Fasteners — Complete Guide
A comprehensive engineering guide to metal fasteners: bolts, nuts, washers, pins, rivets, bolt locking, tightening torque, preload, and strength grades — with full reference tables for selecting the right fastener for every application.
🔗 Fasteners — Complete Guide
🔩 Bolt Types
| Type | Standard | Head | Use |
|---|---|---|---|
| Hex Bolt | ISO 4014 / DIN 931 | External hex | General structure, flanges, machinery |
| Full-Thread Hex Bolt | ISO 4017 / DIN 933 | Hex — fully threaded | Most common. Shear on the thread! |
| Socket Head Cap Screw (SHCS) | ISO 4762 / DIN 912 | Cylindrical + Allen | Engineering, machinery, fixtures — the workhorse |
| Low Head Socket Screw | DIN 7984 / DIN 6912 | 60% lower than DIN 912 | Limited clearance. Limited torque! |
| Button Head Cap Screw (BHCS) | ISO 7380 | Rounded + Allen | Decorative, safety (no sharp edges). Low torque |
| Flat Head Cap Screw (FHCS) | ISO 10642 | 90° conical + Allen | Flush with the surface. ISO=90°, ANSI=82°! |
| Shoulder Bolt | ISO 7379 | SHCS + h6 cylinder | Pivot, guide, spacer. 12.9 |
| Flange Bolt | DIN 6921 | Hex + integral disk | Replaces a washer! 90% automotive. Serrated=locking |
| Stud Bolt | ASTM A193 B7 | No head — threaded both ends | Flanges, pressure vessels. No thread stub protruding from the body |
| Set Screw | DIN 913-916 | Headless — recessed Allen | Locking a part on a shaft. Cup/Cone/Dog/Flat Point |
| Eye Bolt | DIN 580 / DIN 444 | Ring / eye | Lifting, hanging. SWL is stamped! Not for side loads |
| Hi-Lok (Aerospace) | MS21250 | Pin + break-off collar | Built-in torque control. Millions of units in every aircraft |
💡 Selection rule: DIN 912 (Allen) = the engineering default. Hex = when an external wrench is needed. CBORE always with DIN 912. CSK with ISO 10642 (mind the angle!).
▶ YouTube
⬡ Nut Types
| Type | Standard | Characteristic | Use |
|---|---|---|---|
| Hex Nut | ISO 4032 / DIN 934 | H ≈ 0.8d. Grade 8/10/12 | The basic one. 8 with 8.8, 10 with 10.9 |
| Jam Nut | ISO 4035 / DIN 439 | H ≈ 0.5d. Jam Nut | Double locking (below the main nut) |
| Nyloc Nut | DIN 985 / ISO 10511 | Nylon ring — Prevailing Torque | Self-locking. Up to 120°C! 2-3 uses max |
| Flange Nut | DIN 6923 | Integral disk. Serrated optional | Replaces a washer! Load distribution. Automotive standard |
| Coupling Nut | DIN 6334 | H ≈ 3d. Joins two bolts | Extending a stud, joining threaded rods |
| Wing Nut | DIN 315 | Wings — hand-tightened | Frequent opening with no tool. Not for loads! |
| Castle Nut | DIN 935 / ISO 7035 | Slots + Cotter Pin | Positive locking. TRB bearings, wheel axle |
| All-Metal Locknut | DIN 980V / Stover | Distorted crown — All Metal | High temperature (above 120°C — Nyloc melts!) |
| A194 2H Nut | ASTM A194 | Heavy, hardened | Paired with A193 B7 — flanges and pressure vessels |
💡 Grade matching: Nut 8 → Bolt 8.8 | Nut 10 → Bolt 10.9 | Nut 12 → Bolt 12.9 | A194 2H → A193 B7. A nut too strong = the bolt breaks with no warning!
▶ YouTube
◎ Washers
| Type | Standard | Function | Notes |
|---|---|---|---|
| Standard Flat | ISO 7089 (100HV) | Load distribution, surface protection | Soft — deforms at 10.9+ |
| Hardened Flat | ISO 7090 (200HV) | Load distribution, for high loads | ★ the correct engineering choice |
| Large Flat | ISO 7093 | OD×2 — for elongated holes | Mandatory on slots! Large OD = lower surface pressure |
| Split (Spring) | DIN 127 | Locking? | ⚠️ Does not prevent loosening! Fails the Junker Test. Damages the surface. Prohibited at 10.9+ |
| ★ Nord-Lock | DIN 25201 | Wedge — guaranteed locking | A pair of serrated washers. Passes the Junker Test! Reusable. 10.9/12.9 OK |
| Belleville | DIN 2093 | Spring — maintains preload | Compensates for thermal expansion + gasket creep |
| Tab Washer | DIN 93 | A bendable tab — positive locking | Single-use. A tab on the bolt + a tab on the assembly |
| Wave Spring | Smalley | Spring load at a low height | Bearing preload. Half the height of Belleville |
⚠️ DIN 127 spring washer — a myth: studies (Junker, 1969) proved it does not prevent vibration loosening. The sharp edges damage the surface and reduce preload. Alternatives: Nord-Lock (wedge), Nyloc (nylon), Loctite (chemical), Safety Wire (aerospace). DIN 127 = an old habit, not engineering.
