How to Choose Hex Nuts: Style, Property Class, Locking Type and Finish

The regular hex nut is covered by ISO 4032 (style 1) and DIN 934. Both have a height of roughly 0.8 times the thread diameter, which is enough for the nut to carry the full load of a bolt of the matching class. ISO 4033 (style 2) is a taller version for the highest classes, and ISO 4035 and DIN 936 are thin or jam nuts, about half the height, used as a second nut to lock the first or where space is limited.

A thin nut cannot carry the full load of the bolt. If a drawing says 'hex nut' and the supplier sends thin nuts because they are cheaper, the joint will look the same but strip at a much lower load. For imperial work, ASTM A563 covers carbon and alloy steel nuts and ASTM A194 grade 2H covers heavy hex nuts for high-pressure and high-temperature bolting.

• Regular nut (ISO 4032 / DIN 934): the default for almost every joint.
• Thin nut (ISO 4035 / DIN 936): locking against a first nut, or tight spaces; never as the only nut under heavy load.
• Heavy hex (ASTM A563 / A194 2H): structural, pressure and high-temperature joints with heavy hex bolts or studs.

The nut property class is a single number: 6, 8, 10 or 12. It tells you the stress the nut can carry, in units of 100 MPa. The rule is that the nut class number is the same as the whole-number part of the bolt class: class 6 for bolts up to 6.8, class 8 for 8.8, class 10 for 10.9 and class 12 for 12.9. The bolt side is covered in our hex bolts guide. A higher nut class is safe on a lower bolt class, but a lower nut class on a higher bolt will strip first.

Nuts of class 4 and 5 are made from low-carbon steel and are for light use. Nuts of class 10 and 12 are always heat-treated, and so are most class 8 nuts in the larger sizes. When you get a quotation, check that the class is stated and that the price is for that class, because plain low-carbon nuts and heat-treated nuts can look the same and differ a lot in price.

A plain nut holds by friction from the preload alone, so vibration can loosen it. Where that matters, choose a locking nut. A nylon-insert nut (often called a nyloc) uses a polymer ring to grip the bolt thread and is cheap and reusable a few times, but the nylon limits its working temperature to roughly 120 degrees Celsius. An all-metal prevailing-torque nut has a deformed or slotted top and works at higher temperatures. A flange nut has a built-in washer face that spreads the load, and a serrated version bites into the surface to resist turning.

Spring washers are often used for the same purpose but are much less effective than a proper locking nut on a hardened joint, so for vibration-prone work specify the nut, not just a washer (see our washers guide).

Plain nuts are lightly oiled and suit indoor or protected use. Zinc-plated nuts are common for general fastening. For outdoor use, hot-dip galvanized nuts are used, and here the thread is the critical point: the zinc layer fills the thread, so the nut is tapped oversize before galvanizing so that it fits a galvanized bolt. A galvanized nut ordered from a supplier who tapped it at the normal size will not fit.

Stainless steel nuts (A2 or A4, class 50, 70 or 80, see our stainless steel guide) are prone to galling, where the thread seizes on the bolt. Use a little anti-seize paste or wax and tighten at low speed, never with an impact wrench at full speed.

For a bulk lot of nuts, the practical checks are dimensional and functional. Measure the across-flats and height against the standard, screw a sample of nuts onto a go gauge and a gauge bolt to check the thread, and run a proof load test on samples when the class is 8 or above. Check that the class marking is on the nut face or flank where the standard requires it.

Ask for a certificate that shows the material and the heat treatment. Cheap nuts are often not heat-treated at all, and a hardness reading from a sample lot is a quick way to find out.

• Thread: go gauge turns freely, no-go gauge does not.
• Marking: class number present where the standard requires it.
• Surface: no cracks at the corners, no burrs in the thread, even coating.
• Packing: one class, size and finish per labelled carton.

Global Fasteners Exports is a trading and export company: our hex nuts are made by partner factories and we handle inspection, packing, documents and shipping. Because we buy in volume from those factories, prices are typically up to 30% lower than the market price.

For hex nuts, tell us the bolt class they must match and whether they will be galvanized, so we quote the right class and the oversize tap together with your bolts.

Nuts are small and heavy per piece, so cartons and pallets are filled by weight. Send the standard, class, size range, finish and quantity per size and we confirm packing and the container quantity for those sizes.

• Samples or detailed photos can be arranged before a bulk order; sample and courier costs are normally adjusted against the first order.
• Standard payment terms are 50% advance to confirm the order and the balance before dispatch.
• Documents: commercial invoice, packing list, certificate of origin and bill of lading, plus mill test certificates and inspection reports on request.
• Different products can be combined in one consolidated FCL or LCL shipment.