Fixings, fasteners and threads
Fasteners are where a small specification error becomes an expensive one, because the failure turns up months later as corrosion or a stripped thread rather than immediately. These entries cover the codes stamped on the head, the coatings, the stainless grades and the anchor types - and the mismatches that cause most of the trouble. The fasteners guide and the thread pitch guide go deeper.
Head forms and DIN numbers
- DIN 931
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A hexagon head bolt with a plain shank under the head and thread only on the last portion of its length - a partially threaded bolt.
Buying note. The plain shank is the point: it takes shear load and locates the joint, which a fully threaded fastener does not do as well. Substituting DIN 933 for DIN 931 to use up stock is a real downgrade in a shear application, even though the grade marking matches.
- DIN 933
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A hexagon head set screw, threaded along its entire length. The most common general-purpose hex fastener.
Buying note. “Set screw” here means fully threaded, not the grub screw sense of the phrase. That ambiguity causes regular ordering confusion - if you want a headless screw, say grub screw.
- DIN 934
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The standard hexagon nut, in the common metric sizes and property classes.
Buying note. Match the nut’s property class to the bolt. A class 8 nut on a 10.9 bolt turns the nut into the weak point, so the joint fails by stripping the thread rather than by the bolt yielding - which is both weaker and harder to detect.
- DIN 125 (form A washer)
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The standard flat washer - plain and round. Form A is unchamfered; form B is the chamfered variant. The ISO equivalents are 7089 and 7090 respectively.
Buying note. Washers are the cheapest part of a joint and the most frequently omitted. A flat washer spreads load and protects the surface as the nut turns; without one, a soft or coated surface gets chewed and the achieved clamping force is anyone’s guess.
- Nyloc nut
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A nut with a nylon insert in the collar that grips the bolt thread and resists loosening under vibration.
Buying note. Nylocs are single-use in practice - the insert deforms on first fitting and holds progressively less each time. They also have a temperature ceiling; near heat sources use an all-metal locking nut instead.
- Coach screw
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A heavy screw with a hexagon head and a coarse wood thread, driven with a spanner or socket rather than a screwdriver.
Buying note. Coach screws need a pilot hole sized to the shank core, not to the thread. Driving one into an undersized pilot in hardwood or engineered timber will shear the head off, and extracting the remains is a much longer job than drilling correctly.
- Coach bolt
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A bolt with a domed head and a square collar beneath it that bites into timber, so the bolt cannot rotate while the nut is tightened from the other side.
Buying note. Different item from a coach screw and not interchangeable - a coach bolt passes right through and takes a nut. The domed head also means no purchase for a tool on that side, so plan access to the nut.
- Socket cap screw
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A cylindrical-headed screw driven with a hexagon key, usually to a higher property class than equivalent hex head fasteners.
Buying note. The recess is the weak point. A worn key or an imperial key in a metric socket will round it out, and a rounded socket cap in a counterbore is genuinely difficult to remove. Keep a good set of keys with the fasteners.
- Self-tapping screw
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A screw that cuts its own thread into a pre-drilled hole in sheet metal, plastic or thin material.
Buying note. The pilot hole size is the whole job. Too large and the thread has nothing to bite; too small and the screw shears or distorts the material. Manufacturers publish pilot sizes per screw and gauge - use them rather than the nearest drill in the case.
- Self-drilling (Tek) screw
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A screw with an integral drill point that bores its own hole and forms its own thread in one operation, sized by the thickness of metal it can penetrate.
Buying note. The drill point has a maximum material thickness, and exceeding it means the point burns out before it breaks through, work-hardening the steel as it goes. Match the point number to the actual combined thickness, not to the top sheet.
Strength grades and materials
- EN ISO 898-1
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The standard defining mechanical properties of carbon and alloy steel fasteners - the source of the property class numbers stamped on bolt heads.
Buying note. The marking is on the head. An unmarked bolt has no declared property class, which for anything structural means it cannot be accepted regardless of what the packaging says.
