Fixings and fasteners for metalwork

At a glance

Starting points from Euro Inox, SCI and BSSA guidance, not a design check.

Head and drive: what shows

A fixing is a small part that shows. Say which head, which drive and what finish, or the workshop will choose for you. The four heads on the board are the usual ones for visible metalwork.

• Countersunk sits flush, so it needs a countersunk hole and leaves the plainest face. Button is a low dome and needs no countersink. Socket cap is a cylinder: proud, it is the simplest; sunk in a counterbore with a plug over it, it can be hidden. Decide in the maintenance manual how a plug is lifted.
• Do not assume equal strength. The current ISO titles for button head and countersunk hexagon socket screws both say “with reduced loadability”, and the 2020 edition of ISO 3506-1 added requirements for fasteners whose head or shank lowers their load capacity (BSI). Where a fixing carries load, an engineer chooses the head.
• Drive. A hexagon socket takes a key. Hexalobular (six-lobed) drives are covered by ISO 10664. One drive and one bit size across a project keeps fixing and later maintenance simple (our reading).
• Security drives have a pin in the recess, or a shape that ordinary bits do not fit. They slow casual removal. Special bits are sold, and a public-facing fixing that must be serviced still needs an access plan: supply the bit with the handover tools (handover and O&M pack).
• Finish. State whether the fixing shows natural stainless or a finish to match the metal around it. A plated or coated fixing carries only a thin layer: see ironmongery finish codes and plating and coating thickness.

Reading a stainless fastener: A2, A4 and the class

BS EN ISO 3506 covers corrosion-resistant stainless fasteners. SCI Advisory Desk Note 342 explains the designation: a letter for the steel group (A for austenitic, C martensitic, F ferritic), a number from 1 to 5 for the corrosion resistance, and a property class that describes strength. Austenitic fasteners come in three classes, 50, 70 and 80, which stand for minimum tensile strengths of 500, 700 and 800 N/mm².

• Class 70 is cold drawn and the most widely available. Class 80 is severely cold drawn. Both may show some magnetism. Class 50 is annealed, non-magnetic and has the best corrosion resistance, but machined threads in class 50 are more prone to galling. SCI P291 notes that class 70 and 80 are limited to M24 and below in the tables it reproduces.
• A magnet proves nothing. Class 70 and 80 may be magnetic, and A2 and A4 look identical. Ask for fasteners marked with their grade and class, and the supplier's certificate. BSI says the 2020 edition improved marking and labelling.
• Edition. The current edition is BS EN ISO 3506-1:2020 (BSI), which BSI says added duplex classes, property class 100 for austenitic grades and a grade A8. SCI AD 342 and P291 describe earlier editions, so their numbers are quoted here only to explain the designation.
• Rolled threads are stronger than machined ones and gall less, and machined threads need care in marine settings because of crevice corrosion (SCI P291).

Match the metal, then isolate

The rule is Euro Inox's: where metals are combined, make the fasteners from the more noble material, so the area of the part that can corrode is large and the area of the fixing is small. Galvanic corrosion and dissimilar metals has the full table. For fixings, the points that matter are these.

• Stainless fixings in a larger aluminium or galvanised part are the safe way round. Euro Inox shows virtually no galvanic corrosion even in corrosive conditions.
• A zinc-plated, galvanised or carbon steel screw in stainless or copper alloy is the wrong way round. Euro Inox rates a small carbon steel or zinc part in a large stainless one as poor, and a small zinc part in a large copper one as poor. Euro Inox shows galvanised screws holding a stainless cover rusting and staining it.
• Brass and bronze fixings suit brass and bronze work. Euro Inox's table lists copper, not brass, so check the alloy and its resistance to zinc loss in the brass alloys sheet before you rely on brass screws in a damp place.
• Where the joint can get damp and the metals differ, isolate. Euro Inox says other fastener materials, if used, should be separated from stainless by non-metallic washers and bushes. A washer alone leaves the shank touching the hole, so use a sleeve as well as washers, and a gasket between the parts. Coating the more noble part near the joint helps. Coating only the less noble part does not, because any damage leaves a small anode.
• Dry heated interiors need none of this: without a wet film there is no corrosion cell.

Galling, loosening and thread locking

Stainless fasteners usually fail by seizing or by working loose, not by rusting.

