Engineering Guide · 5 min read
Hex Head Bolts: ASME B18.2.1, ISO 4014, ISO 4017 and DIN
Identify external hex heads, choose hex bolt or cap screw requirements, and compare ISO 4014, ISO 4017 and legacy DIN wrench sizes before ordering.
The short answer: an external hex head gives a wrench or socket six flats to grip, with the clamping load transferred through the surface beneath the head. For inch parts, ASME B18.2.1 distinguishes hex bolts from hex cap screws. For metric parts, ISO 4014 and ISO 4017 identify different thread-length forms. Specify the product name and standard together: “hex bolt” alone leaves both the geometry and thread system open.[1][2][3]
This article belongs to the Bolt and Screw Head Types Guide. It covers conventional external hex heads. The wider integral bearing rim is covered separately in Hex Flange Bolts.
Identify the hexagon, seat and thread length
A conventional hex head has six external wrenching faces and a bearing surface underneath. Its wrench size is the distance across two opposite flats; the thread diameter is a separate measurement. A socket head cap screw instead has a cylindrical outer head and an internal hexagon for a key.
Look underneath as well as above. A hex cap screw's washer face is a formed bearing surface on the head itself, not a separate washer or the broad projecting rim of a flange head. Fastenal identifies that face and the chamfered point as features of its hex cap screw category.[4] Use the specified product requirements to verify a part; a photograph cannot establish its tolerances.
The hex head bolt drawing and dimension tables provide inch dimensions. Use the applicable metric standard when the drawing calls for ISO dimensions.
Choose external hex when tool access supports it
External hex heads suit assemblies with room for a wrench or socket. A hex head can be turned by an open-end wrench from the side or by a socket over the head. That makes it practical on exposed mounting feet, frames and brackets where the tool can reach the flats. A socket also needs room around the head; an open-end wrench needs a usable swing path. Check those spaces on the assembly drawing before choosing the head.
A cylindrical socket head may fit a narrower surrounding pocket, but its key needs access to the recess. Compare the actual head envelope and tool approach in the socket head cap screw guide. Head selection starts with that access and the seat; required preload and material properties still need their own specification.
On a thin bracket or softer housing, check whether the bearing surface can carry the clamp load without excessive indentation. A separate washer lets its diameter and hardness be selected independently. An integral flange offers another arrangement where the design allows it; see why flange bolts and nuts have a wider seat and the washer selection guide.
Separate the inch and metric product forms
Choose the row that matches the drawing. These routes identify product requirements, not interchangeable parts.
| Product form | Reference | What to resolve |
|---|---|---|
| Inch hex bolt or hex cap screw | ASME B18.2.1-2012 (R2021), which covers several inch bolt and screw forms.[1] | Name hex bolt, hex cap screw or the required heavy hex form. Apply that form's head, body, bearing-face and thread-length requirements. |
| Metric hex head bolt with a plain-shank form | ISO 4014:2022; coarse-pitch steel and stainless steel bolts, product grades A and B.[2] | Check the specified thread length for the selected diameter and nominal length. Confirm any required plain-shank length against the drawing. |
| Metric hex head screw threaded up to the head | ISO 4017:2022; coarse-pitch steel and stainless steel screws, product grades A and B.[3] | Select this thread-length form when required; allow for the standard's under-head thread runout and transition. |
The A/B labels in the ISO titles identify product accuracy grades, rather than strength classes such as 8.8 or 10.9. State material and mechanical properties separately; Fastener Grades Explained covers those designations.
Also keep the naming systems separate. ISO 4017's word “screw” identifies its product form; it does not mean that a nut is prohibited. Fastenal describes hex cap screws as usable with a tapped hole or a nut.[4] Describe the mating arrangement and required thread engagement directly.
For a structural steel connection, follow its structural bolting specification. ASME's B18.2.1 scope specifically directs heavy hex structural bolts to B18.2.6; an ordinary hex or heavy hex head is not enough to identify a structural assembly.[1]
Read DIN 931 and DIN 933 before accepting an ISO replacement
Older drawings often use DIN 931 for a plain-shank hex bolt and DIN 933 for a screw threaded up to the head. Check the part and edition: DIN Media lists DIN 931-1:1987-09 and DIN 933:1987-09 as withdrawn, with replacement records leading to the ISO 4014 and ISO 4017 adoptions respectively.[5][6] Retain the drawing's requirement until the permitted replacement is agreed.
A matching thread can still need a different wrench. Würth's DIN-to-ISO comparison lists the following across-flats changes. These are a focused historical comparison, not a complete current dimension table.[7]
| Nominal thread | Legacy DIN across flats | ISO across flats |
|---|---|---|
| M10 | 17 mm | 16 mm |
| M12 | 19 mm | 18 mm |
| M14 | 22 mm | 21 mm |
| M22 | 32 mm | 34 mm |
Recheck the tool size, surrounding clearance and bearing seat when replacing stock. Then compare head height, body and thread-length limits in the required editions. A supplier's combined “DIN / ISO” label leaves those checks with the buyer.
Specify enough to select one hex-head product
Start with a description such as hexagon head bolt — ISO 4014:2022 — M10 × 60 or hex cap screw — ASME B18.2.1-2012 (R2021) — 3/8-16 UNC × 2 in. These are geometry examples, not substitutes for each other. Complete the inquiry with:
- Head and seat: conventional or heavy hex, required bearing-face details, washer arrangement and tool clearance.
- Thread: diameter, pitch or TPI, fit and required thread length; identify any controlled plain shank.
- Length: nominal length measured from beneath the head, plus the mating thickness and thread engagement where they control selection.
- Material and supply requirements: grade or property class, finish, quantity and inspection documents.
Use How to Specify a Bolt for the complete RFQ format and Metric and Inch Callouts for reading the size string.
References
- ASME. ASME B18.2.1-2012 (R2021). Public scope for the inch product forms and separate structural-bolt reference.
- ISO. ISO 4014:2022. Hexagon head bolts, product grades A and B; public scope.
- ISO. ISO 4017:2022. Hexagon head screws, product grades A and B; public scope.
- Fastenal. Hex Bolt and Hex Cap Screw — product types. Manufacturer/supplier descriptions of bearing faces, points and mating arrangements.
- DIN Media. DIN 931-1:1987-09. Withdrawn edition and replacement record naming DIN EN 24014:1992-02 and DIN EN ISO 4014:2001-03.
- DIN Media. DIN 933:1987-09. Withdrawn edition and replacement record naming DIN EN 24017:1992-02 and DIN EN ISO 4017:2001-03.
- Würth. DIN 933/931 and ISO 4014/4017 hexagon head screws. Supplier's DIN-to-ISO across-flats comparison, in millimeters.