What Should You Know About Bolts for Your Fastening Projects?

Bolts are among the most fundamental fasteners, used in everything from building skyscrapers to assembling furniture. Choosing the right bolt can mean the difference between a secure, long-lasting connection and a project that fails. This guide will walk you through the types, materials, sizes, and uses of bolts, helping you pick the perfect one for any job.

Types of Bolts

Hex Bolts

Hex bolts are the most common type, with a six-sided head that fits standard wrenches. They’re used in construction, industrial machinery, and DIY projects, where a strong, easy-to-tighten fastener is needed. Hex bolts come with either partial or full threads and are available in a wide range of materials and sizes.

Socket Head Bolts

Socket head bolts have a cylindrical head with a hexagonal socket (recess) for use with an Allen wrench. They’re ideal for tight spaces where a regular wrench won’t fit, such as in electronics, machinery, and aerospace components. Their low-profile head makes them suitable for applications where appearance matters.

Eye Bolts

Eye bolts have a loop (eye) at one end, designed to attach ropes, chains, or cables for lifting or securing objects. They’re used in construction, marine industry, and rigging, with the eye size varying based on the load they need to handle. It’s crucial to choose eye bolts rated for the specific weight to avoid accidents.

Lag Bolts

Lag bolts (also called lag screws) are large, wood-threaded bolts with a hex head, used to secure heavy objects to wood. They’re common in furniture making, deck building, and construction, where a strong grip in wood is needed. Lag bolts require a pre-drilled hole to prevent splitting the wood.

Machine Bolts

Machine bolts are general-purpose bolts with a square or hex head, designed to be used with nuts. They’re used in machinery, automotive repair, and electrical systems, where a removable fastener is needed. Machine bolts come in various lengths and thread types to fit different applications.

Carriage Bolts

Carriage bolts have a smooth, rounded head with a square section under the head that prevents the bolt from turning when tightened. They’re used in wood and metal applications like fencing, furniture, and agricultural equipment, where a low-profile, secure fastener is required.

Set Screws

Set screws are headless bolts (or with a small head) designed to lock two parts together, such as a pulley on a shaft. They’re used in machinery, tools, and electronics, where a compact fastener is needed to prevent movement. Set screws come with various tip types (flat, pointed, cup) for different gripping needs.

Threaded Rods

Threaded rods are long, fully threaded bolts without a head, used to connect two parts over a distance. They’re cut to length and used with nuts in construction, plumbing, and industrial applications. Threaded rods are ideal for creating custom-length fasteners or securing objects in hard-to-reach places.

Shoulder Bolts

Shoulder bolts have a smooth, unthreaded section (shoulder) between the head and threads, used to pivot or align parts. They’re common in machinery, robotics, and automotive components, where precise alignment is needed. The shoulder acts as a bearing surface, allowing parts to rotate freely.

Flange Bolts

Flange bolts have a built-in flange (wide rim) under the head that acts as a washer, distributing pressure and preventing damage to the material. They’re used in automotive, plumbing, and industrial applications, eliminating the need for a separate washer and ensuring a tight seal.

Material Characteristics of Bolts

Material Types

Steel bolts are strong, affordable, and versatile, used in construction, machinery, and automotive applications. They’re often coated (zinc-plated or black oxide) for basic corrosion resistance.

Stainless steel bolts offer excellent corrosion resistance and durability, making them ideal for outdoor use, marine industry, and food processing. They’re more expensive than steel but last longer in wet or harsh environments.

Zinc-plated bolts are steel bolts coated with zinc to improve corrosion resistance. They’re used in automotive, electrical, and construction applications where moderate protection from rust is needed.

Brass bolts are corrosion-resistant, non-magnetic, and conductive, used in plumbing, electrical systems, and marine applications. They’re softer than steel, making them easy to machine.

Aluminum bolts are lightweight and corrosion-resistant, used in aerospace, automotive, and electronics where weight is a concern. They’re not as strong as steel but offer good performance in low-load applications.

Bronze bolts are strong and corrosion-resistant, used in marine industry, valves, and pumps, where exposure to saltwater or chemicals is common.

Copper bolts are conductive and corrosion-resistant, used in electrical systems and plumbing, especially in historic restoration projects.

Titanium bolts are lightweight, strong, and highly corrosion-resistant, used in aerospace, medical devices, and high-performance applications. They’re expensive but offer superior performance in extreme conditions.

Plastic bolts are lightweight, non-conductive, and corrosion-resistant, used in electronics, plumbing, and outdoor furniture. They’re ideal for applications where metal could cause interference or rust.

Key Properties

Strength: Steel, titanium, and bronze bolts offer high strength, suitable for heavy loads in construction and machinery. Aluminum and plastic bolts are better for light-duty use.

Corrosion resistance: Stainless steel, brass, bronze, and titanium bolts resist rust and chemicals, making them suitable for wet or harsh environments. Zinc-plated steel provides moderate protection.

Hardness: Steel and titanium bolts are hard, resisting deformation under pressure, while brass and aluminum are softer, easier to work with but less durable.

Temperature resistance: Titanium and stainless steel bolts handle high temperatures, used in engines, furnaces, and aerospace applications. Plastic bolts are limited to low temperatures.

Coating: Zinc-plated, black oxide, and electroplated coatings improve corrosion resistance and appearance. Black oxide also reduces glare, making it suitable for military or industrial applications.