▶ YouTube
📌 Pins and Retainers
| Type | Standard | Characteristic | Use |
|---|---|---|---|
| Dowel Pin | DIN 6325 / ISO 8734 | Ground m6, hardened HRC 58 | Precise alignment. 2 pins = full location (round + Diamond!) |
| Roll Pin | DIN 1481 / ISO 8752 | Slotted tube — spring-loaded | Friction locking. Cheap, wide tolerance. Hammer installed |
| Taper Pin | DIN 1 / ISO 2339 | 1:50 taper | Self-centering + friction lock. Handles, wheels |
| Clevis Pin | DIN 1444 / ISO 2341 | Pivot + hole for a cotter pin | Joints, cylinders, links. Double shear |
| Cotter Pin | DIN 94 / ISO 1234 | Split — bendable legs | Locks a Castle Nut, Clevis Pin. Single-use! |
| External Retaining Ring | DIN 471 | Circlip — in a shaft groove | Prevents axial movement. Limited load! For locking bearings |
| Internal Retaining Ring | DIN 472 | Circlip — in a bore groove | Holds a shaft/bearing in a housing |
| E-Clip | DIN 6799 | Side-installed — no tool | Fast, for light loads. No groove needed |
💡 The 2-pin rule: Pin 1 (round) locates, pin 2 (Diamond / Slotted) sets orientation and compensates for distance tolerance. Without Diamond — the pins struggle to enter (Over-Constraint). On the drawing: a round pin + a Diamond pin on the same BCD.
▶ YouTube
⊙ Rivets
| Type | Access | Strength | Use |
|---|---|---|---|
| Solid Rivet | Two-sided | ★ Highest | Aerospace structure. AN470 (Universal), AN426 (CSK) |
| Blind (Pop) Rivet | One-sided! | Low | Sheet metal, plastic, signage. Not structural! Broken stem inside |
| Structural Blind Rivet | One-sided | Like solid | Huck / Cherrymax. Stem stays! Aerospace approved |
| Drive Rivet | One-sided | Very low | Nameplates, covers. Hammer-driven |
💡 Rivet vs. bolt: Rivet = permanent joint (removal = drilling). Bolt = disassembles. A solid rivet is reliable through 1,000,000+ cycles (fatigue). A bolt = subject to torque variation. In aerospace: solid rivets for the main structure, Hi-Lok for service areas.
▶ YouTube
🔒 Anti-Loosening Locking Methods
| Method | Type | Temp. | Reusable | Junker Effectiveness |
|---|---|---|---|---|
| ★ Nord-Lock | Mechanical — wedge | Up to 280°C | ✅ Unlimited | ★★★★★ |
| Nyloc Nut | Mechanical — friction | Up to 120°C | 2-3 times | ★★★☆☆ |
| Loctite 243 (blue) | Chemical — anaerobic | Up to 180°C | ✅ Opens with a tool | ★★★★☆ |
| Loctite 271 (red) | Chemical — permanent | Up to 230°C | Requires 250°C heat! | ★★★★★ |
| Safety Wire | Mechanical — positive | Unlimited | ✅ New wire | ★★★★★ |
| Castle Nut + Cotter | Mechanical — positive | Unlimited | ✅ New pin | ★★★★★ |
| Tab Washer | Mechanical — positive | Unlimited | ❌ Single-use | ★★★★☆ |
| DIN 127 Split Washer | Mechanical — friction | Up to 200°C | ❌ Deforms | ★☆☆☆☆ Doesn't prevent! |
🔧 Tightening Torque Table — Metric Bolts
T = K × F × d | K=0.2 dry, K=0.15 oiled, K=0.12 MoS₂ | Preload = 70% of Rp0.2×As
| Size | As [mm²] | 8.8 | 10.9 | 12.9 | |||
|---|---|---|---|---|---|---|---|
| Dry [Nm] | Oiled [Nm] | Dry | Oiled | Dry | Oiled | ||
| M4 | 8.78 | 2.9 | 2.2 | 4.1 | 3.1 | 4.8 | 3.6 |
| M5 | 14.2 | 5.8 | 4.4 | 8.2 | 6.2 | 9.6 | 7.2 |
| M6 | 20.1 | 10 | 7.5 | 14 | 10.5 | 16.5 | 12.4 |
| M8 | 36.6 | 25 | 19 | 35 | 26 | 41 | 31 |
| M10 | 58.0 | 49 | 37 | 69 | 52 | 81 | 61 |
| M12 | 84.3 | 86 | 65 | 120 | 90 | 140 | 105 |
| M16 | 157 | 210 | 157 | 295 | 221 | 346 | 260 |
| M20 | 245 | 410 | 308 | 580 | 435 | 680 | 510 |
| M24 | 353 | 710 | 530 | 1000 | 750 | 1170 | 880 |
💡 Flange tightening sequence: star/cross pattern in 3 stages (30% → 70% → 100%) + a final check pass. Sequential (circular) tightening is prohibited! The first bolt causes the part to distort. Gasket: re-torque after 24h hot (gasket creep).
