Fasteners & Threads

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Those are 10.9s in zinc flake now, so the old dry K factor is junk, rerun the torque-tension study before you reset the nut runners.

  TranslationThe bolts have a new coating with different friction, so the torque values on the automatic tightening tools must be recalculated from fresh measurements of how much tension each torque produces.

• Snug the splice, match-mark it, then turn-of-nut a half turn on every bolt and leave the marks for the inspector.

  TranslationPull the steel connection together first, paint reference lines across each nut and bolt end, rotate each nut half a turn to pretension the bolts, and leave the lines so the inspector can confirm the rotation.

• GO plug ran in fine but the NO GO walked right in too, the holes are big on pitch diameter.

  TranslationThe tapped holes accepted both gauges, which means the internal threads are oversize at the diameter that controls fit, so they fail inspection.

• Somebody ugga-dugga'd the stainless flange with the rattle gun and now half the nuts are galled solid.

  TranslationA worker ran the stainless nuts on fast with an impact wrench and no lubricant, and the threads cold-welded together.

• Station twelve is passing rehits, so pull the rundown curves and see if we're hiding cross-threads.

  TranslationThe assembly tool is approving fasteners that were already tight, so check the recorded tightening data for signs that some screws were started crooked.

• Three lines on the head, no maker's mark and no mill cert, that lot isn't going anywhere near the crane.

  TranslationThe bolts claim to be Grade 5 but carry no manufacturer's identification or material paperwork, so they cannot be trusted for a lifting application.

• Bolts are necked down and every torque stripe has walked off, that joint's been slipping for weeks.

  TranslationThe bolts have stretched past yield and the paint marks show the nuts have rotated, which means the joint lost preload and has been moving in service.

Bolts, screws, studs, nuts, washers, rivets and inserts, and what each one is actually for.

• All-threadalso threaded rod, ready rod
  Shop name for threaded rod: steel bar threaded end to end, cut to length for hangers, anchors and long studs. Shop talk
• Belleville washeralso disc spring, conical spring washer
  A dished, conical spring washer that flattens under load and keeps clamp force on a joint as it settles or expands.
• Boltalso through bolt, hex bolt
  A headed, externally threaded fastener meant to be tightened by turning a mating nut rather than its own head.
• Cap screwalso hex cap screw, HHCS
  A precision-headed screw, hex or socket, made to close tolerances and usually driven into a tapped hole.
• Castle nutalso castellated nut, slotted nut
  A slotted nut with notches across its crown that take a cotter pin through a hole drilled in the bolt.
• Helical insertalso Heli-Coil, wire thread insert
  A coil of diamond-section wire screwed into an oversize tapped hole to repair a stripped thread or armor a soft one.
• Huck boltalso lockbolt, Huck
  Shop name for a lockbolt: a grooved pin with a collar swaged on by a tool, giving a joint that will not vibrate loose. Shop talk
• Jam nutalso thin nut, check nut
  A thin nut, roughly half to two-thirds standard height, run up against another nut or part to stop it turning.
• Nutalso hex nut
  An internally threaded part that mates with a bolt or stud, graded so the bolt breaks before the nut strips.
• Nylocalso nylock, nylon lock nut
  Shop name for a nylon-insert lock nut, the hex nut with a colored plastic ring captured in one end. Shop talk
• Pop rivetalso blind rivet, POP rivet
  Shop name for a blind rivet: a hollow rivet set from one side by pulling a mandrel that snaps off when tight. Shop talk
• Prevailing-torque nutalso locknut, all-metal locknut
  A nut built to resist turning on the thread before it seats, using a nylon insert or deliberately distorted threads.
• Rivnutalso rivet nut, nutsert
  A blind threaded insert crushed into a hole in sheet or tube so a screw can be fitted from one side. Shop talk
• Screw
  An externally threaded fastener tightened by turning its head, into a tapped hole or a thread it makes itself.
• Set screwalso grub screw, socket set screw
  A headless screw, usually socket driven, whose point bears on a shaft to fix a pulley, collar or knob in place.
• Split lock washeralso lock washer, spring washer
  A split, helical spring washer sold to stop loosening, though it does little once a joint is properly tightened.
• Studalso stud bolt, double-end stud
  A headless rod threaded at both ends or along its full length, used with nuts or screwed into a tapped hole.
• Tek screwalso Teks, self-drilling screw
  A self-drilling screw with a drill-bit point that bores its own hole in steel and then threads it in one pass. Shop talk
• Thread-forming screwalso thread-rolling screw, trilobular screw
  A screw that displaces material to form its own mating thread in a plain pilot hole, without cutting chips.

