Fastener Glossary

Nominal diameter #

Also called nominal size

The reference diameter by which a thread is named — the 4 in M4, in mm. The actual major diameter of an external thread is made slightly smaller, within tolerance.

Pitch #

Also called thread pitch

The axial distance between adjacent threads. Coarse-pitch M4 is 0.7 mm; a smaller pitch on the same size is a fine thread.

Coarse and fine thread #

Also called coarse pitchfine pitch

A coarse thread uses the standard pitch for its diameter; a fine thread uses a smaller one. Coarse threads may be written without pitch (M4); fine threads always state it (M8×1).

Pitch diameter #

Also called effective diameter

The diameter of the imaginary cylinder where thread ridge and groove widths are equal. It governs how external and internal threads fit and is the basis of thread tolerances and gauging. A coating adds roughly four times its thickness to it.

Tensile stress area #

Also called stress areaAs

The cross-section used to calculate tensile load on a thread — larger than the root section, smaller than the nominal one, from a formula based on pitch and minor diameters. Coarse M4: 8.78 mm², M8: 36.6 mm².

Thread tolerance class 6g / 6H #

Symbols for the permitted size range of a thread. The number is the tolerance grade, the letter the position — lower case for external (6g), upper case for internal (6H). 6H/6g is the usual pairing.

Incomplete thread #

Also called thread run-out

The part of a thread, at the point or near the head, where the thread is not fully formed. It does not carry load properly, so only the full thread counts towards engagement length.

Fully and partially threaded #

Also called full threadpartial thread

Fully threaded means the thread runs close to the head; partially threaded leaves a plain shank under the head. Thread length varies by standard and part number, so check the drawing.

Screw designation #

Also called designation

The ordered statement of recess, head, type, size, pitch, length, material and finish, e.g. 'cross-recessed pan head machine screw M4×0.7×10, A2-70'. Length is measured differently by head type (countersunk from the top of the head).

Machine screw #

Also called machine screw

A screw fastened into a pre-made internal thread (tapped hole or nut), in metric and inch series. Cross-recessed metric machine screws in Korea follow KS B 1023, and our standard products are mainly made to its annex dimensions; inch screws follow ASME B18.6.3 and similar.

Tapping screw #

Also called self-tapping screw

A screw that forms its own mating thread as it enters a pilot hole. The KS B 0248 annex defines thread types 1 to 4 (type 3 with metric thread, type 4 a type 2 thread with a sharp point); in practice type 1 (sharp point) and type 2 (blunt point) are the most used. Choose by base material and hole, and confirm by trial assembly.

Trilobular thread-forming screw #

Also called TAPTITE

A screw with a rounded-triangular (trilobular) section that forms the mating thread by plastic deformation, producing few chips. Type B has a pitch similar to type 2 tapping screws, type C a metric machine-screw pitch. TAPTITE® is a registered trademark of REMINC.

SEMS screw #

Also called screw and washer assembly

A screw supplied with a spring and/or plain washer pre-assembled and captive. It saves assembly time and prevents missing washers. The thread is rolled after the washer is fitted, so the washer cannot fall off.

Hexagon socket head cap screw #

Also called socket head cap screwSHCSAllen bolt

A bolt with a cylindrical head and hexagon socket, tightened with a hex key. The compact head suits tight spaces and counterbores; high grades such as 10.9 and 12.9 are common. Korean standard: KS B 1003.

Hexagon head bolt #

Also called hex bolthex cap screw

A bolt with a hexagon head tightened with a spanner. Partially threaded hex bolts follow KS B 1002 (based on ISO 4014); fully threaded ones follow KS B ISO 4017.

Pan head #

Also called pan

A head with a gently domed top and flat bearing face — the most common machine-screw head.

Countersunk (flat) head #

Also called flat headCSK

A head with a flat top and conical underside that sits flush in a countersink. Length is measured from the top of the head.