Size and Specifications of Bolts

Key Dimensions

Thread size (diameter of the threads) and thread pitch (distance between threads) determine compatibility with nuts and tapped holes. Metric thread (e.g., M8 x 1.25) and imperial thread (e.g., ¼-20) are the two main standards, with unified thread standard (UTS) used in the U.S.

Length is measured from the underhead to the tip, varying based on the application. Longer bolts are used for thick materials, while shorter ones work for thin sheets.

Head size (width across flats for hex bolts) determines the wrench size needed. Larger heads provide more torque but take up more space.

Standards

Bolts follow ANSI standards (U.S.) and ISO standards (international), ensuring consistent sizing and performance. These standards define thread types, strength ratings, and dimensions, making it easy to find replacement bolts across brands.

Application Areas of Bolts

Construction

In construction, steel and stainless steel bolts (hex bolts, carriage bolts) secure beams, frames, and structural components. Threaded rods are used to anchor objects to concrete, while lag bolts attach wood to metal or concrete.

Automotive

The automotive industry uses a variety of bolts, including flange bolts, socket head bolts, and machine bolts, to assemble engines, transmissions, and body parts. Stainless steel and zinc-plated bolts resist corrosion from fluids and weather.

Aerospace

Aerospace relies on lightweight, high-strength bolts made from titanium and stainless steel. Socket head bolts and shoulder bolts are used in aircraft engines and frames, where precision and reliability are critical.

Industrial Machinery

Industrial machinery uses heavy-duty bolts (hex bolts, machine bolts) to secure gears, motors, and structural parts. Steel and alloy bolts handle high torque and vibration, ensuring machinery operates safely.

Electronics

Electronics uses small bolts (socket head, set screws) made from brass, aluminum, or plastic to assemble circuit boards, casings, and components. These bolts are non-conductive and compact to fit in tight spaces.

Furniture

Furniture making uses carriage bolts, machine bolts, and lag bolts to assemble frames, tables, and chairs. Brass and zinc-plated bolts are popular for their appearance and corrosion resistance.

Plumbing

Plumbing systems use brass and stainless steel bolts (flange bolts, machine bolts) to connect pipes, valves, and fixtures. These bolts resist corrosion from water and chemicals, ensuring leak-free connections.

Electrical

Electrical systems use copper and brass bolts to connect wires, panels, and equipment. Their conductivity and corrosion resistance make them safe for electrical applications.

Marine

The marine industry uses stainless steel, bronze, and titanium bolts to withstand saltwater corrosion. Eye bolts, carriage bolts, and flange bolts secure decks, rigging, and equipment on boats and offshore platforms.

DIY Projects

DIY projects use hex bolts, lag bolts, and machine bolts for everything from building shelves to repairing tools. Zinc-plated steel bolts are affordable and versatile for most home projects.

Installation and Use of Bolts

Installation Tools

Wrenches (open-end, box-end, adjustable) are used to tighten hex bolts and carriage bolts. Box-end wrenches provide a better grip than open-end ones.

Socket wrenches with hex sockets are ideal for socket head bolts and tight spaces, allowing for high torque application.

Torque wrenches ensure bolts are tightened to a specific torque (force), preventing over-tightening (which can break the bolt) or under-tightening (which can cause loosening). They’re essential in automotive, aerospace, and construction.

Screwdrivers (flathead, Phillips) are used with bolts that have slotted or cross-recessed heads, though these are less common than hex or socket heads.

Safety Precautions

Proper alignment: Ensure bolts are straight before tightening to prevent bending or breaking. Misaligned bolts can damage threads and weaken the connection.

Torque specifications: Follow manufacturer guidelines for torque to avoid overloading the bolt. Use a torque wrench for critical applications like automotive engines and aerospace components.

Thread locking: Use thread-locking compounds (e.g., Loctite) on bolts in vibrating environments (machinery, automotive) to prevent loosening.

Storage: Store bolts in a dry place to prevent rust. Keep them organized by size and material for easy access.

Removal methods: Use the correct tool to avoid stripping the head. For stuck bolts, apply penetrating oil and let it sit before attempting to loosen.

Inspection: Check bolts regularly for corrosion, wear, or loosening. Replace damaged bolts promptly to prevent equipment failure.

BBjump's View: As a sourcing agent, we match bolts to specific needs—titanium for aerospace, stainless steel for marine use, zinc-plated for general projects. We ensure compliance with ANSI/ISO standards, offering the right thread types and coatings to balance strength, corrosion resistance, and cost for clients.

FAQs

1. What’s the difference between metric and imperial bolts?

Metric bolts use millimeters for diameter and thread pitch (e.g., M10 x 1.5), while imperial bolts use inches (e.g., ½-13). They’re not interchangeable, so always match the bolt to the nut or tapped hole’s standard.

2. Which bolt material is best for outdoor use?

Stainless steel is the best choice for outdoor use, as it resists rust and corrosion from rain and humidity. For coastal areas with saltwater, titanium or bronze bolts offer superior corrosion resistance.

3. How do I choose the right bolt length?

The bolt should extend at least one thread past the nut when tightened. For through-bolts, measure the thickness of the materials being joined and add the nut thickness plus 1-2 threads. For lag bolts, use a length that penetrates the wood by at least ½ the thickness of the material being attached.