Fastener Terminology & Technical Glossary

Bolt, Screw, Stud & Anchor Basics

Start here if you’re trying to decide whether what you need is technically a bolt, a screw, a stud or an anchor.

Bolt: A headed fastener used with a nut or tapped hole. Typically tightened by holding the bolt head and turning the nut.

Cap Screw: A screw with a precision shank and tighter dimensional control. Often used when the bolt head itself is doing the clamping.

Machine Screw: Smaller, fully threaded screw used with a nut or in tapped metal, plastic or machinery components.

Wood Screw: Screw with sharp, coarse threads that cut into wood. Often has a smooth shank under the head to pull boards together tightly.

Sheet Metal Screw: Self-tapping screw for thin metal or plastic. Threads run to the tip and cut their own mating threads.

Self-Drilling Screw: Sheet metal screw with a drill point that makes its own pilot hole before the threads engage.

Stud: A rod threaded on both ends or along the full length. One end is fixed into a base or housing; the other end uses a nut.

Heavy-Duty & Specialty Types

Terms you’ll see on structural drawings, concrete details and heavier assemblies.

Lag Screw (Lag Bolt): Large wood screw with a hex head used where high pull-out strength is needed, such as decks, ledgers and framing.

Structural Bolt: High-strength bolt meeting structural standards (such as ASTM A325 or A490) for steel-to-steel connections.

Anchor: Fastener that creates a secure point in concrete, masonry or hollow walls. Can be mechanical (expansion) or adhesive (chemical).

Anchor Bolt: Bolt embedded in concrete to attach structures, equipment or columns. May be L-shaped, J-shaped, headed or threaded rod set in epoxy.

Sex Bolt / Chicago Screw: Two-piece fastener with a male screw and female threaded barrel used to clamp materials together with a smooth finished look.

Diameter, Pitch & Series

How we describe the size and spacing of threads on inch (UNC, UNF) and metric fasteners.

Major Diameter: The outside diameter measured over the tops of the threads. This is the “size” of the fastener (for example, 1/4" or M10).

Minor Diameter: The diameter at the root of the threads. Helps determine tap drill size and affects strength.

Pitch (Metric): Distance from one thread crest to the next in millimeters, such as 1.5 mm on M10 × 1.5.

Threads Per Inch (TPI): Number of threads in one inch of length for inch fasteners. Higher TPI = finer thread.

Coarse, Fine & Extra-Fine: For a given diameter, coarse (UNC) has fewer threads per inch, fine (UNF) has more, and extra-fine (UNEF) has the most.

Unified Thread (UNC / UNF / UNEF): Standard inch thread system defined by ASME/ANSI. Specifies diameters, pitches and tolerances.

Metric Thread: Thread system using millimeter diameters and pitch. Sizes written as M diameter × pitch (for example, M8 × 1.25).

Fit, Length & Engagement

These terms explain how far a fastener goes into the joint and how tightly the threads fit together.

Thread Class (2A/2B, 3A/3B): Tolerance system that describes how loose or tight the external (A) and internal (B) threads fit. Higher class = tighter fit.

Grip Length: The unthreaded shank portion that passes through the materials being clamped. Ideally equals total thickness of the joint.

Thread Engagement: Amount of thread overlap between the male and female threads. More engaged threads generally mean a stronger joint.

Tap Drill Size: Drill bit size used before tapping a hole so the tap can cut full threads without excessive torque.

Nominal Length: Stated length of the fastener. For most bolts, measured from under the head to the tip; for flat heads, measured from the top of the head.

Head Styles

These names describe the shape of the head and how it bears on the material.

Hex Head: Six-sided head driven with a wrench or socket. Very common for bolts and lag screws.

Socket Head Cap Screw: Cylindrical head with recessed hex drive. High-strength fastener often used where space is limited.

Pan Head: Low, rounded head with flat bearing surface underneath. Common on machine and sheet metal screws.

Flat Head (Countersunk): Conical head designed to sit flush with or below the surface after countersinking.

Button & Truss Heads: Domed heads with a wide bearing surface that spread load over a larger area on thin or soft materials.

Flange Head: Head with a built-in washer face that distributes load without needing a separate washer.

Drive Types

The recess in the head that matches your screwdriver or bit. Choosing the right drive can reduce stripping and cam-out.

Slotted: Single straight slot. Simple but easy for the driver to slip out under higher torque.

Phillips: Cross-shaped recess designed to center the bit. Can cam out if heavily overtightened.

Pozidriv: Improved cross-style drive with extra ribs that reduce cam-out when used with a proper Pozidriv bit.

Torx / Star: Six-lobed star-shaped drive that transfers torque efficiently and resists cam-out. Often labeled T15, T25, etc.

