How to Choose Hex Bolts: Standard, Property Class, Finish and Supplier

Ordering an M12 hex bolt without naming a standard leaves the supplier free to choose (our DIN vs ISO vs ASTM comparison explains how the systems map to each other). The four standards you will meet most often are DIN 931 (partial thread), DIN 933 (full thread), ISO 4014 (partial thread) and ISO 4017 (full thread). DIN 931 and ISO 4014 are the partial-thread bolts with a plain shank under the head; DIN 933 and ISO 4017 are threaded all the way to the head. In the United States and for structural work you will also see ASTM A307 for low-strength bolts and ASTM A325 and A449 for high-strength ones.

The DIN and ISO bolts are not identical. The two sets have the same thread, but several sizes have a different distance across the flats. For example an M10 bolt is 17 mm across the flats to DIN 931 and 16 mm to ISO 4014, and an M12 is 19 mm against 18 mm. If the bolts will be tightened with a fixed socket, an automatic tool or an existing jig, tell the supplier which standard your tooling is made for.

• Partial thread (DIN 931 / ISO 4014): the unthreaded shank carries shear load and gives a more accurate grip length. Use it where the bolt is loaded across the joint.
• Full thread (DIN 933 / ISO 4017): easier to adjust and to stock, and the better choice when the joint is mostly in tension or the plate stack is thin.
• Imperial bolts (UNC / UNF, ASTM): a different thread family, so never mix them with metric nuts, even when the diameter looks close.

The two numbers on the bolt head tell you its strength. The first number multiplied by 100 is the minimum tensile strength in MPa, and the second is the yield strength as a fraction of tensile. A 4.6 bolt has a minimum tensile strength of 400 MPa and yields at about 240 MPa. An 8.8 bolt is 800 MPa tensile and about 640 MPa yield. A 10.9 bolt is 1000 MPa and 900 MPa, and a 12.9 bolt is 1200 MPa and about 1080 MPa.

Class 4.6 is made from low-carbon steel for non-critical fixings. Class 8.8 is medium-carbon steel, quenched and tempered, and is the standard choice for machinery, frames and general engineering. Class 10.9 is used where space is limited and the bolt must carry more load, such as in vehicles and heavy equipment. Class 12.9 is reserved for special applications and is more sensitive to hydrogen embrittlement and to notch damage, so it is rarely the right choice for plated hex bolts.

Paying for a higher class than the design needs is wasted money, and a lower class than the design needs is a safety risk. If the drawing says 8.8, ask for 8.8 with a mill test certificate that shows the tensile and hardness results, not an unmarked bolt described as 'high tensile'.

A bolt and nut work as a pair. The nut class is the bolt class number without the decimal: a class 8 nut goes with an 8.8 bolt, a class 10 nut with a 10.9 bolt and a class 12 nut with a 12.9 bolt. A nut of equal or higher class can be used, but a lower-class nut will strip before the bolt reaches its rated load, even though the joint seems to tighten correctly.

Our hex nuts guide covers nut styles and locking types. Buy bolts and nuts together from one supplier where you can, and check that the finish is the same on both. A hot-dip galvanized bolt, for example, needs a nut that has been tapped oversize to allow for the zinc layer. A standard nut on a galvanized bolt will not turn, and a galvanized nut on a plain bolt will be loose.

The finish decides how long the bolt lasts outside the factory. Plain or black oxide bolts have almost no corrosion protection and suit only dry indoor use with oil. Electro-zinc plating (clear, yellow or blue passivated) adds a thin layer, usually in the range of 5 to 8 micrometres, which is enough for indoor and sheltered use. Hot-dip galvanizing gives a layer many times thicker, normally 40 micrometres or more, and is the usual choice for outdoor structures, fencing, towers and coastal work.

Zinc flake coatings such as Dacromet and Geomet are used where a thin, even coating is needed without the hydrogen embrittlement risk of electroplating, which matters for bolts of class 10.9 and above. Stainless steel (A2 or A4) is chosen when the bolt itself must not rust, and is covered in our stainless steel fasteners guide.

• Indoor, dry: plain or zinc plated.
• Outdoor, inland: hot-dip galvanized, or zinc flake.
• Coastal, chemical or food plant: A4 stainless steel, or hot-dip galvanized with a protective paint system.
• Class 10.9 and 12.9 with a plated finish: ask the supplier how hydrogen embrittlement is controlled (baking after plating, or a zinc flake coating instead).

Most disputes over hex bolts come from a few repeat causes. The marking on the head does not match the certificate, the thread is out of tolerance because of galvanizing, the length was measured differently (under the head versus overall), or a mix of two property classes is packed in one carton. All of these can be prevented by writing the order precisely.

• State the length as 'under the head' and give the tolerance if it matters.
• Ask for head marking: the manufacturer's mark and the property class must be visible on every bolt of class 4.6 and above.
• Pack different classes, sizes and finishes in separate, labelled cartons.
• Agree the thread tolerance (6g is standard for external metric threads) and whether it applies before or after coating.

For general-purpose bolts a visual check and a sample of dimensional measurements is normal. For classes 8.8 and above, ask for a mill test certificate (EN 10204 type 3.1 where the order requires it) showing the heat number, chemical analysis and mechanical results. Hardness, tensile and proof load tests on sample bolts from the lot are the proof that the head marking is true.

Check the thread with go and no-go gauges, measure the across-flats and head height against the standard, and confirm the coating thickness if a galvanized or zinc finish was ordered. See how to read a mill test certificate. Do not accept a certificate that is not linked to your lot by a heat or batch number, because it proves nothing about the bolts in your carton.

Global Fasteners Exports is a trading and export company: our hex bolts 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 bolts, an enquiry that lists the standard (DIN or ISO), partial or full thread, class, finish and a quantity per size gets an exact quotation, and a drawing or sample is enough for a special length.

Hex bolts move in full containers or palletized LCL lots. Send the standard, property class, size range, finish and quantity per size, and we confirm the container quantity for your exact sizes, since the pieces per container change a lot between M8 and M24.

• 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.