Engineering Guide · 4 min read
Bolt and Screw Head Types: ASME and ISO Identification Guide
Identify hex, socket, button, countersunk, pan and flange heads. Match the profile, drive and bearing surface to the relevant ASME or ISO product standard.
The short answer: identify the head profile, the driving feature and the bearing surface before choosing a product standard. A request for a “cap bolt” still needs those details. A hexagonal drive recess can appear in a cylindrical, button or countersunk head, while a similar-looking head may belong to an inch or a metric product. Use the shape to find the right standard family, then verify the specified dimensions and edition.[1][2][3]
Explore the head-type guides
Start with Socket Head Cap Screws: ASME B18.3, ISO 4762 and DIN 912 for cylindrical socket heads and the legacy DIN designation. The identification chart below covers the wider head family; the linked dimension pages provide the existing inch-series references.
Read the profile, drive and bearing surface separately
Head profile describes the outside shape. Drive describes how the tool turns the screw. Bearing surface describes where the head seats against the joint. “Socket” by itself identifies a drive feature, not one unique head profile.
A low domed button head and a cylindrical socket head can both take a hex key. A pan head with a cross recess is a different combination again. Match the complete head-and-drive description before selecting a standard.[2][4][6]
Match the head to a standard family
This index identifies relevant product standards, not interchangeable parts. The metric references cover the specific products named; the ASME references are inch-series routes.
| Head to identify | Visible features | References to check |
|---|---|---|
| Hex head | External hexagon; bearing surface beneath the head. | ASME B18.2.1-2012 (R2021) for the specified inch product; ISO 4014:2022 for its metric hexagon-head bolt scope.[1][3] |
| Cylindrical socket head | Straight-sided cylindrical head, internal hexagon and flat under-head seat. | ASME B18.3-2026; ISO 4762:2004. DIN 912 is a historical designation discussed in the dedicated guide.[2][7] |
| Button head, hex socket | Low domed profile; identify whether an additional collar is present. | ASME B18.3-2026; ISO 7380-1:2022 without collar, or ISO 7380-2:2022 with collar.[2][4][8] |
| Flat countersunk head, hex socket | Flat top and conical bearing surface; seats in a matching countersink. | ASME B18.3-2026; ISO 10642:2026 for the metric hex-socket product. Check the specified head angle and head dimensions.[2][5] |
| Pan head, cross recess | Rounded top, distinct side wall and flat bearing surface. | ISO 7045:2011 for metric pan heads with type H or Z cross recess. The linked inch chart covers its separately stated product reference.[6] |
| Hex flange head | External hexagon above an integral wider flange. | ASME B18.2.1-2012 (R2021); ISO 15071:2011 for the stated metric small-series product. Identify the required series and bearing face.[1][9] |
The name narrows the search; it does not finish the specification. ASME B18.2.1 contains several bolt and screw products. Likewise, ISO 7380 separates button heads with and without a collar. Keep the exact product form attached to the standard number.[1][4][8]
Check the seat before treating heads as substitutes
A flat countersunk head seats on a cone; a cylindrical socket, pan or conventional hex head bears beneath the head. A matching drive does not make those seating surfaces interchangeable. For countersunk parts, align the head with the countersink and verify the angle and dimensions required by the drawing.[5]
Head shape also affects what a strength marking means for the finished screw. ISO 7380-1 and ISO 10642 explicitly cover products with reduced loadability due to head design. A property-class number alone is therefore a poor way to approve a change from a cylindrical head to a button or countersunk head.[4][5]
Turn an identified head into an order
Record these details together so the supplier can check the same product you have identified:
- Profile and drive: for example, cylindrical socket head with a hexagon recess, rather than “Allen bolt” alone.
- Product standard and edition: select the inch or metric route and the required head variant.
- Thread and length: diameter, pitch or TPI, thread fit and the applicable length measurement.
- Head interface: head diameter and height, tool engagement and the required bearing or countersink details.
- Remaining requirements: the material, properties, finish and documents required by the product standard and project.
Use How to Specify a Bolt for the complete enquiry format, and Metric and Inch Callouts to read the size designation. Headless parts follow a separate route: see Stud Bolts vs Threaded Rod.
References
- ASME. ASME B18.2.1-2012 (R2021). Public scope for the covered inch-series bolt and screw forms.
- ASME. ASME B18.3-2026. Public product listing and scope for inch socket screw products.
- ISO. ISO 4014:2022. Metric hexagon-head bolts, product grades A and B; public scope.
- ISO. ISO 7380-1:2022. Metric hex-socket button heads with reduced loadability; public scope.
- ISO. ISO 10642:2026. Metric hex-socket countersunk heads with reduced loadability; public scope.
- ISO. ISO 7045:2011. Metric pan heads with type H or Z cross recess; public scope.
- ISO. ISO 4762:2004. Metric hexagon socket head cap screws; public scope.
- ISO. ISO 7380-2:2022. Metric hex-socket button heads with collar; public scope.
- ISO. ISO 15071:2011. Metric flange bolts, small series, product grade A; public scope.