Fastener Terms Explained: The Complete Glossary for Screws, Bolts, and Nuts

Screw Types

“A” Sheet Metal Screws — Sharp point with deeper threads for thin sheet; more aggressive than AB.

“AB” Sheet Metal Screws — Hybrid of A and B; pointed tip with finer threads for better start and hold.

Cap Screw — Often used interchangeably with hex head bolt; typically features a washer face. See hex bolts.

Machine Screw — Straight shank, machine thread; used with tapped holes or nuts.

Self-Tapping Screw — Forms threads in the mating material; pilot hole recommended.

Self-Drilling Screw — Includes a drill-point tip; no separate drilling step needed for thin stock.

Set Screw — Headless screw driven from the end; secures pulleys/gears to shafts.

Tap Bolt — Fully threaded hex bolt (threads run the full length of the shank).

Wood Screw — Coarse thread with tapered shank for wood; typically sharp point.

Screw Drive Types

• Slotted (Flat) — Single slot; best for manual driving; prone to cam-out.
• Phillips — Self-centering cross; good for powered drivers; can cam-out at high torque.
• Pozidriv — Phillips-like with extra ribs; more torque, less cam-out (requires correct bit).
• Square (Robertson) — Excellent torque transfer; common in woodworking.
• Hex (Allen) — Six-sided recess; common on socket screws; pairs with hex keys.
• Torx (Star) — Six-point star; high torque, low cam-out; popular in automotive/electronics.

Screw Head Styles

• Flat Head — Countersunk; sits flush with the surface.
• Oval Head — Countersunk with a low dome; decorative flush appearance.
• Pan Head — Rounded top with short vertical sides; general-purpose.
• Truss Head — Extra-wide, low profile to spread load.
• Button Head — Low, rounded head; commonly hex/Allen drive.
• Socket Head Cap Screw — Cylindrical head with recessed hex; high strength in limited space.

Nut Types

Acorn (Cap) Nut — Domed top covers exposed threads.

Jam Nut — Thin nut; locks against a standard nut to resist loosening.

Castle Nut — Slotted nut used with a cotter pin for mechanical locking.

Nylon Insert Locknut (Nyloc) — Nylon ring increases friction to resist vibration.

Flange Nut — Built-in washer face to spread load.

Stover (All-Metal) Lock Nut — Deformed thread for prevailing torque without nylon.

Washer Types

• Flat Washer — Spreads load; prevents pull-through.
• Split Lock Washer — Spring action resists loosening (limited in high-vibration specs).
• Tooth Lock Washer — Internal/external teeth bite to resist rotation.
• Fender Washer — Large OD for thin/soft materials.
• Belleville (Conical) — Spring washer for consistent preload; can be stacked.

Materials

Alloy Steel — Heat-treatable; used for high-strength classes (e.g., 12.9, 10.9).

Carbon Steel — Common structural steel; strength depends on grade and treatment.

Stainless Steel A2-70 — ≈304/18-8; min tensile ~700 MPa; great general corrosion resistance.

Stainless Steel A4-80 — ≈316; min tensile ~800 MPa; better chloride resistance (marine).

Titanium (e.g., Grade 5) — High strength-to-weight; non-magnetic; excellent corrosion resistance.

Brass / Copper Alloys — Good conductivity and corrosion resistance; lower strength.

Finishes & Coatings

Black Oxide — Mild corrosion resistance; low dimensional change; dark appearance.

Zinc Plated (Clear/Yellow) — Economical corrosion protection; chromate color varies.

Hot-Dip Galvanized — Thick zinc layer; strong corrosion resistance; threads may require oversize nuts.

Nickel / Chrome — Decorative; hardness/wear benefits.

Dry Film Lubricant / Wax — Reduces friction and galling; stabilizes torque-tension.

Threading Terms

UNC / UNF / UNEF — Unified coarse/fine/extra-fine inch threads (e.g., 1/4-20 UNC, 1/4-28 UNF).

Metric Thread Pitch — mm between threads (e.g., M6-1.0). See Metric Pitch Table.

TPI (Threads Per Inch) — Count of thread peaks per inch (inch series). See US TPI Table.

Major / Minor / Pitch Diameter — Outer/inner/mean diameters used for thread fit.

Class of Fit — Tolerance system (e.g., 2A/2B, 3A/3B) balancing ease of assembly vs precision.

Helicoil / Insert — Wire or solid inserts used to repair/strengthen tapped threads.

Strength & Testing

Proof Load — Max axial load without permanent set.

Yield Strength — Stress at which permanent deformation begins.

Tensile Strength — Ultimate stress prior to fracture.

Metric Classes — 12.9 (~1220 MPa tensile), 10.9 (~1040 MPa), 8.8 (~800–830 MPa); see metric strength chart.

Stainless Classes — A2-70 (~700 MPa), A4-80 (~800 MPa); see stainless grades.