Machine Design · Fasteners
Bolts are one of the most fundamental elements in mechanical design — yet in practice, they are surprisingly easy to get wrong. With dozens of variations in head shape, thread form, and material, selecting the right bolt for the job is a skill that separates a careful engineer from a careless one.
“From head types to strength grades — everything you need to know about bolts before you put pen to drawing.”
This article covers bolt classification by head type, thread coverage, thread standard, and material — the essentials for confident part selection and drawing annotation.
1. Bolt Head Types — Classified by Fastening Method
The shape of a bolt head determines what tool is used to drive it and how much clearance is needed around the fastening point.
| Type | Drive Tool | Characteristics & Applications |
|---|---|---|
| Hex Bolt | Spanner / Open-end wrench | The most common bolt type. Used where high clamping force is needed and there is ample clearance around the head. |
| Socket Head Cap Screw (Allen Bolt) |
Hex key (Allen key) | Hexagonal recess in the head. The go-to choice for jigs, fixtures, and general machine assembly. Best practice is to countersink a counterbore so the head sits flush with or below the surface. |
| Flat Head Screw (Countersunk) |
Hex key / Phillips driver | Head sits completely flush with the surface after fastening. Requires a countersink (cone-shaped recess). Used where protruding heads would interfere with passing parts or mating surfaces. |
| Pan Head Screw | Phillips (+) / Flathead (−) driver | Low-load applications: precision instruments, electrical panels, thin covers. Easy to drive but not suited for high-torque joints. |
| Set Screw (Headless / Grub Screw) |
Hex key (internal socket) | No head — fully threaded along the entire body. Threaded completely into tapped holes to lock pulleys and gears onto motor shafts, remaining invisible when installed. |
2. Thread Coverage — Fully Threaded vs. Partially Threaded
Whether threads run the full length of the shank or only part of it affects both the application and the structural integrity of the joint.
- Fully Threaded (Full Thread): Threads extend from directly below the head all the way to the tip. Typically used on shorter bolts. Allows the bolt to be tightened all the way regardless of thread engagement depth.
- Partially Threaded (Partial Thread): Threads cover only the lower portion of the shank; the upper shank near the head remains a smooth cylinder. The unthreaded shank is significantly stronger in shear than a threaded section, making this type the preferred choice for load-bearing joints that require precise positional alignment.
3. Thread Standards and Direction
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Metric (M) vs. Unified Inch (UNC / UNF)
Korea, Japan, and Europe primarily use ISO metric threads (e.g., M6, M8). Imported machinery — particularly in aerospace and piping — may still use inch-based threads. The two systems have different pitches and are not interchangeable. Forcing a metric bolt into an inch-threaded hole (or vice versa) will strip the threads immediately. -
Coarse Thread vs. Fine Thread
A drawing that simply saysM10means coarse thread (default pitch: 1.5 mm) — faster to assemble, better for general use.M10×1.0specifies a fine thread with a tighter pitch. Fine threads are used in thin-walled parts, vibration-prone environments where loosening is a concern, or applications requiring precise axial adjustment. -
Right-Hand Thread vs. Left-Hand Thread (LH)
Nearly all bolts use right-hand threads — clockwise to tighten. Left-hand threads (marked L or LH) tighten counterclockwise. They are used in specific rotating assemblies — such as a fan blade retaining bolt or a bicycle’s left pedal — where the direction of rotation would otherwise cause a right-hand thread to self-loosen over time.
4. Bolt Materials and Heat Treatment
| Material | Appearance | Characteristics & Applications |
|---|---|---|
| SCM Steel (Chromoly / Alloy Steel) Heat-treated Black Bolt |
Black (oxide finish) | Industry standard for socket head cap screws. Quenched and tempered for maximum hardness. Primary fastener for load-bearing machine frames and jig structures. |
| Stainless Steel (SUS) | Silver (polished) | Corrosion-resistant — essential for food processing, pharmaceutical machinery, wet environments, and cleanrooms. Caution: tightening two stainless fasteners together under high torque can cause galling (cold welding / seizure), making them impossible to remove. |
| Mild Steel / Zinc-plated (e.g., SS400) |
Silver (zinc plating) | Common hex and cross-head bolts found at hardware stores. Lower tensile strength — not suitable for primary structural or high-load joints. |
You may have seen a drawing note that reads “Use bolt strength grade 10.9 or higher.” Here is what that means:
- First number (10): Tensile strength. 10 × 100 = 1,000 N/mm² — the stress at which the bolt breaks.
- Second number (9): Yield strength ratio. 90% of 1,000 N/mm² = 900 N/mm² — the stress at which the bolt permanently deforms (stretches) and can no longer spring back elastically.
In general industrial equipment and jigs, grade 10.9 or 12.9 high-tensile bolts are standard for safety-critical joints.
In equipment subject to frequent vibration or impact, bolts will work themselves loose over time. Consider the following countermeasures at the design stage:
- Spring Washer — maintains constant spring tension against the bolt head, resisting rotation
- Double Nut (Jam Nut) — a second nut locked against the first creates mechanical interference that prevents self-loosening
- Nylon Insert Nut (e.g., Nyloc / Prevailing Torque Nut) — a nylon ring grips the thread and resists rotation without relying on clamping force alone
- Thread-locking compound (e.g., Loctite 243 / 277) — apply to threads before assembly; specify on the drawing with a note such as “Apply thread locker, Loctite 243, before installation”
Determining the correct tap depth is one of those recurring questions on the shop floor. While precise formulas exist, the practical rule of thumb is:
- General rule — 1.5D to 2D: Tap to a depth equal to 1.5 to 2 times the bolt’s nominal diameter (D). For an M8 bolt, that means 12–16 mm of thread engagement. (Steel: 1–1.5D; Aluminum or soft materials: 1.5–2D)
- Stock bolt length rule: Bolts come in standard increments (typically every 5 or 10 mm). If the tap depth does not align with available bolt lengths, the assembler will struggle to find the right bolt or will stack extra washers. Design your tap depth with stock lengths in mind.
- Most intuitive reference — nut thickness: If you tap slightly deeper than the thickness of the equivalent standard nut for that bolt size, you have more than enough thread engagement. With 4–5 threads engaged, the bolt shank will fail in tension before the internal threads strip.
Wrapping Up
We have covered bolt types, basic sizing standards, strength grades, and loosening prevention — the essential knowledge for practical mechanical design. When preparing drawings, keep in mind the load conditions the part must withstand, how the mating surface will be machined, and whether the assembler will have enough wrench clearance. That level of forethought is what separates a smart design from one that creates problems on the shop floor.