Complete Guide to Nuts, Bolts, and Rivnuts: Sizing, Types, and Installation

Understanding Different Types of Nuts and Bolts

Nuts and bolts form the backbone of mechanical fastening systems across countless applications, from household furniture to automotive assemblies and industrial machinery. Understanding the various types available helps you select the right fastener for your specific project requirements, ensuring structural integrity and long-term reliability.

Common Bolt Types

Hex bolts, also known as hex cap screws, feature a hexagonal head and are the most widely used bolt type in construction and mechanical applications. They provide excellent torque capability and can be tightened with standard wrenches or sockets. Carriage bolts have a rounded head with a square section underneath that prevents rotation when tightening the nut, making them ideal for wood-to-wood or wood-to-metal connections. Lag bolts, sometimes called lag screws, have a pointed tip and coarse threads designed to bite into wood without requiring a pre-drilled hole in softer materials.

Eye bolts feature a circular loop instead of a traditional head, allowing them to serve as anchor points for cables, ropes, or chains. Machine screws are precision-threaded fasteners designed to thread into tapped holes or be secured with nuts, commonly used in electronics and appliances. Stud bolts are headless threaded rods with threads on both ends, typically used when one end screws into a tapped hole while the other accepts a nut.

Nut Varieties and Their Applications

Hex nuts are the standard six-sided nuts that complement hex bolts and provide reliable fastening for general applications. Lock nuts incorporate features that resist loosening from vibration, including nylon inserts (nylock nuts), deformed threads, or prevailing torque designs. Wing nuts feature two large tabs that allow for hand tightening without tools, perfect for applications requiring frequent assembly and disassembly.

Cap nuts have a domed top that covers the bolt end, providing a finished appearance while protecting threads from damage and preventing injury from sharp edges. Flange nuts include an integrated washer-like base that distributes load over a wider area and eliminates the need for a separate washer. Coupling nuts are elongated internally threaded fasteners that join two threaded rods or bolts together, commonly used in lighting fixtures and structural applications.

Specialty Fastener Types

T-bolts have a T-shaped head that slides into slots commonly found in workbenches, machine tables, and extruded aluminum framing systems. U-bolts form a U-shape with threads on both ends, used to secure pipes, tubes, or cables to surfaces. Anchor bolts are embedded in concrete or masonry to provide strong attachment points for structural elements. Shoulder bolts feature a smooth cylindrical section between the head and threads, serving as a precision shaft for rotating components while providing clamping force.

How to Determine and Measure Bolt Size

Accurate bolt sizing is essential for proper fit, adequate strength, and successful project completion. Bolt dimensions follow standardized systems that specify diameter, thread pitch, and length, with measurements varying between imperial and metric systems.

Understanding Bolt Size Designation

In the imperial system, bolt sizes are designated by diameter in fractions of an inch or gauge numbers for smaller sizes. Common fractional sizes include 1/4", 5/16", 3/8", 1/2", and larger. Bolts smaller than 1/4" use numbered sizes from #0 to #12, with #8 and #10 being particularly common in household applications. The metric system uses millimeter measurements, with popular sizes including M3, M4, M5, M6, M8, M10, and M12, where the number indicates the nominal diameter.

Thread pitch refers to the distance between adjacent threads. Imperial bolts use threads per inch (TPI), with designations like "1/4-20" indicating a 1/4-inch diameter with 20 threads per inch. Metric bolts specify pitch in millimeters, such as "M10 x 1.5" for a 10mm diameter bolt with 1.5mm between threads. Coarse threads are standard for general applications, while fine threads provide greater precision and resistance to vibration loosening.

Measuring Bolt Diameter

To measure bolt diameter accurately, use digital calipers or a micrometer for the most precise results. Place the measuring tool across the widest part of the threaded shank, measuring from outer thread to outer thread. For imperial bolts, compare your measurement to standard fractional sizes, rounding to the nearest common size. For metric bolts, the measurement should closely match the nominal diameter specification.