Head styles, recess drives and the wrenches and keys that turn them, plus what happens when they slip.

• Allen keyalso Allen wrench, hex key
  Shop name for an L-shaped hex key used to drive socket head cap screws and set screws. Shop talk
• Cam-outalso cam out, cammed out
  The driver bit climbing out of a screw recess under torque, chewing up the recess and wearing the bit.
• Flat headalso FHCS, countersunk screw
  A countersunk screw head that sits flush in a cone-shaped seat, 82 degrees for inch screws and 90 for metric.
• Heavy hexalso heavy hex nut, heavy hex bolt
  A hex head or nut wider across the flats than the regular series, used for structural and pressure bolting.
• Phillipsalso cross recess, Phillips head
  A cross-shaped recess drive with tapered walls, the default drive on sheet metal, wood and small machine screws.
• Rattle gunalso impact gun, impact wrench
  Shop nickname for an impact wrench, from the hammering racket it makes running fasteners on and off. Shop talk
• Robertsonalso square drive
  A square-socket screw drive, standard in Canadian construction, with color-coded bit and handle sizes.
• Rounded offalso rounded, chewed up
  Describes a hex head, nut or socket recess whose corners have worn smooth so wrenches just slip. Shop talk
• SHCSalso socket head cap screw, socket head
  Socket head cap screw: a round-headed, high-strength screw driven with a hex key into a tapped hole.
• Torxalso star drive, hexalobular
  A six-lobed star drive that transmits more torque with less cam-out than cross or hex recesses.

Thread series, pitch, diameters, classes and gauges that decide whether two threads go together.

• Chasealso chase the threads, thread chaser
  To run a tap, die or thread chaser through an existing thread to clean and reform it without cutting it oversize. Shop talk
• Class 3A/3Balso 3A, 3B
  The tightest standard inch thread fit, with no allowance on the external thread, used for precision hardware.
• Cross-threadalso cross-threaded, cross-threading
  To start a nut or screw crooked so its thread cuts across the mating thread instead of following it. Shop talk
• Flankalso thread flank
  Either sloping side face of a screw thread, the surface that actually carries load between bolt and nut.
• ISO metric threadalso metric thread, M thread
  The 60-degree metric screw thread, written as M plus diameter and pitch in millimeters, such as M10 x 1.5.
• Leadalso thread lead
  The distance a thread advances along its axis in one full turn, equal to pitch times the number of starts.
• Left-hand threadalso LH thread, reverse thread
  A thread that tightens counterclockwise, used where normal rotation would loosen a right-hand fastener.
• Major diameteralso outside diameter, OD
  The largest diameter of a screw thread, over the crests of an external thread or the roots of an internal one.
• Minor diameteralso root diameter
  The smallest diameter of a screw thread, at the roots of an external thread or the crests of an internal one.
• Percent threadalso thread percentage, percentage of thread
  How deep an internal thread is compared with full theoretical depth, set mainly by the tap drill size.
• Pitchalso thread pitch
  The distance from one thread crest to the next, measured parallel to the screw axis.
• Pitch diameteralso PD, effective diameter
  The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.
• Righty-tighty, lefty-looseyalso righty tighty lefty loosey
  Rhyme reminding that standard threads tighten when turned clockwise and loosen counterclockwise. Shop talk
• Thread classalso class of fit
  A number-and-letter grade, such as 2A or 3B, that sets how much tolerance and allowance an inch thread has.
• Thread engagementalso length of engagement, LE
  The axial length over which external and internal threads fully overlap, which sets stripping strength.
• Thread plug gaugealso GO/NO GO gauge, thread plug
  A hardened GO and NO GO gauge for internal threads: the GO end must screw in, the NO GO end must not.
• TPIalso threads per inch
  Threads per inch: the count of thread crests in one inch of length, used to state the pitch of inch threads.
• UNCalso coarse thread, UNRC
  Unified National Coarse: the standard coarse inch thread series, such as 1/4-20, 3/8-16 and 1/2-13.
• UNFalso fine thread, UNRF
  Unified National Fine: the inch thread series with more threads per inch, such as 1/4-28, 3/8-24 and 1/2-20.
• UNRalso UNRC, UNRF
  Unified National Round root: an external inch thread with a required rounded root, for better fatigue life.