▶ YouTube
📐 Preload — Tightening Theory
🎯 Why Preload?
• The bolt doesn't feel external loads as long as Preload > external load
• Without preload: every load cycle = a fatigue cycle on the bolt → fracture!
• With preload: the clamped parts absorb 90%+ of the load → the bolt stays "alive"
• Target: Preload = 70-90% of Sy × As
• Without preload: every load cycle = a fatigue cycle on the bolt → fracture!
• With preload: the clamped parts absorb 90%+ of the load → the bolt stays "alive"
• Target: Preload = 70-90% of Sy × As
📊 The Problem: 90% Goes to Friction!
• ~50% of the torque → bolt head/nut friction
• ~40% → thread friction
• Only 10% → bolt tension (Preload)!
• Therefore: K-Factor (lubrication condition) controls everything
• Same torque, lubricated = 60% higher preload than dry!
• ~40% → thread friction
• Only 10% → bolt tension (Preload)!
• Therefore: K-Factor (lubrication condition) controls everything
• Same torque, lubricated = 60% higher preload than dry!
🔧 Tightening Methods (precise → less)
1. Torque-Angle — Snug + angle = ±5% Preload ★★★★★
2. Yield-Point (TTY) — to yield = ±3% ★★★★★
3. Controlled Torque — calibrated wrench = ±25% ★★★☆☆
4. Turn-of-Nut — steel construction = ±15% ★★★★☆
5. Stretch — elongation measurement = ±1% ★★★★★
6. Ultrasonic — Bolt Gauge = ±1% ★★★★★
2. Yield-Point (TTY) — to yield = ±3% ★★★★★
3. Controlled Torque — calibrated wrench = ±25% ★★★☆☆
4. Turn-of-Nut — steel construction = ±15% ★★★★☆
5. Stretch — elongation measurement = ±1% ★★★★★
6. Ultrasonic — Bolt Gauge = ±1% ★★★★★
⚠️ Common Failures
• Insufficient Preload — the #1 cause of vibration loosening
• Over-Tightening — a 12.9 bolt + Anti-Seize = breakage!
• Unknown Friction — dry vs. lubricated = ±60% variance
• Wrong Sequence — sequential instead of star pattern = distortion
• Reusing TTY — a bolt that has yielded plastically = single-use!
• Over-Tightening — a 12.9 bolt + Anti-Seize = breakage!
• Unknown Friction — dry vs. lubricated = ±60% variance
• Wrong Sequence — sequential instead of star pattern = distortion
• Reusing TTY — a bolt that has yielded plastically = single-use!
💪 Strength Grades — ISO 898 / ASTM
Metric Bolts — ISO 898-1
Decoding X.Y: X×100 = UTS [MPa], Sy = X×Y×10 [MPa]. Stamped on the bolt head.
4.6
UTS 400 · Sy 240
Soft. Cheap fasteners
5.8
UTS 520 · Sy 415
Medium. General
8.8 ★
UTS 800 · Sy 640
The structural standard
10.9
UTS 1040 · Sy 940
Hardened. Machinery
12.9
UTS 1220 · Sy 1080
Maximum. ⚠️ HE risk
Stainless Steel — ISO 3506
A2-70
304 · UTS 700 · Sy 450
General. ⚠️ Weaker than 8.8!
A4-80
316 · UTS 800 · Sy 600
Marine/chemical. Cold Worked
Inch Bolts — SAE / ASTM
Grade 2
≈4.6. No head markings
Grade 5
≈8.8. 3 lines on the head
Grade 8
≈10.9. 6 lines on the head
A193 B7
4140 Q&T. Flanges/pressure