Round Head Bolts
Round head bolts, also known as button head bolts, are fasteners distinguished by a smooth, domed head with no wrenching flats. They closely resemble carriage bolts but lack the square neck under the head that carriage bolts use to grip and prevent rotation during installation. The rounded, low-profile head provides a finished, tamper-resistant appearance on the exposed side of a connection while the nut is tightened from the opposite end.
Applications
Round head bolts are commonly used in wood connections, where the broad, smooth head bears against the timber surface without digging in and offers a clean, low-profile finish. They are a practical choice wherever a carriage bolt style head is desired but the square neck is unnecessary or undesirable, and where a tamper-resistant head without wrenching flats adds value. Typical uses span timber and marine construction, playground equipment, steel to steel connections, railroad, and general connections in which appearance and a non-protruding head matter.
Manufacturing
Portland Bolt manufactures button head bolts from 1⁄2” through 11⁄4 ” diameter in most ASTM, AASHTO, and SAE specifications. Special tooling can be created to produce nonstandard head dimensions, button head bolts larger than 11⁄4” in diameter, or bolt heads with your company’s name and/or logo.
Stock List
Standard Thread Length
Bolt lengths ≤ 6″: Thread length = (2 X diameter) + 1⁄4“
Bolt lengths > 6″: Thread length = (2 X diameter) + 1⁄2“
Round Head Bolt Dimensions
| Bolt Diameter | C | H | ||
|---|---|---|---|---|
| Head Diameter | Head Height | |||
| Max | Min | Max | Min | |
| 1⁄2 | 1.094 | 1.032 | 0.270 | 0.250 |
| 5⁄8 | 1.344 | 1.219 | 0.344 | 0.313 |
| 3⁄4 | 1.594 | 1.469 | 0.406 | 0.375 |
| 7⁄8 | 1.844 | 1.719 | 0.469 | 0.438 |
| 1 | 2.094 | 1.969 | 0.531 | 0.500 |
| 11⁄4 | 2.850 | 0.531 | 0.500 | |
*Dimensions per ASME B18.5 1990 (R 1998) cover diameters up to 1". Bolts over 1" use custom head dimensions. | ||||
FAQs
A round head bolt, also called a button head bolt, is a fastener with a smooth, domed head and no wrenching flats. It is similar to a carriage bolt but has no square neck beneath the head, and it is most often used in wood connections.
Both bolts share a rounded, domed head with identical geometry, but a carriage bolt has a square neck under the head that grips into wood and stops the bolt from spinning during tightening. A round head bolt has no square neck, so it relies on the friction of the connection to prevent rotation.
Round head bolts are commonly used in wood connections where a smooth, low-profile head is desired. They provide a clean, tamper-resistant finish and are a good fit when a carriage bolt style head is wanted without the square neck.
Portland Bolt produces round head bolts in most ASTM, AASHTO, and SAE specifications, including A307A, A449, A354, F593, F3125, and SAE J429. Because they are usually used with wood, they are most often supplied in standard ASTM A307.
Portland Bolt manufactures round head bolts from 1/2″ diameter through 1-1/4″ diameter as standard. With special tooling, diameters larger than 1-1/4″ can also be produced.
Round head bolts are offered in plain black and hot-dip galvanized finishes. Hot-dip galvanizing adds corrosion resistance for outdoor or wet environments.
Round head bolts can be produced in high strength grades including F3125, A449, A354, and SAE J429 Grades 5 and 8.
Yes. Portland Bolt can create special tooling to produce nonstandard head dimensions, oversized heads, or bolt heads stamped with a company name or logo.
For bolts 6″ and shorter, standard thread length is (2 x diameter) + 1/4″. For bolts longer than 6″, it is (2 x diameter) + 1/2″.
Round head bolt head dimensions follow ASME B18.5 1990 (R 1998) for diameters up to 1″. Bolts larger than 1″ use custom head dimensions.