Property classes, SAE and ASTM grades, proof load and the markings that prove what a bolt can carry.

• 18-8 stainlessalso A2 stainless, 304 stainless
  Fastener shorthand for austenitic 300-series stainless, about 18 percent chromium and 8 percent nickel.
• A325also A325 bolt, F3125 Grade A325
  The workhorse high-strength structural bolt: heavy hex, quenched and tempered, 120 ksi minimum tensile.
• B7also A193 B7, B7 stud
  The chromium-molybdenum alloy stud grade that is the default for pipe flange and pressure vessel bolting.
• Grade 8also G8, six-line bolt
  Shop shorthand for the strongest common inch bolt, and loosely for anything overbuilt; six lines on the head. Shop talk
• Hardware-store gradealso no-name bolt
  Dismissive term for unmarked, soft, zinc-plated bolts of unknown origin with no certified strength. Shop talk
• Head markingalso head stamp, grade markings
  The grade symbols and maker's mark stamped on a bolt head, the main clue to its strength once it leaves the box.
• Proof loadalso proof stress, proof test
  The tensile load a full-size fastener must hold without measurable permanent stretch, about 90 percent of yield.
• Property classalso metric grade
  The metric strength grade of a bolt or nut, stamped on the head as a number such as 8.8, 10.9 or 12.9.
• SAE gradealso SAE J429 grade
  The inch bolt strength system, chiefly Grades 2, 5 and 8, identified by radial lines on the head.
• Tensile strengthalso ultimate tensile strength, UTS
  The maximum pulling load a fastener can take before breaking, stated as stress over its tensile stress area.

Torque, tension, clamp load and the methods, tools and habits used to get a joint tight and keep it there.

• Anti-seizealso never-seize, anti-seize compound
  A grease loaded with metal or ceramic particles that stops threads from galling, rusting or bonding under heat.
• Bolt stretchalso elongation
  The elastic elongation of a bolt under preload, measured directly as the most accurate check of tension.
• Bolt-upalso flange bolt-up, bolting up
  The assembly and tightening of a gasketed flange or bolted joint as a job, done in a pattern over several passes. Shop talk
• Break loosealso crack loose, bust loose
  To get a tight or seized fastener to start turning in the loosening direction, the hardest moment of removal. Shop talk
• Breakaway torquealso audit torque, residual torque
  The torque needed to start a tightened fastener turning again, used to audit joints and rate locking features.
• Clamp loadalso clamping force, clamp force
  The compressive force a tightened fastener puts on the joint members, holding them together in service.
• Click wrenchalso clicker, micrometer torque wrench
  Shop name for an adjustable torque wrench that clicks and briefly gives way when the set torque is reached. Shop talk
• Double-nuttingalso double nut, jamming two nuts
  Locking two nuts together on a stud so a wrench on one of them can screw the stud in or out. Shop talk
• DTIalso direct tension indicator, load-indicating washer
  Direct tension indicator: a hardened washer with bumps that flatten when the bolt reaches required tension.
• Embedmentalso seating loss, settling
  Preload lost soon after tightening as surface roughness under the head, nut and threads flattens out.
• Gutentightalso good-n-tight, German torque spec
  Mock-German torque spec meaning tightened by feel until it seems good and tight, with no wrench reading. Shop talk
• Hard joint
  A bolted joint that goes from snug to final torque in very little rotation, typically under about 30 degrees.
• Nut factoralso K factor, K
  The empirical constant K in T = KDF that lumps all the friction and geometry between torque and tension.
• Nut runneralso nutrunner, DC tool
  A powered tightening spindle, handheld or fixtured, that drives nuts and bolts to a controlled torque or angle.
• Preloadalso bolt tension, initial tension
  The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
• Rehitalso re-hit, double hit
  Running a power tool on a fastener that is already tight, giving a false good reading or extra torque. Shop talk
• Rundownalso run down, tightening cycle
  One complete tightening cycle by a powered tool on an assembly line, from first thread to final torque or angle.
• Self-looseningalso vibration loosening, backing off
  A tightened nut or bolt turning loose by itself, usually because sideways slip in the joint lets it walk off.
• Snug tightalso snug, snugged
  The first stage of bolting, where the joint plies are pulled into firm contact before final tightening.
• Squirteralso squirter DTI
  Shop name for a direct tension indicator filled with colored silicone that squirts out when the bolt is tight. Shop talk
• TC boltalso twist-off bolt, tension control bolt
  Tension control bolt: a structural bolt with a splined tip that shears off once the right tension is reached.
• Threadlockeralso Loctite, blue
  An anaerobic adhesive applied to threads that hardens between metal surfaces to stop loosening and seal.
• Torque stripealso torque seal, witness mark
  A paint line drawn across a tightened nut, bolt and joint to show it was torqued and reveal any later movement. Shop talk
• Torque-tensionalso torque-tension relationship, T = KDF
  The relationship between the torque applied to a fastener and the preload it produces, dominated by friction.
• Torque-to-yieldalso TTY, stretch bolt
  Tightening by torque plus angle until the bolt just yields, giving consistent high preload; the bolt is single-use.
• Turn-of-nutalso part-turn method, turn of the nut
  A pretensioning method that snugs the nut, match-marks it, then turns it a fixed fraction of a turn.
• Ugga duggaalso ugga-dugga'd, ugga duggas
  Imitation of an impact wrench, meaning tightened by impact until it sounded done rather than to a torque spec. Shop talk

