How to Choose Threaded Rods and Stud Bolts: DIN 975, DIN 976 and ASTM A193 B7

A threaded rod (DIN 975) is a bar with a thread along its whole length, usually supplied in one-metre or longer lengths and cut to size on site. It is used for hangers, supports, anchors in resin and general assembly. A stud bolt (DIN 976) is a rod cut to a fixed length with a continuous thread, and a double-end stud has a thread at each end with a plain section in the middle, or a tap end screwed into a tapped hole and a nut end.

For anchoring a rod in concrete see our anchor bolts guide. Most users call all of these 'studs', but the terms matter in a purchase order. Say whether you need a rod in bar length or a cut stud, the full thread length, and whether the thread is continuous.

Commercial DIN 975 rod is usually made from low-carbon steel in property class 4.6 or 4.8, with a low tensile strength of around 400 to 500 MPa. It is cheap and fine for hangers and light fixings. It is not suited to bolting a flange. For load-bearing use, specify a rod of property class 8.8, or higher, and ask for a certificate.

For flanges and pressure equipment in refineries, power plants and chemical plants, the specification is ASTM A193. Grade B7 is a chromium-molybdenum alloy steel, quenched and tempered, with a minimum tensile strength of 125 ksi (about 860 MPa) and is the most widely used grade. Grades B8 and B8M are 304 and 316 stainless steel (see our stainless steel guide), with a lower strength but corrosion resistance. ASTM A320 covers studs for low-temperature service, such as Grade L7 for cryogenic and cold-climate pipelines.

• DIN 975 class 4.6 / 4.8: hangers, supports, light fixings.
• Class 8.8 / 10.9 rod: structural and machine use.
• ASTM A193 B7: flanges, valves and pressure vessels up to high temperature.
• ASTM A193 B8 / B8M: corrosive service where stainless is needed.
• ASTM A320 L7: low-temperature and cryogenic service.

A stud is only as good as the nut that goes on it. B7 studs take ASTM A194 grade 2H heavy hex nuts, B8 studs take A194 grade 8 and B8M studs take A194 grade 8M. For L7 studs the nut is A194 grade 4 or 7. Our hex nuts guide explains nut classes in general. A commercial nut on a B7 stud will strip before the stud reaches its rated load. Order the nuts with the studs, and with the same coating.

Where PTFE or Xylan coatings are used on both parts to reduce friction and give corrosion protection, the thread allowance already allows for the coating, so do not add a second coating on top.

For a flange joint the stud length is the total of the two flange thicknesses, the gasket thickness, the two nut heights, and the protrusion beyond each nut, normally one to two threads. The result is rounded up to the nearest standard length. A short stud leaves the nut with too little engagement, and a stud that is too long causes interference with neighbouring parts.

Always say how the length is measured. On a double-end stud the length is the overall length, not the length of the thread on one end. When you reorder from an old stud, measure the whole stud and not only the threaded part.

Metric studs normally use a coarse ISO thread with tolerance 6g, and imperial studs use UNC, or 8UN (eight threads per inch) for sizes of 1 inch and above in pipe flange bolting. For fine-thread and special-pitch studs, state the pitch in the order. Rolled threads are stronger than cut threads, because the grain of the steel follows the thread form, so ask for rolled threads on a stud that carries dynamic or high loads.

Check the thread with gauges and the end of the stud for chamfer and burrs. Studs cut from long bars should be deburred and chamfered at each end, so that nuts start easily.

Plain B7 studs are supplied with a light oil and suit protected, inland service. Zinc plating gives mild protection. Hot-dip galvanizing is used for outdoor structural rods, with oversize nuts. PTFE or Xylan coatings are common on flange studs in offshore and chemical plants, because they resist corrosion and reduce friction, giving a more reliable tightening torque. Stainless studs are used without a coating.

For pressure equipment ask for an EN 10204 3.1 certificate that shows the heat number, chemical analysis, tensile, yield, elongation and hardness results, and the heat treatment, which for B7 is quenching and tempering. Make sure the markings on the stud (the grade B7 and the manufacturer's mark) agree with the certificate. For stainless B8 and B8M, check the class (class 1 solution-treated or class 2 strain-hardened), because they have very different strength levels.

Global Fasteners Exports is a trading and export company: our threaded rods and studs 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 rods and studs, state the grade, diameter, overall length and thread length, and whether matching nuts and a coating are needed; for flanges, send the flange rating and we work out the length.

Rods are shipped in bundles of full or cut lengths, and studs in cartons or bags by size. Send the standard, grade, diameter, length and thread length, coating, and the matching nuts, and we confirm the packing and lead time.

• 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.