- Property class (8.8, 10.9, 12.9)
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The two-part number on a bolt head. The first figure relates to nominal tensile strength, the second to the ratio of yield to tensile strength. Higher numbers mean stronger and harder.
Buying note. Higher is not automatically better. 12.9 fasteners are hard and correspondingly more brittle, and more susceptible to hydrogen embrittlement when electroplated. For general site work 8.8 is the sensible default; go higher only where the design calls for it.
- Proof load
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The tensile load a fastener can carry without measurable permanent deformation - effectively the working ceiling, below the yield point.
Buying note. Torque figures in tables are calculated to achieve a target preload, usually a percentage of proof load, on a clean, dry or specified-lubrication thread. Lubricate a thread that the table assumed dry and the same torque delivers far more preload - which is how bolts get snapped by the book.
- A2 stainless (304)
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The common austenitic stainless grade, roughly equivalent to type 304. Good general corrosion resistance in ordinary atmospheric conditions.
Buying note. A2 is not a marine grade. In chloride environments - coastal sites, quaysides, road salt, swimming pools - it pits, and pitting is hard to spot until the fastener fails. If salt is in the picture, use A4.
- A4 stainless (316)
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Austenitic stainless with added molybdenum, roughly equivalent to type 316. Substantially better resistance to chlorides and industrial atmospheres than A2.
Buying note. A4 costs meaningfully more and it is worth it near salt water. A4 and A2 look identical, so mixed stock is a real risk - keep them physically separated and labelled. See marine and ports.
- Galling (cold welding)
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The tendency of stainless threads to seize and fuse under pressure as they are tightened, wrecking both fastener and nut so neither can be undone.
Buying note. This surprises people who assume stainless is the easy option. Prevent it with an anti-seize compound, by tightening slowly, and by avoiding power drivers on stainless nuts. Once a stainless fastener galls it usually has to be cut off.
- Galvanic corrosion
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Accelerated corrosion where two dissimilar metals are in electrical contact in the presence of moisture, with the less noble metal corroding preferentially.
Buying note. The classic site error is a stainless fastener into aluminium or galvanised steel in a wet environment - the fastener is fine, and the structure around it corrodes. Isolate with a sleeve or washer, or match the materials.
Coatings and finishes
- BZP (bright zinc plated)
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A thin electroplated zinc coating with a bright passivated finish. The standard low-cost finish on general-purpose fasteners.
Buying note. BZP is an indoor and light-duty finish. The coating is thin enough that outdoor exposure produces white corrosion within months and red rust not long after. Using BZP outdoors to save money is the most common false economy in fixings.
- HDG (hot-dip galvanised)
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A thick zinc coating applied by immersing the fastener in molten zinc, giving far greater corrosion protection than electroplating.
Buying note. The coating is thick and inconsistent, so HDG nuts are tapped oversize to fit HDG bolts. That means HDG nuts and plain bolts are a sloppy, unreliable fit - buy them as a matched pair, not from separate bins.
- Sherardised
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A zinc diffusion coating applied by tumbling parts in zinc powder at high temperature, producing an even matt grey layer that follows the thread form closely.
Buying note. The advantage over HDG is dimensional: sherardising does not build up in threads, so standard nuts still fit. Frequently specified in rail and civils work for exactly that reason, and worth asking about when HDG thread clearance is causing trouble.
- Zinc flake coatings
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Applied coatings of zinc and aluminium flakes in a binder, cured rather than plated, giving high corrosion resistance at low coating thickness. Sold under various proprietary names.
Buying note. Because they are not electroplated, they avoid the hydrogen embrittlement risk that affects high-strength plated fasteners. That makes them a common choice at 10.9 and above where the alternative would be plating a hard fastener.
- Salt spray hours
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A comparative laboratory corrosion figure, quoting hours in a salt fog chamber before red rust appears.
Buying note. Treat it as a way of ranking coatings against each other, not as a prediction of service life. Real exposure involves wet-dry cycling, UV and abrasion that the chamber does not reproduce, so 1,000 hours does not mean 1,000 hours on a quayside.
