Choosing between a bolt, cap screw, stud, or rivet is not a detail — the wrong fastener type in a high-stress joint leads to fatigue failure, loss of clamp load, or assembly problems that require expensive redesigns.
Fasteners are the most common mechanical components in any machine, yet their selection is often informal — whatever is available in the shop, or what the previous version of the design used, or whatever the drafter defaulted to. A systematic understanding of fastener types, strength grades, and when each is appropriate makes you a significantly more effective mechanical designer. This guide covers threaded fasteners and rivets with an emphasis on practical selection criteria.
Bolt vs Cap Screw vs Stud: The Fundamental Distinction
These three terms are often used interchangeably, but they describe fundamentally different assembly configurations:
Bolt: A fastener with a head on one end and threads on the other, designed to be used with a nut. Both the bolt head and the nut rotate during assembly. Bolted joints allow assembly and disassembly from either side and are the standard for through-hole connections where access is available from both sides. ISO 4014 (hex bolt, partially threaded) and ISO 4016 are the fundamental standards.
Cap screw (machine screw): A fastener with a head that threads directly into a tapped hole in one of the joined members. Only the head end needs to be accessible — the threaded end anchors into the component. Cap screws are used when only one side is accessible, or when a clean design without a protruding nut is desired. ISO 4762 (socket head cap screw, the ubiquitous hex socket bolt) is one of the most widely used fastener standards in machine design.
Stud: A fully-threaded rod with no head. One end threads into a tapped hole (often with interference fit or thread adhesive for permanent retention), and the assembly component threads over the other end and is secured with a nut. Studs are used where the tapped component is difficult to align during assembly (engine cylinder heads, pipe flanges, pressure vessels), where frequent disassembly would damage the tapped threads if a cap screw were used directly, or where a stud provides a register (alignment) feature during assembly. ISO 939 and ISO 4028 cover studs.
Head Types and Drive Systems
Hex head (ISO 4014/4017): the standard for high-strength structural bolts. Requires open-end or socket wrench. Maximum torque application, visible inspection. Dominates in structural and heavy equipment applications.
Socket head cap screw / Allen bolt (ISO 4762): the dominant fastener in machine design. High bearing surface area under the head, compact head height, high torque capability with Allen (hex) key or bit driver. Available in property class 12.9 as standard. Ideal for precision machinery, tight spaces, and clean appearance. The socket drive is recessed, keeping the head below the surface when countersunk.
Button head cap screw (ISO 7380): low-profile rounded head with socket drive. Good for panels, covers, and fixtures where head protrusion must be minimized. Lower torque capacity than socket head — not suitable for high-clamp-force joints. Property class 10.9 maximum in button head form.
Flat head / countersunk (ISO 10642): tapered bearing surface seats in a countersunk hole, resulting in flush surface. Used for covers, panels, and any application requiring a flush face. The countersink angle (90° for metric per ISO, 82° for imperial) must match the fastener to develop proper clamp load.
Flange head bolt (ISO 4162): integral serrated flange replaces separate washer, distributing load and providing some vibration resistance. Widely used in automotive and general assembly where washer handling is burdensome.
Strength Grades: ISO 8.8, 10.9, 12.9 and SAE Grade 5, 8
Metric fastener strength is designated by a property class (ISO 898-1): the first number × 100 gives nominal ultimate tensile strength in MPa; the product of both numbers × 10 gives nominal yield strength (0.2% proof stress) in MPa.
