Fastener Types
The first step is knowing what category of fastener you need. Each type has a distinct function.
| Type | What it does | Common uses |
|---|---|---|
| Bolt / Cap Screw | Threaded fastener tightened with a nut or into a threaded hole | Structural connections, machinery assembly, flanges |
| Nut | Internally threaded to mate with a bolt | Paired with bolts; hex, lock, flange, and wing styles available |
| Washer | Flat disc that distributes load under a bolt head or nut | Prevents surface damage, compensates for oversized holes |
| Anchor | Expands inside concrete or masonry to hold loads | Attaching equipment, shelving, or structures to concrete floors/walls |
| Rivet | Permanent blind fastener — set from one side only | Sheet metal, enclosures, light fabrication where disassembly isn't needed |
| Seal / Gasket | Creates a pressure-tight or fluid-tight joint between flanges | Piping, pressure vessels, high-temp or chemical environments |
Thread System: Imperial vs. Metric
Imperial and metric fasteners are not interchangeable — forcing a metric bolt into an imperial nut (or vice versa) will strip threads. Always confirm which system your application uses before ordering.
Imperial (Unified Thread Standard — UNC / UNF)
Written as diameter – threads per inch × length, for example 3/8″–16 × 2″. The "16" means 16 threads per inch. UNC (Unified Coarse) is the most common; UNF (Fine) has more threads per inch and is used where vibration resistance matters.
Metric (ISO)
Written as M[diameter] × [pitch] × [length], for example M10 × 1.5 × 40mm. The pitch is the distance in millimeters between thread crests — smaller pitch = finer thread.
How to tell them apart without a gauge
- Place the bolt against a ruler: metric diameters are whole millimeter numbers (8mm, 10mm, 12mm); imperial diameters are fractions of an inch (3/8″, 1/2″, 5/16″).
- Count threads over a 1-inch span with a ruler — imperial gives you threads-per-inch (e.g., 16); metric gives you a distance between crests in mm.
- A thread gauge (pitch gauge) is the most reliable tool — the blades will only seat flush on matching threads.
- Metric bolts usually have "8.8" or "10.9" stamped on the head. Imperial Grade 5 bolts have 3 radial lines; Grade 8 bolts have 6 lines.
| Imperial | Metric equivalent (approx.) |
|---|---|
| 1/4"-20 | M6 × 1.0 |
| 5/16"-18 | M8 × 1.25 |
| 3/8"-16 | M10 × 1.5 |
| 1/2"-13 | M12 × 1.75 |
| 5/8"-11 | M16 × 2.0 |
| 3/4"-10 | M20 × 2.5 |
How to Measure Diameter
Fastener diameter always refers to the major (outer) diameter of the threads — not the wrench size or head width.
Using calipers (recommended)
Measure across the widest point of the threads with a digital or vernier caliper. The reading is the nominal diameter. Round to the nearest standard size: if you read 9.9mm, the bolt is M10; if you read 0.373″, it's 3/8″.
Using a drill bit (field method)
A drill bit that slides over the shank (not the threads) with minimal play is approximately the correct diameter. This is less precise but works in a pinch.
Bolts vs. nuts
- Bolt diameter: measure the outside of the thread (the shank).
- Nut diameter: measure the inside of the thread (the bore). The nominal size should match the bolt it pairs with.
- Washer: specify the bolt size it's used with, not the washer's own OD or ID — e.g., a "3/8″ washer" fits a 3/8″ bolt.
| Nominal size | Actual major diameter | Notes |
|---|---|---|
| M6 | 6.0 mm | Metric |
| M8 | 8.0 mm | Metric |
| M10 | 10.0 mm | Metric |
| M12 | 12.0 mm | Metric |
| M16 | 16.0 mm | Metric |
| 1/4" | 0.250" (6.35 mm) | Imperial |
| 3/8" | 0.375" (9.53 mm) | Imperial |
| 1/2" | 0.500" (12.7 mm) | Imperial |
| 5/8" | 0.625" (15.9 mm) | Imperial |
How to Measure Length
Bolt length is measured differently depending on head style. The rule: length is the portion of the bolt that goes into the material.
| Head style | Measure from… | Measure to… |
|---|---|---|
| Hex / Socket cap | Underside of head (bearing surface) | Tip of bolt |
| Flat / countersunk head | Top of head (it sits flush) | Tip of bolt |
| Carriage bolt | Underside of the dome head | Tip of bolt |
| Pan / Button head | Underside of head | Tip of bolt |
| Stud (no head) | One end | Other end (full body length) |
Practical tip
Add up the thickness of all the parts being clamped, then add 1–2 thread pitches to ensure the nut fully engages. For through-bolts: material thickness + washer thickness + nut height + 2–3 threads of protrusion past the nut.