Small countersunk head #

Also called reduced countersunk head

In our catalogue, a countersunk head with smaller diameter and height than standard (company standard), for thin parts or small countersinks. 'Small head' dimensions vary by maker — check the drawing.

Raised countersunk (oval) head #

Also called oval head

A countersunk head with a domed top that stands proud of the surface after countersinking. Length is measured from the edge between dome and cone.

Truss head #

Also called trussmushroom head

A wide, low head — about 1.3 times the diameter of a pan head of the same size — that spreads load on thin sheet, plastic or slotted holes.

Binding head #

Also called binding

Narrower than truss, wider and lower than pan — used inside electronics, on PCBs and brackets where a truss head is too wide.

Pan head with washer face #

Also called pan washer head

A pan head with an integral washer-shaped collar underneath (company standard), to increase bearing area without a washer. Check the collar diameter separately from the head diameter.

Cross recess #

The cross-shaped drive recess for a screwdriver bit. ISO 4757 defines type H (Phillips type) and type Z (Pozidriv type), in recess sizes 0 to 4. The number depends on size and head, and KS annex and ISO products of the same size can take different numbers. Our cross recesses are type H.

Spring lock washer #

Also called split lock washerhelical spring washer

A split ring washer that deflects elastically as it is tightened. Once flattened under full preload its effect against vibration loosening can be limited (NASA-STD-5020B advises against using it as a secondary locking feature), so where loosening matters, rely on preload control and a dedicated locking design.

Plain washer #

Also called flat washer

A flat ring placed under a head or nut to enlarge the bearing area and protect the surface, lowering surface pressure on soft parts or oversized holes.

Preload #

Also called bolt tension

The axial tension produced in a bolt when it is tightened, which clamps the parts together with an equal force. Whether a joint is properly tight is judged by preload, not torque.

Tightening torque #

Also called installation torque

The turning moment applied when tightening. Much of it is spent on thread and under-head friction, so only part produces preload — the same torque can give very different preload when friction changes.

Coefficient of friction #

Also called μ

The value that links torque to preload, treated separately for the thread (μG) and under-head contact (μK). It varies over a range with finish, lubrication and mating material.

Torque coefficient (K) #

Also called nut factorK factor

The K in the short-form equation T = K·F·d (torque = K × preload × nominal diameter). It lumps friction and geometry into one number and is not the same as the friction coefficient μ.

Loosening torque #

Also called breakaway torqueremoval torque

The torque needed to start undoing a tightened screw. It depends on preload, friction and time, and is usually lower than the tightening torque.

Breaking torque #

Also called torsional strengthultimate torque

The torque at which a held screw breaks in torsion. For small stainless screws ISO 3506-1 sets a minimum breaking torque — a lower limit, not a permitted tightening torque.

Thread engagement length #

Also called length of engagement

The axial length over which external and internal threads actually mesh and carry load. It is not the same as tap depth or insertion depth, and must be longer in softer base materials.

Pilot hole #

Also called pre-drilled holecore hole

The hole drilled or punched before driving a tapping screw or cutting a thread. For tapping screws it depends on material, thickness and hole-making, so no single table fits — decide by trial. KS hole sizes are test conditions.

Clearance hole #

Also called through hole

The unthreaded hole a bolt passes through, chosen from ISO 273 fine, medium or coarse series. A larger hole reduces under-head contact area and changes surface pressure and friction.

Countersink and counterbore #

Also called countersinkcounterborespot face

Machining a seat for the head at the hole entry — conical (countersink) for countersunk heads, cylindrical (counterbore) for socket head cap screws. Wrong angle or depth leaves the head proud or seated on one side.

Galling #

Adhesion of the external and internal thread surfaces under friction during tightening, so the screw will neither turn further nor come out. Common with stainless on stainless; managed through speed, lubrication and material pairing.

Thread insert #

Also called helical coil insertHelicoil

A coil or bush inserted into soft material such as aluminium to reinforce the internal thread where engagement cannot simply be lengthened.

