Engineering Guide · 8 min read
Hex Flange Bolts: ASME B18.2.1, ISO 4162 and DIN 6921
Why use hex flange bolts? Compare load spreading, washer handling and bearing surfaces, then match ASME B18.2.1, ISO 4162 and legacy DIN 6921 specifications.
The short answer: a hex flange bolt puts a wider bearing face under an external hexagon head. Designers use it to spread the clamping force over a larger supported area and, where the joint permits, eliminate a separate flat washer from assembly. The benefit depends on the surface beneath the flange: its available area, strength and finish. 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.
For the design purpose shared by flange bolts and flange nuts, start with Flange Bolts and Nuts: Why the Flange Matters. This article focuses on selecting the bolt and its product standard.
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 wider seat is there to spread load. At the same clamping force, a larger effective contact area gives a lower average pressure on the clamped part. That matters when a narrow bolt head would indent a thin steel bracket or a softer mating material. If the surface beneath the head permanently compresses, the joint loses thickness and some bolt preload can be lost. Nord-Lock describes this loss through settlement of surface irregularities and, over time, creep of susceptible materials.[8] A suitably supported flange can help reduce local indentation; the bearing face and the part beneath it still need to withstand the design load.
The other benefit is one less loose component to handle. When an approved flange-head design replaces a bolt and a load-spreading flat washer, the assembler feeds and positions one piece instead of two. There is no separate washer to drop or omit. This can simplify repetitive assembly and work in awkward locations, even when a bolt with a correctly selected washer would provide an equally suitable bearing area.
The choice becomes clearer when it is tied to the actual part being fastened:
| Assembly situation | Why consider a flange head? | What decides the choice? |
|---|---|---|
| Sheet-steel guard or machine bracket | A wider seat can spread the clamping force; an integral flange reduces loose-part handling. | Enough flat support around the hole, suitable sheet thickness and acceptable indentation under preload. |
| Aluminum housing or mounting part | Lower mean contact pressure may help protect the softer bearing surface. | Allowable bearing pressure, temperature and settlement. A suitably sized, hardened washer may be the better interface. |
| Slotted hole, oversized hole or nearby edge | The flange offers a wider outline, but only the supported part of its underside carries load. | Check the minimum supported area at every permitted position. Use a properly designed washer or plate if the flange cannot bridge the opening adequately. |
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 separate washer gives more freedom to design the interface. Its outside diameter, thickness, hardness and material can be selected independently of the bolt head. That is useful where a drawing calls for a larger bearing area, a hardened seat or a replaceable contact surface. Check the space as well: a flange needs clearance beside the head and a flat seat beneath it, so it may interfere with an edge, recess or nearby bend that accepts an ordinary hex head.
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.
Recheck tightening instructions when changing the head-and-washer arrangement. The bearing interface contributes friction during tightening, so a change in the contact geometry, coating or washer arrangement can change the torque needed for a target preload. Fastenal lists under-head friction, surface finish and washer presence among the variables affecting the torque–tension relationship.[9] Use an assembly specification validated for the selected fastener and mating surface; matching thread size and strength class is insufficient for carrying over a torque value.
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.
- Nord-Lock Group. Safe Bolted Joint Design, printed page 6, “Settlement and creep.” Explains how deformation of contact surfaces reduces preload.
- Fastenal. Bolted Joint Design, revision March 4, 2009, PDF pages 9–10. Describes friction and assembly variables in torque-controlled tightening.