Thread forms and pitch
- ISO metric coarse
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The default metric thread form, where each nominal diameter has one standard coarse pitch - M10 at 1.5 mm, M12 at 1.75 mm and so on.
Buying note. Coarse is the right default for site work: more tolerant of damage and dirt, faster to assemble, less prone to cross-threading. The full table is in the thread pitch guide.
- ISO metric fine
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Metric threads with a smaller pitch than coarse for the same diameter - M10×1.25 or M10×1.0 rather than M10×1.5.
Buying note. Fine threads give better resistance to vibration loosening and finer adjustment, and are common in automotive and machine assemblies. They also cross-thread easily and are unforgiving of dirt, so they are a poor choice for outdoor site fixings.
- Pitch
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The distance from one thread crest to the next, in millimetres for metric threads. Measured with a thread gauge, or across a known number of crests with a rule.
Buying note. The most useful field trick: measure across ten crests and divide by ten - far more accurate than trying to measure one. If a nut “nearly” fits, that is almost always a pitch mismatch, and forcing it destroys both parts.
- UNC and UNF
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Unified National Coarse and Unified National Fine - the imperial thread forms common on American machinery and older equipment.
Buying note. UNC and metric threads of similar size will start together and then bind, which feels like dirt in the thread rather than a mismatch. Keep imperial fasteners visually separated from metric stock; mixed bins cause damaged components.
- BSW and BSF (Whitworth)
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British Standard Whitworth and British Standard Fine - the older imperial thread forms with a 55-degree flank angle, found on pre-metric British machinery and structures.
Buying note. Whitworth spanner sizes refer to the bolt diameter, not the across-flats measurement, which is why a Whitworth spanner set looks mislabelled to anyone used to metric. Genuinely relevant on heritage plant and older infrastructure.
- BSP and NPT
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Pipe thread forms. BSP is British Standard Pipe, in parallel and taper variants; NPT is the American taper standard. Both are sized by nominal bore rather than by outside diameter.
Buying note. The flank angles differ, but do not try to tell them apart by pitch: at the two commonest sizes, 1/2″ and 3/4″, BSP and NPT are both 14 threads per inch. That coincidence is exactly why they get mixed, start together convincingly and then weep. A half-inch BSP fitting also does not measure half an inch across the thread, which trips people up when identifying loose fittings.
- All-thread (studding)
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Plain bar threaded along its entire length, cut to size on site and used with nuts and washers to make up fixings of arbitrary length.
Buying note. Cut it with an abrasive disc and you will deform the end thread and have to chase it. Run a nut onto the bar past the cut line first, then cut, then wind the nut back off - it reforms the thread as it goes.
- Thread engagement
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The length of thread actually in contact between fastener and tapped hole or nut, which determines whether the joint fails by bolt yield or by thread stripping.
Buying note. The rule of thumb is a minimum engagement of about one diameter in steel, and more in softer materials such as aluminium. A bolt protruding by a couple of threads may look adequate and be well short of it.
Anchors into masonry and concrete
- Through bolt
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A one-piece expansion anchor pushed through the fixture into a hole drilled through both, which expands against the hole wall as the nut is tightened.
Buying note. Quick and strong in sound concrete, and poor in hollow or weak substrates where expansion has nothing to bear against. It also imposes expansion force on the base material, so it needs adequate edge distance - too close to an edge and it will spall the concrete out.
- Sleeve anchor
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An anchor with an expanding sleeve over its length, spreading the expansion force over a larger area of the hole.
Buying note. More tolerant than a through bolt in brick and blockwork, because the load is spread rather than concentrated. Still an expansion anchor, so still not the answer in hollow block - that needs a resin anchor with a sleeve.
- Drop-in anchor
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An internally threaded shell set flush into a drilled hole and expanded with a setting tool, into which a bolt is then threaded.
Buying note. The advantage is a flush, removable fixing - useful where plant is periodically unbolted. The catch is that it must be set with the correct tool; hammering a bolt in instead leaves it partially expanded and it will pull out under load.