| ISO Property Class | Min UTS (MPa) | Min Yield / Proof Load (MPa) | SAE Grade Equivalent | Typical Material | Marking |
|---|---|---|---|---|---|
| 4.6 | 400 | 240 | Grade 2 (approx.) | Low carbon steel | 4.6 |
| 8.8 | 800 | 640 | Grade 5 | Med carbon steel, Q&T | 8.8 |
| 10.9 | 1040 | 940 | Grade 8 | Alloy steel, Q&T | 10.9 |
| 12.9 | 1220 | 1100 | No direct equivalent | Alloy steel, Q&T | 12.9 |
| A2-70 (stainless) | 700 | 450 | — | 304 stainless | A2-70 |
| A4-80 (stainless) | 800 | 600 | — | 316 stainless | A4-80 |
Selection guidance: use property class 8.8 as the default for general machine design — it provides good strength, broad availability, and is not susceptible to hydrogen embrittlement (unlike 12.9). Specify 10.9 when joint size is constrained and higher clamp force is needed. Reserve 12.9 for applications where space is absolutely limited and the designer understands hydrogen embrittlement risk and has specified the appropriate baking treatment. Never use plain class 4.6 bolts in dynamic (fatigue) applications — the low proof load allows preload relaxation and joint slip under vibration.
Thread Engagement Length
A tapped hole must have sufficient thread engagement to develop the full tensile strength of the bolt without stripping the internal thread. The minimum thread engagement length depends on the strength of the tapped material relative to the bolt:
• Tapped in steel with similar strength to the bolt: minimum 1.0 × bolt diameter (1D)
• Tapped in cast iron or lower-strength steel: minimum 1.5D
• Tapped in aluminum alloy: minimum 2D for standard thread; consider Heli-Coil (wire thread insert per ISO 8085) or Keensert for repeated assembly — these bring the thread engagement strength close to steel tap performance
• Tapped in plastic: minimum 3–4D; use coarse thread; consider heat-set or ultrasonic inserts for thermoplastics
Rivets: Permanent, Lightweight, and Fatigue-Friendly
Rivets create permanent joints that cannot be disassembled without destruction. Their advantages over bolts in appropriate applications are significant: no protruding threads to catch, no concern about bolt loosening in vibration, can join materials that cannot be tapped (thin sheet metal, composites), and excellent fatigue performance because the joint is loaded in shear rather than tension-bearing on thread flanks.
Solid rivets (ISO 1051) are the traditional aerospace and structural rivet, installed by bucking with a backing hammer to form a shop head. Require access to both sides. Used extensively in aircraft structure, bridges, and boilers.
Blind rivets (ISO 15977–15983, “pop rivets”) are installed from one side only using a mandrel pull — the mandrel is pulled through the rivet body, expanding the blind end to clamp the joint, then the mandrel breaks. Widely used for sheet metal assemblies, electrical panels, and any single-side-access application. Grip range, material compatibility (steel, aluminum, stainless), and mandrel retention (standard vs. sealed) should be verified for each application.
Structural blind rivets (Huck BobTail, Avdel, Gesipa PolyGrip): higher shear and tensile strength than standard blind rivets, with better consistency. Used in automotive body structures, rail vehicles, and structural aluminum assemblies where rivet integrity must be verifiable.
Torque and Preload: Getting the Joint Right
A bolted joint only functions as designed if the correct preload (clamping force) is achieved during assembly. The torque-preload relationship is: F = T / (K × d), where F is clamp force (N), T is applied torque (N·m), d is nominal bolt diameter (m), and K is the nut factor (approximately 0.20 for as-received dry steel; 0.15 for lubricated; 0.25 for zinc-plated unlubricated). The nut factor accounts for friction in the thread and under the nut face — it is the largest source of scatter in torqued joint preload, typically ±25–30% for production assembly.
For critical joints, specify a torque value based on 70–75% of bolt proof load with the appropriate K factor, and note the lubrication condition (dry, machine oil, thread compound). Never reuse 12.9 bolts in torque-to-yield applications — once yielded, the bolt has reduced fatigue life.
Conclusion
The bolt, cap screw, stud, and rivet each have distinct use cases driven by access constraints, assembly frequency, strength requirements, and load type. Specifying the correct fastener type, strength grade, thread engagement length, and torque value on the drawing — not just leaving it to the shop floor — is a fundamental part of the mechanical designer’s job. When in doubt about critical joints, consult Shigley’s Mechanical Engineering Design (Chapter 8) or VDI 2230 (systematic calculation of highly stressed bolted joints) for rigorous analysis methods.
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