# Flat washers

Plain-washer geometry, dimensional series, material and hardness designations, manufacturing features, and limits of bearing-area calculations.

Reviewed: 2026-10-30

Tags: components, fasteners, washers

Dueemme Sidol's size-8 ISO 7089 example gives **8.4–8.62 mm** bore and **15.57–16 mm** outside diameter. Using **16 mm OD** and **8.4 mm ID** gives **145.6 mm²** nominal ring area. A hypothetical **1,000 N** over full uniform contact gives **6.87 MPa** average pressure. Check the real support footprint, workpiece opening and washer hardness separately. [Dueemme Sidol dimensional tables](https://www.dueemmesidol.it/en/products/standard-fasteners/washers-plates/iso-7089/).

## Identity and function

A flat washer provides a bearing surface around a fastener. Its shape and dimensions are separate from the mating thread. [Boker’s washer guide, pp. 2–3](https://www.bokers.com/userfiles/file/bokers_washer_guide.pdf).

Boker’s describes flat washers for load distribution. Actual support depends on the mating geometry and contact area. [Boker’s fender washers](https://www.bokers.com/washers/custom-washers/fender-washers/).

## Nominal size and geometry

Nominal metric size identifies the mating bolt diameter, not the washer bore. Dueemme Sidol’s ISO 7089 reference illustrates this with size 8: bore 8.4–8.62 mm, outside diameter 15.57–16 mm, thickness 1.4–1.8 mm, nominally 1.6 mm. These are that product reference’s limits, not dimensions shared by everything called “M8.” [Dueemme Sidol dimensional tables](https://www.dueemmesidol.it/en/products/standard-fasteners/washers-plates/iso-7089/)

| Feature | Meaning | Conditional consequence — geometric synthesis |
| --- | --- | --- |
| Inside diameter, ID or `d1` | Clearance opening, not a thread | Passage depends on the actual fastener geometry, including relevant transitions. |
| Outside diameter, OD or `d2` | Outer footprint | Changing OD changes surrounding clearance and possible bearing coverage. |
| Thickness, `s`, `h` or `t` | Axial material dimension | It contributes to stack height; a nominal value alone does not bound the assembled stack. |
| Flatness, parallelism and edge condition | Shape beyond the three size dimensions | Equal ID, OD and material thickness need not mean equal seating envelopes. |

Boker’s specification form treats OD, ID and thickness tolerances separately. Seastrom’s workmanship reference separately addresses concentricity, flatness, burrs and measurement methods. Thus, three size dimensions do not fully describe a manufactured washer. [Boker’s dimensional fields](https://www.bokers.com/fastquote/), [Seastrom workmanship acceptance standards](https://www.seastrom-mfg.com/standards.aspx)

## Series and related forms

The ISO titles distinguish these metric plain-washer families. “Small,” sometimes described as narrow, “normal” and “large” identify series, not strength ratings. Public publisher pages establish the identities, not complete dimensional tables.

| Reference | Identified family |
| --- | --- |
| [ISO 7092:2000](https://www.iso.org/standard/13674.html) | Small series, product grade A |
| [ISO 7089:2000](https://www.iso.org/standard/13666.html) | Normal series, product grade A |
| [ISO 7090:2000](https://www.iso.org/standard/13669.html) | Chamfered normal series, product grade A |
| [ISO 7093-1:2000](https://www.iso.org/standard/13677.html) | Large series, product grade A |

A product-grade designation, hardness designation and bolt property class are different descriptors; a series name does not supply all three.

Fender washers have comparatively large OD relative to ID. Even one manufacturer’s sources disagree on the name boundary: Boker’s guide says more than three times ID, whereas its product page uses at least 2.5 times ID. The page’s rendered search table also includes rows below its stated threshold. These snapshots support neither a universal ratio nor an assumption that every displayed result satisfies the stated filter. [Boker’s guide, p. 3](https://www.bokers.com/userfiles/file/bokers_washer_guide.pdf), [Boker’s fender page and search results](https://www.bokers.com/washers/custom-washers/fender-washers/)

Other forms introduce different features:

- Shoulder washers, called step or flange washers in Boker’s terminology, have an integral cylindrical sleeve. “Flange washer” therefore needs its product context. [Boker’s guide, p. 2](https://www.bokers.com/userfiles/file/bokers_washer_guide.pdf)
- Finishing or countersunk washers seat the fastener head in a shaped recess; Boker’s offers flanged, unflanged and rolled-flange styles. This differs from the edge chamfer of a plain-washer family. [Boker’s finishing washers](https://www.bokers.com/washers/custom-washers/finishing-washers/)
- Spring washers are intended to deflect: SPIROL’s disc springs are conical components loaded axially. Lock washers form another category; ring-shaped geometry alone establishes no locking performance. [SPIROL disc springs](https://www.spirol.com/product/disc-springs/), [Boker’s guide, pp. 2–3](https://www.bokers.com/userfiles/file/bokers_washer_guide.pdf)
- Captive plain washers belong to screw-and-washer assemblies. ISO 10673:2009 covers small, normal and large steel-washer series for metric assemblies under ISO 10644. Its separate scope prevents assuming that a loose-washer reference completely specifies the captive part. [ISO 10673 abstract](https://www.iso.org/standard/44117.html)

Precision shim washers overlap in shape with fastening washers but can serve spacing or wear functions. SPIROL lists thrust washers, compression rings, piston rings and cylinder rings among its engineered products, with case-hardened and through-hardened options for thrust washers. Those functions and treatments do not follow from a generic flat-washer label. [SPIROL precision shim washers](https://www.spirolshims.com/washers-spacers.html)

