Engineering Guide · 5 min read
Button Head Screws: ASME B18.3 and ISO 7380-1 / -2
Choose hex socket button heads with or without a collar. Compare ASME B18.3 and ISO 7380, low-profile clearance, bearing support and reduced loadability.
The short answer: a button head is a low rounded head that remains above the mounting surface. In the hex-socket family, ISO 7380-1 covers the metric form without a collar and ISO 7380-2 the form with a collar; ASME B18.3 covers the specified inch socket-screw forms. Choose the low profile for its clearance and surface shape, then check the finished screw's loadability. Both ISO button-head standards explicitly address reduced loadability caused by the head design.[1][2][3]
This article is part of the Bolt and Screw Head Types Guide. It covers hexagon-socket button heads: identify the drive separately when a catalog calls another rounded screw a “button head.”
Identify the dome and any integral collar
A button head has a low dome and a flat bearing surface beneath it. A hexagon socket is the internal six-sided recess used by the key. With the collar variant, a wider washer-like rim projects beneath the dome as part of the head. The collar is integral; it is not a captive or separate washer.
A cylindrical socket head has a taller, straight-sided outer profile; a pan head has a distinct side wall and rounded top. Neither “round head” nor “Allen screw” is a sufficient product description. Use the button head drawing and inch dimension tables to check the site's stated ASME product form, including head diameter, height and socket dimensions.
Use the low dome where protrusion matters
A button head offers a lower profile than a cylindrical socket head. On a removable equipment cover or a bracket where hands pass close to the assembly, a low, rounded projection can be easier to accommodate than a taller cylindrical head. It still stands proud of the surface. If the drawing needs a flush face, the mating part needs a suitable countersunk design instead.
Check the full envelope: head diameter, height above the surface and room to put the key squarely into the socket. A low head can fit the available height yet extend too far sideways into a neighboring part. When tool engagement or required clamp load controls the choice, compare the actual requirements with a cylindrical socket head cap screw before changing the head.
A collar provides a wider integral seat where the mating surface supports it. On thin sheet, check the supported contact area and the sheet's resistance to indentation; a slot, nearby edge or curved surface can remove support beneath the rim. Keep any washer required by the drawing. The flange bolts and nuts guide explains the same bearing-area principle, and the washer guide covers reasons to retain a separate washer.
Match the standard to the collar and thread system
The two ISO parts separate the head variants. A/B product accuracy grades and mechanical property classes are different designations; the grade A below concerns product accuracy.
| Head and thread route | Product reference | Selection detail |
|---|---|---|
| Inch hex-socket button head | ASME B18.3-2026.[1] | Select the button-head form within the inch socket-screw standard, with the specified inch thread and edition. |
| Metric button head without collar | ISO 7380-1:2022; steel and stainless steel, coarse threads M3–M16, product grade A.[2] | Check this head's dimensions and reduced-load requirements. |
| Metric button head with collar | ISO 7380-2:2022; steel, coarse threads M3–M16, product grade A.[3] | Specify the collar form and its supported seat; retain the reduced-load requirements. |
A stainless collar screw offered commercially needs its own agreed material and property requirements: ISO 7380-2:2022's public scope specifies steel. Select the product actually required instead of assuming that every material listed beside “7380” has the same scope.
For a German adoption, DIN EN ISO 7380-1:2023-04 names ISO 7380-1:2022 and replaces the 2011 German edition.[4] Preserve the part number and edition in an old drawing's designation; a shortened “DIN / ISO 7380” listing omits the collar distinction.
Keep the head's load limit in the specification
A property-class number alone does not establish the finished button head's tensile capacity. The rounded head and the socket cut into it leave a different load-carrying section from a cylindrical socket head. ISO 7380-1 and -2 identify reduced ultimate tensile load due to that design, with a leading zero in the relevant property-class marking. Bossard's steel collar-screw listing, for example, uses 010.9 and explicitly identifies reduced loadability.[2][3][5]
The collar broadens the seat beneath the head; it does not remove that head-design limit. For a specified clamp load, check the selected screw's minimum load requirements and approved assembly instructions. Bossard's technical guidance also distinguishes tightening requirements for reduced-loadability products.[6] Carry over a cylindrical-head torque value only after the replacement screw and joint have been checked.
This comparison keeps the two decisions separate:
| Decision | What to check |
|---|---|
| Will the head fit and seat? | Dome height, outside diameter, collar diameter where present, supported flat seat and key access. |
| Will the specified screw meet the joint requirement? | Material and property designation, finished-product load requirements, needed preload and assembly instructions for that head-and-drive form. |
For interpreting mechanical property designations beyond the head form, use Fastener Grades Explained.
Order the head variant rather than just a rounded screw
A geometry description such as hexagon socket button head screw with collar — ISO 7380-2:2022 — M6 × 20 identifies a different product from the same size without a collar. Add the material and the required reduced-load property designation; the example is not a complete strength selection.
- Head and drive: button head, with or without collar, hexagon socket and exact standard edition.
- Thread and length: diameter, pitch or TPI, fit, length measured from beneath the bearing surface, and required thread length.
- Mating detail: head clearance, flat seat, any slot or edge, and the specified washer arrangement.
- Supply requirements: material, mechanical properties and head-load requirements, finish, quantity and inspection documents.
Complete the inquiry using How to Specify a Bolt; read metric and inch sizes with the size-callout guide.
References
- ASME. ASME B18.3-2026. Public listing and scope for inch socket-screw products.
- ISO. ISO 7380-1:2022. Hexagon socket button heads, material and thread scope, and head-design loadability note.
- ISO. ISO 7380-2:2022. Hexagon socket button heads with collar, steel scope and reduced-loadability note.
- DIN Media. DIN EN ISO 7380-1:2023-04. Current German adoption and replacement of the 2011 edition.
- Bossard. BN 11252 — button head screws with collar. Supplier product data identifying steel 010.9 and reduced loadability; product data, not a universal material substitution.
- Bossard. Preload and tightening torques (2025), printed page 66. Reduced-loadability guidance for the listed head-and-drive forms, including the collar product BN 11252; no generic torque values reproduced here.