- Resin (chemical) anchor
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A threaded stud or bar bonded into a drilled hole with a two-part resin, which cures to transfer load through adhesion rather than expansion.
Buying note. Strongest option, works close to edges and in hollow substrates, and completely dependent on hole preparation. The hole must be cleaned of dust properly - blow, brush, blow - and cure time extends significantly in cold weather. Most resin anchor failures are cleaning failures.
- Edge distance and spacing
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The minimum permitted distance from an anchor to a free edge of the base material, and between adjacent anchors, published by the anchor manufacturer.
Buying note. Anchor performance figures are only valid at or above these distances, and they are frequently ignored on site because the steelwork sits where it sits. If the geometry forces a short edge distance, change the anchor type rather than accepting a reduced capacity by eye.
- Embedment depth
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How deep the anchor sits in the base material. Load capacity is stated at a specified embedment.
Buying note. Drilling shallow because the drill bottomed out on aggregate is the everyday version of this problem, and the resulting anchor may carry a fraction of its rated load while looking identical from outside. Mark the drill with tape and check.
Assembly and torque
- Torque
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The rotational force applied to a fastener, measured in newton metres (Nm). It is a proxy for the clamping force actually achieved, not a direct measurement of it.
Buying note. Most of the applied torque is consumed by friction under the head and in the threads, so surface finish, lubrication and cleanliness change the resulting preload dramatically. This is why the same torque figure gives different clamping force on a plated bolt and a plain one.
- Torque wrench
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A wrench that indicates or limits the torque applied, so fasteners can be tightened to a specified figure repeatably.
Buying note. Click wrenches drift out of calibration and should be checked periodically and stored wound back to their lowest setting, not at the last figure used. Never use one to undo a fastener - that is how the mechanism gets wrecked.
- Washer types
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Form A is the standard flat washer; form C is larger and thicker; penny (repair) washers have a much larger outside diameter; spring washers are split and sprung.
Buying note. Penny washers exist to spread load across thin or soft sheet - without one, the fastener pulls straight through. Spring washers are much less effective against vibration than most people assume; for a joint that genuinely vibrates, use a proper locking nut or a thread locker.
- Thread locker
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An anaerobic adhesive applied to a thread before assembly, curing in the absence of air to resist loosening. Sold in grades from low-strength removable to high-strength permanent.
Buying note. Grade selection matters more than brand. A high-strength grade on a fastener that will need removing means heat or destruction later. Also check the temperature range and note that most need clean, dry threads - applied over oil, they do very little.
- Flange nut
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A nut with an integral flange at its base, spreading the load like a washer and often serrated on the underside to resist loosening.
Buying note. Convenient because it removes a component from the assembly, but the serrations bite into the surface. On a coated, painted or galvanised surface that means damaging the protection at exactly the point where water will sit.
- Shear nut
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A security nut with a sacrificial head designed to snap off at a predetermined torque, leaving a smooth cone that ordinary tools cannot grip.
Buying note. Genuinely effective against casual removal, and equally effective against you. Removal needs a specialist gripper or destruction, so do not use them anywhere requiring routine maintenance access.
Standards, schemes and regulations are described here as general buyer education. Nothing on this page is a claim that any TES-supplied product is certified to a particular standard, or that TES holds a scheme it is not listed as holding on our accreditations. Where a marking matters commercially, check the product’s own certification and the wording of your own specification.
The rest of the glossary
Procurement & compliance - PQQs, assurance schemes, site paperwork and stock arrangements.
PPE & safety standards - The EN standards and markings behind hi-vis, footwear, gloves and RPE.
Tools, abrasives & consumables - Drill shanks, abrasive discs, socket drives and workshop chemicals.
Need this checked against a real specification?
Definitions only get you so far. Send us the actual requirement - a PPE schedule, a fixings list, a PQQ question or a marking you cannot make sense of - and the TES team will come back with what matches it. Trade pricing is on the B2B portal.