## Material, finish and hardness

Flat washers can be metallic or nonmetallic. Boker’s lists carbon and spring steels, stainless steel, aluminum, brass and copper, plus polymers including acetal, PTFE, polyester and nylon. Material variety does not establish equivalent load, temperature, chemical or electrical performance. [Boker’s manufacturing FAQ](https://www.bokers.com/faq/)

Material, condition, finish and hardness are distinct. Bossard BN 20732 identifies an unchamfered large-series washer using ISO 7093-1, stainless A2 and 200 HV. A2 identifies material; 200 HV identifies hardness. Its DIN 9021 cross-reference carries an approximation sign rather than an unqualified identity claim. [Bossard BN 20732](https://www.bossard.com/de-de/eshop/scheiben-flach-ohne-fase/scheiben-ohne-fase-breite-ausfuehrung/p/20732/)

Dueemme Sidol’s ISO 7089 reference associates steel 200 HV washers with bolts through class 8.8 and nuts through class 8, and 300 HV washers with bolts through class 10.9 and nuts through class 10. These reference-specific associations are not washer tensile-strength values or whole-joint approval. Its listed steel finishes include plain, zinc-plated and hot-dip-galvanized executions; hardness alone identifies no finish. [Dueemme Sidol materials, classes and finishes](https://www.dueemmesidol.it/en/products/standard-fasteners/washers-plates/iso-7089/)

ISO 898-3:2018 addresses carbon- and alloy-steel flat-washer mechanical and physical properties for use with specified-property-class fasteners. Its abstract includes captive and loose forms, several shapes and thicknesses of 0.2–12 mm. Properties are evaluated at 10–35 °C and may change outside that interval. The abstract’s description of applications from −50 to +150 °C is not a guarantee of unchanged properties throughout that range. Corrosion resistance and weldability are excluded requirements. Stainless and polymer washers cannot inherit these carbon/alloy-steel requirements through similar geometry. [ISO 898-3 abstract](https://www.iso.org/standard/63232.html)

**Synthesis:** hardness, dimensions and chemistry alone establish neither allowable bearing load, corrosion life, galvanic isolation nor tightening torque. Those depend on the actual joint, mating materials, finish, environment and applicable mechanical evidence. These sources provide no universal washer-omission, substitution, reuse or structural-connection approval.

## Manufacturing features and measurement

Seastrom’s workmanship sheets distinguish burnished shear land, fractured region and rollover on punched edges. Their guidance locates feature-size measurement in the cut region and excludes burr from material-thickness measurement. Burr protrusion is therefore distinct from sheet thickness but can affect the physical envelope. Flatness and face parallelism are separate characteristics. The sheets carry the printed date `2-1-92`: these are historical workmanship conventions reproduced by Seastrom, not newly issued universal ISO tolerances. [Seastrom sheets WD1003–WD1006](https://www.seastrom-mfg.com/standards.aspx)

SPIROL describes grinding and lapping as ways to obtain tighter flatness on its engineered rings. This product/process capability does not establish that ordinary fastening washers receive those operations. [SPIROL precision washer finishing](https://www.spirolshims.com/washers-spacers.html)

## Limits of bearing-area arithmetic

**Geometric synthesis:** for a concentric circular ring, `A = π(D² − d²)/4`, where `D` is OD and `d` is ID. Illustrative values `D = 16 mm`, `d = 8.4 mm` give `A ≈ 145.6 mm²`. These match one boundary pair in the size-8 reference above, not a tolerance-envelope minimum or measured part. [Dimension source](https://www.dueemmesidol.it/en/products/standard-fasteners/washers-plates/iso-7089/)

With the additional hypothetical assumption of full, uniform contact carrying 1,000 N, `F/A ≈ 6.87 N/mm²`, or 6.87 MPa. This is average nominal pressure, not safe load. A workpiece hole larger than the washer bore, unsupported span, misalignment or incomplete seating invalidates the full-contact assumption. Local pressure, washer bending, indentation and preload retention are not calculated. Increasing OD increases geometric area for fixed ID; it does not prove a proportional increase in joint capacity.

## Source versions and coverage limits

Research review: **October 30, 2026 (PDT)**. Retrieval receipts are dated October 31 UTC. These dates are separate from source publication or revision.

The ISO pages identify June 2000 editions for 7089, 7090, 7092 and 7093-1; the last includes a February 2001 corrected version. Their snapshots show 2021 confirmation alongside stage 90.92, “to be revised.” Confirmation does not erase later revision status. These public bibliographic pages are not inspected full standards. [ISO 7089](https://www.iso.org/standard/13666.html), [ISO 7090](https://www.iso.org/standard/13669.html), [ISO 7092](https://www.iso.org/standard/13674.html), [ISO 7093-1](https://www.iso.org/standard/13677.html)

ISO 898-3 is edition 1, May 2018, confirmed in 2023, with [Amendment 1, August 2020](https://www.iso.org/standard/79531.html). The amendment text was not inspected; no amended class table is reconstructed here. ISO 10673 is edition 2, July 2009, confirmed in 2024; its lifecycle separately records publication in June 2009. [ISO 898-3 metadata](https://www.iso.org/standard/63232.html), [ISO 10673 metadata](https://www.iso.org/standard/44117.html)

Boker’s four-page guide has no visible publication/revision date; embedded PDF creation/modification timestamps are April 15, 2020, which do not establish publication. The fender, Bossard and SPIROL pages are undated technical snapshots. Dueemme Sidol embeds an October 1, 2026 update timestamp, which is webpage metadata, not a new ISO edition or product certification. Direct HTTP requests to ISO returned 403 while the web research tool supplied readable publisher snapshots. New requests do not establish origin freshness. Full normative tables, lot properties, service-life predictions and assembly-specific friction/torque data remain outside the evidence.

## Related reference

[Hardware and component references](../index.md)