Property class (steel) #

Also called grade

The mechanical-property grade of steel bolts and screws. The first number ×100 is nominal tensile strength (N/mm²); the second gives nominal yield as a fraction of it (8.8 → 800, 80 %). Old 'T' marks are not in current standards.

Stainless property class #

The grade of stainless fasteners. A2 or A4 is the steel group (A2 ≈ 304 type, A4 ≈ 316 type); the number ×10 is minimum tensile strength (N/mm²). The same 304 material reaches 50, 70 or 80 depending on cold work.

Tensile strength #

Also called Rmultimate tensile strengthUTS

Maximum load in a pull-to-failure test divided by the stress area. The basic item of a property class, verified by tensile testing the finished part.

0.2 % proof stress #

Also called Rp0.2

The stress that leaves 0.2 % permanent strain after unloading, used instead of yield strength where there is no clear yield point. For full-size bolts ISO 898-1 and ISO 3506-1 treat the same concept as Rpf (stress at 0.2 % non-proportional elongation); formal determination needs extension measured as the standard specifies.

Proof load #

Also called Spproof load stress

A test load that a bolt must withstand with permanent elongation within limits after release. Each property class has a proof load stress (Sp).

Hardness #

Resistance to indentation, measured on Vickers (HV), Rockwell (HRC) and similar scales. Each property class has a permitted range; too high raises the risk of brittle failure.

Austenitic stainless steel #

Also called 18-8

Chromium-nickel stainless steel, typically 304 and 316. Good corrosion resistance and formability; it cannot be hardened by quenching, so strength comes from cold work, which can also make it slightly magnetic.

304Cu stainless #

Also called 304Cu

A 304-type austenitic stainless with added copper to reduce work hardening and improve cold heading. It falls in the A2 group and is the material of our A2 products. XM7 (STS XM7) is also a copper-bearing cold-heading grade, but a separate grade with a different composition specification.

Cold heading carbon steel #

Also called SWCH

Carbon steel wire for cold heading bolts and screws. The number is carbon content (10 ≈ 0.10 %), the suffix A means aluminium-killed. Our carbon steel parts use 10A, and some 18A.

SCM435 (Cr-Mo steel) #

Also called chromium-molybdenum steelchromoly

A chromium-molybdenum alloy steel with good hardenability — the typical material for 10.9 and 12.9 bolts, quenched and tempered for strength with toughness. It is a different material from boron steels such as 51B20.

Boron steel (51B20) #

Also called boron-added steel51B20

Carbon steel with a very small boron addition to raise hardenability, so it responds well to quenching and tempering with little alloying — used for 8.8 and 10.9 bolts. It is a different material from Cr-Mo steel (SCM435).

Work hardening #

Also called strain hardeningcold work

The increase in hardness and strength when metal is deformed at room temperature. Austenitic stainless bolts reach A2-70 and A2-80 this way, which also makes forming harder and can introduce magnetism.

Magnetism (stainless) #

Austenitic stainless is only weakly magnetic, but cold work turns some of it into martensite and a magnet may stick. ISO 3506-1 has no magnetism criterion; confirm material by certificate and analysis, not a magnet.

Cold heading #

Also called cold forging

Forming the head and shank from wire at room temperature with dies and punches. Faster and less wasteful than machining, and the metal grain flow is not cut.

Thread rolling #

Also called rolled thread

Forming the thread by rolling the blank between dies — no chips, and the thread surface is work-hardened. Our screw threads are rolled.

Case hardening #

Also called carburisingcarburizing

Heat treatment that diffuses carbon into the surface and then quenches it, hardening only the skin. Used on carbon steel tapping screws, which must form threads in the mating part.

Quenching and tempering #

Also called Q&Thardening and tempering

Hardening by quenching followed by tempering to restore toughness. Required for property classes 8.8 and above; 4.8 gets its strength from cold work without heat treatment.