If precision tools aren't available, a bolt gauge provides a quick and reliable alternative. These gauges feature holes corresponding to standard bolt sizes—simply test-fit the bolt through progressively sized holes until you find the correct match. A combination square with a ruler can also work for larger bolts, though with reduced precision. When measuring worn or damaged bolts, take multiple measurements along the thread length to account for any deformation.

Determining Thread Pitch

Thread pitch gauges are specialized tools with multiple blades, each matching a specific thread configuration. To use one, hold various blades against the bolt threads until you find a perfect match where the blade teeth align precisely with the thread valleys. The gauge will indicate the thread pitch, either in threads per inch for imperial or millimeter pitch for metric fasteners.

Without a thread gauge, you can count threads manually. For imperial bolts, use a ruler to mark exactly one inch along the threaded portion, then count the number of thread peaks within that span. For metric bolts, measure the distance across ten threads with calipers, then divide by ten to calculate the pitch in millimeters. This method works best with clean, undamaged threads and good lighting.

Measuring Bolt Length

Bolt length measurement depends on head style. For hex bolts, carriage bolts, and other fasteners with prominent heads, measure from directly under the head to the end of the threads—do not include the head in the measurement. For flat head screws and countersunk fasteners that sit flush with the surface, measure the entire length including the head, as this represents the depth of hole required.

When selecting bolt length for an application, account for the combined thickness of materials being joined, plus enough length for washers, the nut, and at least two to three threads extending beyond the nut face after full tightening. Insufficient thread engagement compromises joint strength, while excessive length may interfere with adjacent components or create safety hazards.

Quick Reference for Common Bolt Sizes

Installing Rivnuts Without Specialized Tools

Rivnuts, also called rivet nuts or threaded inserts, provide strong threaded connections in thin materials like sheet metal, plastic, or composite panels where traditional nuts would be impractical. While dedicated rivnut installation tools make the process easier, you can successfully install rivnuts using common hand tools found in most toolboxes.

Understanding Rivnut Mechanics

A rivnut consists of a cylindrical threaded body with a flange on one end. When installed, the opposite end of the body is compressed and expanded, creating a bulge that clamps the material between the flange and the expanded section. This mechanical action creates a permanent threaded anchor point that can accept bolts or screws multiple times without degradation, unlike self-tapping screws that can strip with repeated use.

The installation process requires pulling force to draw the rivnut body through itself while something prevents rotation, causing the body to collapse and expand. Purpose-built tools accomplish this with threaded mandrels and leverage systems, but alternative methods can achieve the same result with patience and improvisation.

Method One: Using a Bolt, Washer, and Wrench

This approach is the most accessible method for installing rivnuts without dedicated equipment. Start by drilling a hole in your workpiece that matches the rivnut's outer body diameter—refer to the rivnut packaging or specifications for the exact size. Deburr the hole edges to ensure the rivnut flange sits flat against the material surface.

Thread a bolt that matches the rivnut's internal threads through a standard flat washer large enough to span beyond the rivnut flange. The washer acts as a spacer and bearing surface. Screw this bolt-washer assembly into the rivnut until the washer contacts the flange, leaving a small gap. Insert the rivnut into the prepared hole from the installation side, ensuring the flange seats properly against the material.

Hold the bolt head stationary with one wrench while turning the nut with a second wrench to tighten it against the washer. As the nut advances toward the washer, it pulls the rivnut body upward through the hole while the washer prevents the flange from moving. This compression causes the blind end to collapse and expand, securing the rivnut. Continue tightening until you feel significant resistance and observe that the flange has drawn tight against the material surface. Remove the bolt and washer assembly to reveal the installed threaded insert.

Method Two: Modified Bolt with Locking Nuts

For improved control during installation, use a longer bolt with two nuts instead of just a bolt and washer. Thread both nuts onto the bolt several inches from the end, then thread the bolt into the rivnut. Position one nut on each side of the rivnut flange, effectively sandwiching the flange between them. This configuration provides better stability and prevents the rivnut from spinning during installation.

