Hex Nut

How to choose hex nuts: thread, class and finish

A hex nut has to match the bolt three ways. Get the thread right first, then the strength, then the finish for where it will live.

1. Thread: the size (M10 fits a 10mm bolt, 1/2 inch a 1/2 inch bolt) and the pitch. Metric coarse is the everyday thread; fine pitch, UNC, UNF and BSW are different threads and do not mix.
2. Strength: match the nut class to the bolt. Class 8 nuts go on class 8.8 bolts and class 10 on 10.9, as Hobson's torque sheets pair them for nyloc nuts; Hobson's commercial bolt and nut assemblies pair class 4.6 bolts with class 5 nuts. For imperial, the general practice is grade 5 nuts on grade 5 bolts and grade 8 on grade 8.
3. Finish: zinc plated for dry, indoor work; hot dip galvanised or stainless outdoors (stainless only in the sea spray zone); 316 stainless within 2m of a pool. Match the bolt's finish, so the nut does not corrode first.

Metric hex nut sizes (M6, M8, M10, M12 and up) with spanner sizes

Every metric coarse thread hex nut size we carry. The spanner size depends on the standard the nut is made to, and AS 1112 and DIN 934 differ at M10, M12, M14 and M22. Most steel nuts are AS 1112, and the Bumax stainless nuts are ISO 4032, which takes the same spanners as AS 1112. The other stainless and the nylon nuts are DIN 934, and so are the plain class 12 nuts in M10, M12, M16, M20, M27, M36, M42 and M48; the plain class 12 M24 and M30 nuts are Soko AS 1112. Select a size to open it in the selector.

Fine pitch metric hex nuts

Fine pitch nuts have more threads per length than coarse nuts of the same size, for fine thread bolts and studs. Check the pitch on the bolt before you order: an M12 x 1.25 nut will not go on an M12 x 1.50 bolt. The big M39 to M100 nuts are fine pitch too.

Imperial hex nuts: UNC, UNF, BSW and ACME

Imperial nuts are sized by the bolt diameter in inches, or by gauge number for the smallest (#2 to #12). UNC is the coarse thread and UNF the fine; BSW is the older British Whitworth thread; ACME nuts (6 threads per inch) are a trapezoidal thread for lead screws and adjusters. Check the bolt before you order, as the threads do not mix.

Thin hex nuts (jam nuts)

A thin nut is a lower hex nut, also called a jam nut or lock-nut. Hobson's brass thin nut sheet explains the correct use: fit the thin nut under the full nut. As the full nut is tightened, the thin nut deforms and locks on the thread, which is why a softer thin nut works better.

For AS 1252 structural bolt assemblies tightened snug tight and subject to vibration, Hobson's lock-nut advice is a hot dip galvanised AS 1112.4 thin nut of property class 04. Fit it above the structural washer and finger tighten it, fit the class 8 full nut, then tighten the full nut while holding the lock-nut with a thin spanner. The thin nut is working as a locking washer and the strength comes from the class 8 nut, so a class 8 lock-nut is not needed. Hobson notes lock-nuts are generally not needed in fully tensioned joints, and if one is used there, only the part turn method applies and no DTIs or torque methods should be used with it.

Hex nut finishes and materials: which suits the job

Hobson's corrosion guide for metal connectors gives the finish by exposure: zinc plated is only for enclosed work in very low and low corrosion areas, and anything exposed to the weather needs hot dip galvanised or 304 or 316 stainless (stainless only in the sea spray zone), with 316 within 2m of a pool edge. Use the same rule for the nut and bolt in that job.

• •Zinc plated: a 5 micron zinc coating with a clear passivate, for indoor and non-corrosive conditions.
• •Hobkote (class 5, M10 to M20): a 5 to 8 micron zinc coating with a special passivation that Hobson says gives 33% more corrosion resistance than zinc plated.
• •Hot dip galvanised: a 50 micron coating for long term outdoor work. Hobson notes no zinc whiskers have been found on its hot dip galvanised range, which suits sensitive sites such as data centres, unlike electroplated zinc.
• •Zinc flake silver: a 10 micron coating with high corrosion resistance and no risk of hydrogen embrittlement. Hobson's ZFS hex nut sheet lists 1,000+ hours salt spray and corrosion category 4 (ISO 9223) fastening, and the nuts suit plain, electroplated and zinc flake coated bolts.
• •Zinc black passivate (class 5, M3 to M8): for electronics and appliances, and to match black screws and washers.
• •Plain steel: no coating, supplied lightly oiled.
• •304 and 316 stainless: 316 has better resistance near the sea. Neither 304 nor 316 is made to sit in seawater. Stainless threads can gall; Hobson suggests lubricating the threads (moly or Xylan type coatings help) or using a different stainless grade for the bolt and the nut. The 316 nuts with a 1PX1 moly coating (M12 to M36) are one option.
• •Neptune (A4-80) and Bumax (A4-80 and A4-100): high strength 316 stainless nuts. Bumax nuts are made by Bufab in Sweden, with full traceability and 3.1 certificates available for each item.
• •Brass: Hobson lists brass as corrosion resistant, non-sparking, non-magnetic and highly conductive, for electrical, lighting, automotive and marine work.
• •Nylon (natural, metric M2 to M30 and #10 to 1 inch UNC): Hobson notes nylon absorbs moisture, which lowers its tensile strength and stiffness.

Hex nut proof loads by class

A nut's class sets the proof load it must take in testing without stripping. These are test values from Hobson's hex nut properties sheet, not working loads: the load a joint can carry depends on the bolt, the joint and how it is tightened.

Left-hand hex nuts and heavy hex nuts

• •Left-hand hex nuts tighten anticlockwise, for rotating parts where a right-hand thread could undo itself. We carry M16 to M36 in plain class 5, M16 to M24 in zinc plated class 5, M16 to M36 in hot dip galvanised class 8, and M5 to M16 in 316 stainless. Set thread direction to Left-hand in the selector.
• •Heavy hex nuts to ASTM A194 (grades 2H, 2HM, 3, 4L, 6, 7, 7L, 7M, 8, 8M, 8MA and 8T, plus duplex and super duplex to ASTM A1082) are for stud bolt and flange work in petrochemical, oil, gas and mining plant. They are thicker with larger across-flats than standard nuts, and are covered in detail on the heavy hex nut page.

Hex nuts in Adelaide

Allmat supplies hex nuts, bolts, washers and threaded rod to builders, fabricators, mechanics, farmers and trade across Adelaide and South Australia. Add the nuts to a quote with the bolts and washers they go on, and we will price the lot.

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