How fasteners are headed, threaded, heat treated and finished, and how coatings change their behavior.

• Bakealso embrittlement relief, relief bake
  To heat plated high-strength fasteners soon after plating so absorbed hydrogen escapes before it cracks them.
• Cold headingalso cold forming, heading
  Forming bolt and screw heads from wire at room temperature by striking it in dies at high production rates.
• HDGalso hot-dip galvanized, galv
  Hot-dip galvanized: coated by dipping in molten zinc, leaving a thick, rugged layer for outdoor steel.
• Headeralso cold header, former
  The machine that cold heads fasteners, feeding wire, cutting blanks and striking them in a series of dies.
• Overtappingalso overtapped nut, oversize tapped
  Tapping a nut oversize after galvanizing so it will fit over the thick zinc on a hot-dip galvanized bolt.
• Passivationalso passivate, citric passivation
  An acid treatment that strips free iron from stainless fasteners and restores their chromium oxide film.
• Quench and temperalso Q&T, hardened and tempered
  The hardening heat treatment behind strength-grade bolts: heat, quench rapidly, then reheat to toughen.
• Thread cuttingalso cut thread
  Making a thread by removing metal with a tap, die, single-point tool or thread mill.
• Thread rollingalso rolled thread, roll threading
  Forming threads by squeezing a blank between hardened dies, displacing metal instead of cutting it away.
• Upsetalso upsetting, gathering
  To thicken part of a wire or bar by squeezing it along its length, the basic move that forms a fastener head.
• Zinc flakealso zinc flake coating, Geomet
  A baked-on coating of zinc and aluminum flakes in a binder, giving high corrosion resistance without plating hydrogen.
• Zinc platingalso electroplated zinc, clear zinc
  An electroplated zinc layer, usually with a clear, blue or yellow passivate, the cheapest common fastener finish.

How fasteners crack, strip, gall and seize, and the paperwork and checks that catch bad parts.

• Easy-outalso screw extractor, EZ-out
  Shop name for a spiral screw extractor driven backward into a drilled broken bolt to unscrew it. Shop talk
• Fatiguealso fatigue failure, fatigue crack
  Cracking that grows a little with every load cycle until a bolt breaks far below its static strength.
• Gallingalso gall, galled
  Adhesive wear where thread surfaces cold-weld and tear under load, seizing a nut partway down a bolt.
• Head burstalso burst, head crack
  A split on the edge of a cold-headed bolt head, caused by metal being stretched past its ductility.
• Hydrogen embrittlementalso HE, hydrogen-assisted cracking
  Delayed brittle cracking of hard steel fasteners caused by hydrogen absorbed from plating, cleaning or corrosion.
• Mill certalso MTR, material test report
  Shop name for the material test report tracing steel back to its mill heat, chemistry and test results. Shop talk
• Necked downalso necked, stretched
  Describes a bolt visibly thinner in one spot because it was stretched past yield and started to neck. Shop talk
• Spinneralso spun, free spinner
  A fastener that just turns without tightening or loosening, because its thread, insert or captive nut gave up. Shop talk
• Thread strippingalso stripped thread, thread shear
  Failure where the threads of a nut, tapped hole or bolt shear off instead of the bolt breaking.
• Twisted offalso snapped, broke off
  Describes a bolt or screw that snapped in torsion while being tightened or removed, leaving the stub behind. Shop talk

Frequently asked questions

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