Insert the rivnut into your prepared hole and tighten the outer nut against the flange while holding the inner nut stationary. The mechanical advantage of this setup reduces the force required and gives you better feedback about the installation progress. You'll feel the rivnut begin to compress and lock into place. Once fully seated, carefully back out the installation bolt without disturbing the newly set rivnut.

Method Three: Threaded Rod and Socket Approach

For multiple installations or larger rivnuts, a threaded rod with a deep socket can create a more comfortable tool arrangement. Cut a section of threaded rod matching the rivnut's internal threads, at least six inches long for adequate grip. Thread a nut onto one end to serve as a handle, and attach a deep socket to the opposite end using another nut as a spacer to create the proper offset.

The deep socket serves as a guide that centers over the rivnut flange and distributes force evenly. Thread the rod into the rivnut, insert the assembly into the hole, and rotate the rod using the handle nut while the socket bears against the material surface. This method works particularly well for overhead installations or confined spaces where using two wrenches would be awkward.

Critical Installation Tips

• Always verify hole size before installation—too small and the rivnut won't insert properly, too large and it won't grip adequately
• Use cutting oil or lubricant on the installation bolt threads to reduce friction and prevent galling during the installation process
• Ensure the material thickness falls within the rivnut's specified range—too thin and it won't grip, too thick and it won't fully expand
• Keep the installation bolt perpendicular to the work surface throughout the process to prevent binding or crooked installation
• Stop tightening immediately when you feel the flange contact the surface firmly—over-tightening can strip the threads or damage the rivnut
• For aluminum or soft materials, use extra caution to avoid pulling the rivnut completely through the workpiece
• Test the installation by threading a bolt in and out several times to verify the threads are clean and properly formed

Troubleshooting Common Installation Problems

If the rivnut spins in the hole during installation, it indicates either an oversized hole or insufficient grip before the expansion phase begins. Try using a slightly larger rivnut designed for the next hole size up, or add a small amount of thread-locking compound to the hole perimeter before insertion to create temporary resistance.

When the installation bolt strips before the rivnut fully sets, you're likely using a bolt made from soft material or one with damaged threads. Replace with a grade 5 or higher bolt, and verify the thread pitch matches exactly—mixing fine and coarse threads will cause immediate stripping. If the rivnut flange deforms or bends during installation, reduce tightening force and ensure your washer or socket fully supports the flange perimeter rather than concentrating pressure at the center.

Selecting the Right Fastener for Your Application

Choosing appropriate nuts and bolts requires evaluating multiple factors including load requirements, material compatibility, environmental conditions, and accessibility for installation and future maintenance. Making informed selections ensures safe, reliable assemblies that perform as intended throughout their service life.

Material Grade and Strength Considerations

Bolt grade markings indicate tensile strength and material composition. In the imperial system, grade 2 bolts are standard low-carbon steel suitable for non-critical applications, grade 5 offers medium strength for automotive and general construction use, and grade 8 provides high strength for demanding structural and mechanical applications. The bolt head displays radial lines corresponding to the grade—grade 5 shows three lines, grade 8 shows six lines.

Metric bolts use property class numbers like 4.6, 8.8, and 10.9, where the first number multiplied by 100 gives tensile strength in megapascals. Class 8.8 and 10.9 are most common for general mechanical and structural applications. Stainless steel bolts, designated 18-8 or by specific alloys like 304 or 316, provide excellent corrosion resistance but lower tensile strength than comparable carbon steel grades, requiring larger sizes for equivalent load capacity.

Environmental and Corrosion Protection

Outdoor applications, marine environments, and chemical exposure require careful material selection to prevent corrosion failure. Zinc-plated fasteners offer economical protection for dry indoor environments and limited outdoor exposure. Hot-dip galvanized bolts provide superior corrosion resistance for structural outdoor applications, though the thick coating may affect fit in precisely sized holes.

