Bolts and nuts
Bolts and nuts are the most standardised product you buy, and for that reason the easiest to order incorrectly. A DIN 931 and an ISO 4014 are the same article on paper, but at M10, M12, M14 and M22 the width across flats differs. Switch supplier without knowing that and you receive bolts your existing spanners do not fit.
Taiwan is the world's third largest exporter of fasteners. The cluster around Kaohsiung holds more than 700 screw factories and accounts for roughly 70 per cent of Taiwanese exports. For standard DIN work in 8.8 and 10.9 that means a quality level European buyers have known for decades, without the anti-dumping duty that sits on Chinese fasteners.
Standards and dimensions
- DIN 931 / ISO 4014
- Hex bolt, partially threaded. Width across flats differs at M10, M12, M14 and M22.
- DIN 933 / ISO 4017
- Hex bolt, fully threaded. DIN runs to M52, ISO to M64.
- DIN 912 / ISO 4762
- Socket head cap screw, usually 10.9 or 12.9.
- DIN 7991 / ISO 10642
- Countersunk socket screw, for a flush finish.
- DIN 6921
- Flange bolt with integrated bearing face, spreading the clamp load.
- DIN 603
- Carriage bolt with domed head and square neck.
- DIN 934 / ISO 4032
- Hex nut. The DIN nut is slightly lower than the ISO version.
- DIN 985 / ISO 10511
- Nylon insert lock nut. Not to be reused.
- DIN 6923
- Flange nut, replacing nut plus washer in a single part.
- DIN 125 / ISO 7089
- Plain washer. DIN 9021 has a larger outside diameter.
- DIN 127
- Spring washer. In modern joints usually replaced by correct preload.
- Many of these DIN standards are formally superseded by DIN EN ISO, but the trade still orders by DIN number.
Property classes
- 4.6
- 400 MPa tensile, 240 MPa yield. Unhardened carbon steel, light structures.
- 5.8
- 500 MPa tensile, 400 MPa yield. Cold worked, common in threaded rod.
- 8.8
- 800 MPa tensile, 640 MPa yield. The workhorse class in steel fabrication and machine building.
- 10.9
- 1040 MPa tensile, 940 MPa yield. Alloy steel, quenched and tempered. Watch hydrogen embrittlement when electroplated.
- 12.9
- 1220 MPa tensile, 970 MPa proof stress. Toolmaking and highly loaded machine parts.
- Nuts to ISO 898-2
- The nut number is the highest bolt class the nut may safely mate with. A class 8 nut does not belong on a 10.9 bolt.
- A2-70 and A4-80
- Stainless, 700 and 800 MPa respectively. Austenitic stainless does not reach 10.9.
Coatings and corrosion
- Electro-zinc plated (ISO 4042)
- 5 to 15 µm. Inexpensive, for interior and light exterior use. Passivation now trivalent chromium.
- Hot-dip galvanised (ISO 10684)
- Over 40 µm. Exterior and structural steelwork. Nuts are tapped oversize, so never mate an oversize nut with an undersize bolt.
- Zinc flake (ISO 10683)
- Geomet, Delta-Protekt, Dacromet. Thin layer, high resistance, and no hydrogen embrittlement risk.
- Ruspert and black oxide
- Limited protection, largely cosmetic or for dry interiors.
- Salt spray testing (ISO 9227)
- A comparative quality test, not a service life prediction. Read the hours as relative ranking.
- Hydrogen embrittlement
- At 10.9 and above, electroplating requires baking at 200 to 230 °C shortly after plating. Zinc flake avoids the problem entirely.
Applications
- Steel fabrication: DIN 931 and 933 in 8.8, hot-dip galvanised, with EN 10204 3.1 material certificates.
- Machine building: DIN 912 in 10.9 or 12.9, black or plated, in smaller batches with tight tolerances.
- Solar mounting systems: flange bolts and nuts in A2 or A4, usually against aluminium profile.
- Trailer and bodywork: carriage bolts DIN 603 and lock nuts DIN 985 in 8.8.
- Greenhouse and agricultural structures: hot-dip galvanised, given permanent moisture exposure.
- Maintenance and MRO: broad ranges in smaller quantities, where consolidating into one container pays.
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