Engineering Guide · 5 min read
Hex Flange Bolts: ASME B18.2.1, ISO 4162 and DIN 6921
Identify hex flange bolts, compare ASME and ISO specifications, and follow DIN 6921 to EN 1665. Check the flange series, bearing face and washer arrangement.
The short answer: a hex flange bolt combines an external hexagon head with a wider, integral flange beneath it. The outline identifies the head family; the product still needs a standard, series and bearing-face description. ASME B18.2.1 covers inch hex flange screws among several head forms. Metric standards include ISO 15071 and ISO 4162, while a legacy DIN 6921 callout leads to a separate EN replacement history.[1][2][3][4]
This is part of the Bolt and Screw Head Types Guide. For an inquiry, distinguish the integral flange from a separate washer assembly.
Identify the integral flange before choosing a standard
Look for six external wrench flats above a circular flange that projects beyond the hexagon. The flange is formed as part of the head. A conventional hex bolt sitting on a loose washer can have a similar outline from above, so inspect the side and the joint between the components.
The flange provides an integral bearing face and a wider head envelope. That changes the space needed around the head and the interface with the clamped part. When replacing a hex bolt and washer, compare the entire head-and-seat arrangement, including any washer requirements on the drawing. The washer selection Guide explains why washers can serve different functions.
For inch-series dimensions and the reference drawing, open Hex Flange Bolt Dimensions — ASME B18.2.1. Check the flange diameter, head height, wrench size and under-head length against the required edition.
Match the head to the exact standard and series
ASME and ISO publish product requirements; they group the covered products differently. ASME B18.2.1 includes conventional hex heads and hex flange screws in one inch-series document. ASME calls the inch product a hex flange screw; the ISO titles use hexagon bolt with flange. Keep that product name with the document number so the supplier selects the correct form.[1]
| Product standard | Reference and scope | Detail to retain |
|---|---|---|
| Inch hex flange screw | ASME B18.2.1-2012 (R2021), within its covered inch bolt and screw forms.[1] | The flange-head product form, inch thread designation and required edition. |
| Metric small series | ISO 15071:2011, hexagon bolts with flange, small series, product grade A, M5–M16.[2] | The specified small-series product and its complete dimensional requirements. |
| Metric small series, different driving-feature requirements | ISO 4162:2012, product grade A with the external hexagon—the driving feature engaged by the wrench—following product grade B; M5–M16.[3] | The external hexagon's specified dimensions and tolerances. |
| Metric heavy series | EN 1665:1997; the current German adoption is DIN EN 1665:1998-11, with DIN EN 1665 Berichtigung 1:2007-07.[5] | The heavy-series designation and the drawing's required edition and amendments. |
Product grade is different from strength class. The A/B distinction in the ISO titles concerns product accuracy requirements; ISO 4759-1:2000 addresses tolerances for product grades A, B and C.[7] A strength designation such as 8.8 or 10.9 is a separate field. ISO 4162's product-grade-B driving feature does not mean that the bolt has a lower strength class than an ISO 15071 bolt.[2][3]
Likewise, small series and heavy series identify product series, not a strength ranking. Compare the head and flange dimensions for the required nominal size; a matching thread size alone does not establish that the head will fit the available clearance.
Follow DIN 6921 through its actual replacement history
DIN 6921 remains common on supplier listings and older drawings. DIN Media records DIN EN 1665:1998-02 as replacing DIN 6921:1983-06 and DIN 6922:1983-06. That 1998-02 adoption was then replaced by DIN EN 1665:1998-11.[4][5]
This history leads to EN 1665's heavy-series product, rather than making DIN 6921 another name for every ISO flange bolt. For a legacy drawing, confirm whether the order must retain DIN 6921 or may use the named replacement, then compare the actual head dimensions.
Specify the bearing face and the washer arrangement
A smooth underside and a serrated underside are different purchasing descriptions. Serrations are a designed contact feature, not a feature implied by the word flange. State the required underside on the inquiry and provide a detail drawing for a particular tooth or locking design.
The mating surface matters. NASA's Fastener Design Manual describes serrated-face heads and nuts that engage the bearing surface during tightening, with restrictions where surface damage or corrosion is a concern.[6] For painted, coated or sensitive parts, check that the selected design and assembly instructions suit that surface.
A flange also does not settle the locking requirement or remove every reason for a washer. Keep a specified washer or locking device in the assembly until the drawing owner approves a change. For vibration-related selection, continue with Do Lock Washers Work?.
Write an inquiry that selects one flange-head product
Make the head description specific enough to check against both the product standard and the mating part:
- Head form and standard: hex flange screw or bolt, exact document, edition and required small/heavy series.
- Head interface: flange diameter, wrench size, head height and any space restriction around the head.
- Bearing face: smooth or the specified serrated design; include the mating material and surface finish.
- Thread and length: diameter, pitch or TPI, fit, nominal under-head length and required thread length.
- Material and properties: the specified material, strength/property class, finish and inspection documents.
- Assembly: retain the stated washer/locking arrangement and attach the mating-detail drawing where it controls selection.
For example, hex flange screw — ASME B18.2.1-2012 (R2021) and hexagon bolt with flange — ISO 4162:2012 identify different product specifications before size and material are added. Complete the inquiry with How to Specify a Bolt, and use the fastener grades guide to keep product accuracy and strength designations separate.
References
- ASME. ASME B18.2.1-2012 (R2021). Public title and scope for inch bolt and screw forms, including hex flange screws.
- ISO. ISO 15071:2011. Small-series flange bolts, product grade A; public scope.
- ISO. ISO 4162:2012. Small-series flange bolts, product grade A with a product-grade-B driving feature; public scope.
- DIN Media. DIN EN 1665:1998-02 — replacement history. Records replacement of DIN 6921:1983-06 and DIN 6922:1983-06, followed by the 1998-11 edition.
- DIN Media. DIN EN 1665:1998-11. Current German heavy-series adoption of EN 1665:1997; lists the 2007 correction.
- NASA. Fastener Design Manual, NASA RP-1228 (1990), printed page 8, serrated-face heads and nuts. Historical technical guidance on serrated bearing-surface interaction.
- ISO. ISO 4759-1:2000. Tolerances for bolts, screws, studs and nuts, product grades A, B and C; public title and status.