Hydrogen embrittlement #

Delayed fracture, such as heads snapping off some time after tightening, caused by hydrogen absorbed during pre-treatment or plating. The risk rises with hardness; KS B 1066 (a translation of ISO 4042:1989) identifies steel bolts and screws of 1,000 MPa tensile strength or above, or over 320 HV, as at risk.

Baking (hydrogen relief) #

Also called de-embrittlementhydrogen relief

Heating parts after plating, before chromating, to drive out absorbed hydrogen. It reduces, but does not eliminate, the risk of hydrogen embrittlement. We bake our plated 10.9 and 12.9 parts after plating.

Zinc electroplating #

Also called zinc plated

Electrodepositing zinc on steel for corrosion protection, usually followed by a chromate conversion coating that sets colour (clear, yellow, black) and corrosion resistance. Thickness must stay within thread tolerance — thicker is not better.

Chromate conversion coating #

A thin conversion film applied over zinc plating, based on hexavalent or trivalent chromium. Colour alone cannot tell them apart; for RoHS, confirm by the treatment specification and analysis.

Nickel plating #

Also called nickel plated

An electroplate with an even silver lustre, common on visible electronics parts. Thickness is measured with nickel-specific magnetic gauges or XRF, not standard magnetic gauges.

Acid cleaning #

Also called acid cleaned

Degreasing and cleaning stainless screws, then chemically brightening them to an even finish — the standard shipped condition of our stainless parts (semi-bright silver). It is not passivation and not a galling remedy.

Passivation #

Also called passivated

Treating stainless steel in nitric or citric acid solutions to remove free iron and help form an even chromium oxide (passive) film. We normally supply acid cleaned; passivation is carried out separately on customer request, to the required specification.

Coating thickness #

Also called plating thickness

The thickness of the coating, controlled in μm. Too thin loses corrosion resistance; too thick stops the thread fitting. It varies by location — thicker at head edges and the point — so measuring positions are fixed.

RoHS #

Also called EU RoHS

The EU directive restricting lead, cadmium, hexavalent chromium and other substances in electrical and electronic equipment. It applies to a screw as a component of such equipment; finish colour does not show compliance.

Go/No-Go gauge #

A pair of gauges that checks a thread is within tolerance. The Go gauge must pass the full thread freely; the No-Go must stop within the specified turns. Rings for external threads, plugs for internal.

Inspection and test report #

Also called inspection reporttest reportcertificate

A general term for documents reporting inspection and test results against requirements. For fasteners the core is the product report — dimensions, appearance and thread gauging, mechanical tests such as tensile, torsion and hardness, and coating thickness — sometimes with third-party test or substance analysis results when the customer asks. A mill certificate covers only the raw material. Ours are issued only from inspection data of the lot actually delivered.

Mill certificate #

Also called mill test reportMTREN 10204 3.1

A document issued by the steelmaker giving the chemical composition and mechanical properties of the raw material. It confirms the material, but not the strength, dimensions or finish of the finished part, so read it together with the product report.

Certificate of Conformance (CoC) #

Also called C of Ccertificate of conformity

A supplier's statement that the delivered goods meet the order specification and requirements — a declaration of conformity, unlike a test report of measured values. We issue a CoC for every item shipped to defence customers; for other orders, please ask.

Lot traceability #

Also called traceability

The ability to trace a shipped lot back to its material, processes and inspection records, which narrows the scope when a problem occurs. All our products are traceable from raw-material coil barcode to shipped lot.

KS annex dimensions #

Also called annex

Standards such as KS B 1023 (machine screws) and KS B 1032 (tapping screws) have an ISO-aligned main body, with the former Korean dimensions kept in an annex. Most products made in Korea follow the annex, whose head dimensions differ from ISO.

Company standard (JK Standard) #

Also called in-house standard

Our stock items are produced and controlled to a company standard that follows the relevant KS. Parts that must strictly conform to KS (e.g. defence) are produced separately. Pan-with-washer-face and small countersunk heads and SEMS washer dimensions are also company standards.