0.188 | 0.219 | 0.25 | 0.313 | 0.375 | 0.438 | 0.5 |
| s max | 0.199 | 0.26 | 0.324 | 0.388 | 0.452 | 0.515 | 0.642 | 0.768 | 0.895 | 1.022 |
| s min | 0.185 | 0.245 | 0.307 | 0.368 | 0.431 | 0.492 | 0.616 | 0.741 | 0.865 | 0.99 |
| k1 max | 0.125 | 0.156 | 0.187 | 0.219 | 0.25 | 0.281 | 0.344 | 0.406 | 0.469 | 0.531 |
| k1 min | 0.094 | 0.125 | 0.156 | 0.188 | 0.219 | 0.25 | 0.313 | 0.375 | 0.438 | 0.5 |
| r max | 0.031 | 0.031 | 0.031 | 0.031 | 0.031 | 0.031 | 0.062 | 0.062 | 0.062 | 0.062 |
Application of ASME/ANSI B 18.5 Round Head Square Neck Bolts:
One-Side Installation and Anti-Rotation Connections
ASME/ANSI B18.5 Round Head Square Neck Bolts are useful in assemblies where the bolt head needs to remain stationary while the nut is tightened from the opposite side. The square neck engages with the material around the hole, reducing the need to hold the bolt head with a separate tool. This makes the design suitable for through-bolted brackets, mounting frames, access panels, and fabricated assemblies where installation from one side is preferred.
Sheet Metal and Fabricated Panels
The rounded head and square neck configuration is commonly used for sheet-metal assemblies, metal enclosures, equipment panels, guard panels, and fabricated frames. The round head provides a relatively smooth exposed surface, while the square neck helps maintain the bolt position during tightening. The selected neck dimensions should correspond with the prepared hole and material thickness.
Wood and Metal-to-Wood Connections
Round head square neck bolts are also widely suited to wooden structures and wood-to-metal connections, where the square neck can embed into the material and resist rotation. Typical applications include timber frames, outdoor structures, agricultural constructions, benches, gates, fencing hardware, and heavy-duty wooden assemblies. Carbon steel versions with an appropriate protective finish can be selected for outdoor use where additional corrosion protection is required.
Equipment Frames and Mounting Structures
For industrial and commercial equipment, these bolts can be used to join support frames, mounting brackets, protective guards, conveyor structures, storage systems, and material-handling assemblies. The fastening method is particularly convenient where the rear side of the bolt is accessible only for nut installation and holding the bolt head with a wrench would be inconvenient.