Material Selection
Material choice affects strength, corrosion resistance, weight, and cost. Choose the minimum material that meets your environment and load requirements.
| Material | Strength | Corrosion resistance | Best for |
|---|---|---|---|
| Carbon Steel | Medium | Low (needs coating) | Indoor, dry environments; general construction; most common and lowest cost |
| Alloy Steel (Grade 8 / Class 10.9) | High | Low (needs coating) | High-load structural joints, automotive, heavy machinery |
| Stainless Steel (18-8 / 304) | Medium | Excellent | Outdoor, marine, food service, chemical environments; no coating needed |
| Stainless Steel (316) | Medium | Superior (chloride resistant) | Saltwater, marine, medical, chemical processing |
| Aluminum | Low–Medium | Good (oxidizes naturally) | Weight-critical applications; electronics, aerospace, light fabrication |
| Brass | Low | Excellent | Electrical, plumbing, decorative; non-magnetic, non-sparking |
Grade & Strength Standards
Grade indicates the minimum tensile strength and proof load of the fastener. Using an under-strength bolt in a critical joint can result in failure — always match or exceed the design requirement.
Imperial (SAE) grades
| Grade | Head markings | Tensile strength | Common uses |
|---|---|---|---|
| Grade 2 | No marks | 74,000 psi | Light-duty hardware, non-structural, household |
| Grade 5 | 3 radial lines | 120,000 psi | General machinery, automotive, most structural work |
| Grade 8 | 6 radial lines | 150,000 psi | High-stress joints, suspension components, heavy equipment |
Metric (ISO) property classes
| Class | Head markings | Tensile strength | Common uses |
|---|---|---|---|
| Class 5.8 | 5.8 | 520 MPa (75,000 psi) | Light-duty metric equivalent to Grade 2/5 |
| Class 8.8 | 8.8 | 830 MPa (120,000 psi) | General metric machinery — equivalent to Grade 5 |
| Class 10.9 | 10.9 | 1,040 MPa (151,000 psi) | High-strength metric — equivalent to Grade 8 |
| Class 12.9 | 12.9 | 1,220 MPa (177,000 psi) | Ultra-high strength; socket head cap screws, critical joints |
Stainless grades
| Grade | Alloy | Tensile strength | Notes |
|---|---|---|---|
| 18-8 (A2-70) | 304 stainless | 70,000 psi | Most common stainless; good corrosion resistance, moderate strength |
| 316 (A4-70) | 316 stainless | 70,000 psi | Molybdenum added for chloride/acid resistance; marine grade |
Finish & Coating Guide
Finish affects corrosion protection, appearance, and dimensional tolerances. Some coatings add enough thickness to affect thread fit — always verify torque specs when using thick coatings.
| Finish | Protection level | Appearance | Best for |
|---|---|---|---|
| Plain (bare steel) | None | Silver-grey, will rust | Dry indoor use only; lowest cost; often used with lubrication |
| Zinc Plated | Moderate (200–500 hrs salt spray) | Shiny silver | General indoor/light outdoor; most common on hardware store bolts |
| Hot-Dip Galvanized (HDG) | Excellent (1,000+ hrs) | Dull grey/matte | Outdoor structural, construction, utility poles, bridges; thicker coat |
| Black Oxide | Light (adds oil/wax barrier) | Matte black | Decorative and light corrosion prevention; common on high-strength bolts |
| Mechanically Galvanized | Good (500–700 hrs) | Matte silver | Thinner than HDG; maintains tighter thread tolerances |
| Stainless (no coating needed) | Excellent (self-passivating) | Bright silver | Corrosion resistance is inherent to the alloy |