• Galling is cold welding of two threads under load. BSSA says austenitic stainless steels show a tendency to gall, and that very polished (below 0.25 µm Ra) or very rough (above 1.5 µm Ra) surfaces make it worse. SCI P291 says it is a problem only in limited circumstances: damaged threads, fine threads or tight-fitting thread forms.
• What helps. Use different grades for bolt and nut (SCI P291 gives A2 with A4). BSSA reports that combinations of work-hardened A4-80 and A2-80 fasteners are less prone to seizure than two class 70 parts. Use rolled threads, a PTFE dry-film or anti-galling agent, and undamaged threads.
• Loosening. SCI P291 says that, because there are no corrosion products, stainless nuts and bolts are more prone to working loose under vibration than to seizing. Footrails, handles and stool bases need a locking method.
• Locking methods. Thread-locking adhesive is a single-component compound that cures between clean metal threads and comes in strengths from easily removable to permanent; a secondary source notes that high-strength grades release with heat (around 230 °C) and that less active metals may need a primer, so ask the supplier about stainless. A nylon-insert lock nut keeps its grip to about 121 °C, so it is not for hot parts. Pick one method per joint: a lubricant and a locking adhesive can work against each other (our reading, to confirm with the supplier).
• Serviceable. Hospitality fixings are removed for cleaning and repair. Choose a removable grade of locking and say so in the maintenance manual.

All fixings in austenitic stainless steel to BS EN ISO 3506-1 and -2 (edition [to be confirmed]): bolts, screws and studs [A4-70], nuts [A4-70], washers [A4]. Grade not lower than the stainless steel fixed. Heads [hexagon socket countersunk, flush with the finished face] at [location]; [button head] at [location]. Drive [hexalobular], one size throughout, [__] bits supplied to the client. Fixings marked with grade and class, with the supplier's certificate. Visible fixings finished [natural / to match adjoining metal]. No substitution without approval.

No carbon steel, zinc-plated or galvanised fixings in contact with stainless steel or copper alloy. Where [stainless steel] is fixed to [aluminium / brass / steel] in a damp or exterior location, isolate with a [polyamide] sleeve, washers under head and nut, and a [gasket] between the parts. Sealant is not isolation. Isolation shown on the shop drawings.

Stainless on stainless threads: nut and bolt in different grades [A2 / A4], rolled threads, [PTFE dry-film lubricant] applied, torque [set by the fastener supplier]. At [footrails, handles, stool bases] use [prevailing-torque nuts / removable-grade thread-locking adhesive, ref. __]. Security fixings [pin-in hexalobular] at [public-facing locations]; [2] bits in the handover tool kit; spares [__] of each size.

Standards and sources

• BS EN ISO 3506-1:2020 and ISO 3506-2:2020 (nuts): current; texts not read.
• SCI Advisory Desk Note 342 (basis: 2009 edition); SCI P291 (2001); BSSA SSAS 5.60 (2001)
• Euro Inox Vol. 1 (2005) and Vol. 10 (2009); PD 6484:1979 (not read)
• ISO 4762, ISO 7380-1:2022, ISO 10642, ISO 10664:2014: listings only.

Questions we are asked

What does A2-70 mean?

A is the steel group, austenitic. 2 is the corrosion-resistance group, which SCI says has the corrosion resistance of 1.4301 (304). 70 is the property class, a minimum tensile strength of 700 N/mm2. A4-80 is the same system for a 316-type fastener of class 80.

Should I specify A2 or A4 screws?

A2 for dry and ordinary interiors, A4 where the stainless it fixes is 316 or where salt or chlorides reach the surface. Euro Inox says the fastener grade should be at least equal to the stainless it fixes in moist conditions. For anything that carries load or hangs in a pool hall, an engineer decides.

Why did my stainless nut seize on its bolt?

That is galling: the two threads cold-welded under load. BSSA says austenitic stainless steels tend to gall. Use a different grade for the nut, rolled threads, undamaged threads and a PTFE dry-film or anti-galling agent.

Can I use brass screws with stainless steel?

Euro Inox's table rates copper with stainless as good in ambient air, but it lists copper, not brass, and says nothing about brass screw strength. Check the alloy, and whether it resists zinc loss, with the partner workshop.

Are tamper-proof screws secure?

No. They slow casual removal. Matching bits are sold, so treat them as a deterrent, and supply the bit for maintenance.

Do stainless screws need thread-locking?

Where there is vibration or knocking, yes: SCI P291 says stainless nuts and bolts tend to work loose rather than seize. Pick a removable method so the joint can be serviced, and ask the supplier whether the product suits stainless.