Square / Robertson: Square recess that holds the bit firmly. Popular in wood and deck screws for one-handed driving.

Hex Socket: Internal hex recess used with hex keys or Allen wrenches. Common on socket head cap screws and set screws.

Materials & Strength Grades

These names show up in specs, bolt head markings and product filters when you choose a fastener.

Low Carbon Steel: Common, economical steel used for general-purpose bolts and screws (for example, SAE Grade 2).

Alloy Steel: Steel with added elements (such as chromium and molybdenum) for higher strength. Used in Grade 5, Grade 8 and many socket head fasteners.

Stainless Steel (304 / A2): Corrosion-resistant steel for general outdoor, food service and architectural use.

Stainless Steel (316 / A4): Higher corrosion resistance, especially in marine, chemical and coastal environments.

SAE Grade 2, 5, 8: Inch fastener strength grades. Grade 2 is general-purpose, Grade 5 medium strength (3 radial lines), Grade 8 high strength (6 radial lines).

Metric Property Class 8.8, 10.9, 12.9: Metric strength system; higher numbers mean higher tensile strength and proof load.

Coatings & Corrosion Protection

Coatings are chosen based on how wet, salty or chemically aggressive the service environment is.

Zinc Plated: Thin zinc coating applied by electroplating for basic indoor corrosion protection.

Mechanical Zinc / Mechanical Galvanized: Zinc coating applied by tumbling parts with zinc powder. Often used for structural hardware where thick, uniform coatings are needed.

Hot-Dip Galvanized: Thick zinc coating from dipping parts into molten zinc. Very good for outdoor and exposed structural use.

Black Oxide: Thin conversion coating that provides mild corrosion resistance and a low-glare black appearance. Often used on alloy steel fasteners indoors.

Passivation (Stainless): Cleaning and chemical treatment process that removes free iron from stainless surfaces to help maintain corrosion resistance.

Galvanic Corrosion: Corrosion caused when dissimilar metals touch in the presence of moisture, such as stainless steel on bare carbon steel.

Standards Organizations

The main groups that define dimensions, materials, grades and test procedures for fasteners.

ASTM: ASTM International publishes material and performance standards such as A307, A325, F593 and many more.

SAE: Society of Automotive Engineers. Defines many inch strength grades and automotive fastener requirements.

ISO: International Organization for Standardization. Issues global standards such as ISO 898-1 (metric property classes).

DIN: Deutsches Institut für Normung (German standards). Many metric fastener dimensions originated as DIN numbers.

IFI: Industrial Fasteners Institute. Publishes technical standards and handbooks used across the fastener industry.

ANSI / ASME: American standards bodies that define inch threads, fastener dimensions and tolerances (such as B18 series).

Strength & Test Terms

Numbers from these tests show up in spec sheets and are important when comparing fastener options.

Proof Load: Maximum load that a fastener can support without permanent stretch. Used as a safe working limit for tightening.

Tensile Strength: Maximum stress in tension before the fastener fractures. Often expressed in ksi or MPa.

Yield Strength: Stress level where the fastener begins to permanently deform and not fully spring back.

Hardness: Resistance to indentation, typically measured by Rockwell or Brinell tests. Correlates with strength and wear resistance.

Charpy Impact Test: Measures the energy required to fracture a notched sample. Indicates toughness, especially at low temperature.

Hydrogen Embrittlement: Condition where high-strength steel fasteners become brittle and may crack after plating or pickling if not properly baked.

Torque, Clamp Load & Joint Behavior

Understanding these terms helps you avoid both under-tightening and over-tightening.

Torque: Twisting force applied with a wrench or driver. Usually measured in in-lb, ft-lb or N·m.

Preload / Clamp Load: The clamping force created in the joint when a fastener is tightened. Correct preload keeps the joint closed under service loads.

Torque–Tension Relationship: The link between applied torque and resulting clamp load. Affected by friction, lubrication and finish.

Relaxation: Loss of clamp load over time due to embedding, gasket creep or material settling.

Vibration Loosening: Gradual loss of preload when joint movement causes the nut or screw to rotate loose, if not locked properly.

Loads & Failure Modes

These terms often appear in failure reports and design notes.

Shear: Side-to-side force that tries to slide joined parts past each other rather than pulling them apart.

Tension: Force that pulls the fastener along its axis, trying to stretch it.

Fatigue: Progressive damage from repeated loading and unloading, which can cause cracks even below the static strength of the fastener.

Stress Corrosion Cracking: Cracking that can occur when tensile stress and a corrosive environment act together on susceptible materials.

Over-Tightening: Applying more torque than recommended. Can yield or neck down the fastener and reduce its fatigue life.

Under-Tightening: Not enough torque to create proper clamp load, leading to joint slip, fretting and loosening.