Stainless steel fasteners excel in wet, humid, or corrosive environments, with 316 stainless offering better resistance to chlorides and salt water than 304 stainless. For extreme conditions, consider exotic alloys like Monel, titanium, or silicon bronze. Always match nut and bolt materials to prevent galvanic corrosion when dissimilar metals contact each other in the presence of electrolytes.

Thread Engagement and Joint Design

Proper thread engagement is critical for achieving rated bolt strength. As a general rule, thread engagement depth should equal at least one times the bolt diameter for steel-to-steel joints, 1.5 times diameter for steel bolts into aluminum, and 2 times diameter for steel bolts into softer materials like brass or plastic. Insufficient engagement risks thread stripping under load, while excessive engagement provides no additional strength benefit.

In through-bolted joints where the bolt passes completely through the materials and tightens against a nut, ensure adequate space for the nut and at least two full threads extending beyond the nut face after tightening. For blind holes that don't go all the way through, calculate required hole depth by adding thread engagement length to the unthreaded portion of the bolt that enters the hole, plus extra clearance for debris or incomplete threading at the hole bottom.

Vibration Resistance and Locking Methods

Applications subject to vibration, thermal cycling, or dynamic loads require measures to prevent fastener loosening. Nylon insert lock nuts create friction that resists rotation but can be reused several times before losing effectiveness. All-metal prevailing torque lock nuts use deformed threads or spring elements for higher temperature resistance and longer service life but cost more than nylon insert types.

Thread-locking compounds provide chemical resistance to loosening, available in strengths from low (removable with hand tools) to high (requires heat for removal). Split lock washers create tension and bite into material surfaces but work poorly on soft materials or hardened surfaces. Nord-lock washers use cam surfaces that prevent rotation through wedge action, providing superior vibration resistance for critical applications.

Proper Installation Techniques for Maximum Performance

Correct installation practices are just as important as selecting the right fastener. Improper tightening, inadequate preparation, or poor technique can compromise joint integrity and lead to premature failure, even with high-quality components.

Surface Preparation and Alignment

Clean all mating surfaces thoroughly before assembly, removing dirt, oil, paint, or corrosion that could prevent proper contact or introduce contamination into the joint. Flat washers help distribute load and protect soft materials, but only when they sit flush against clean, flat surfaces. Deburr all holes to prevent raised edges from creating stress concentrations or preventing proper fastener seating.

Ensure bolt holes align properly before attempting to insert fasteners. Forcing bolts through misaligned holes deforms threads and stresses materials, creating weak points in the assembly. Use alignment pins or temporary fasteners to establish correct positioning before installing permanent bolts. In assemblies with multiple fasteners, insert all bolts loosely before beginning final tightening to allow for tolerance variations.

Tightening Sequence and Torque Control

For multi-bolt joints, follow a star or cross pattern when tightening to distribute clamping force evenly and prevent warping or gaps. Begin at the center and work outward, or alternate between opposite bolts. Perform tightening in multiple passes, bringing all fasteners to approximately 30 percent of final torque on the first pass, 60 percent on the second pass, and full torque on the final pass.

Torque specifications ensure adequate clamping force without exceeding the fastener's elastic limit or damaging threads. Use a calibrated torque wrench for critical applications, particularly in automotive, aerospace, or structural assemblies where failure could have serious consequences. When torque specifications aren't available, general guidelines suggest tightening until snug plus a quarter to half turn for small bolts, or until resistance is clearly felt for larger fasteners. Never use impact tools on hardened fasteners or in applications requiring precise torque control.

Thread Lubrication Effects

Friction between threads and under fastener heads consumes 85 to 90 percent of applied torque, with only 10 to 15 percent actually creating clamping force. Lubricating threads reduces friction, allowing a given torque value to produce significantly more clamping force. Standard torque specifications typically assume dry, as-received fasteners with no additional lubrication.

When using thread lubricants, cutting oils, or anti-seize compounds, reduce specified torque values by approximately 25 to 30 percent to achieve equivalent clamping force. Alternatively, consult torque charts specific to lubricated fasteners if available. Never mix lubrication practices within a single joint—use either all dry or all lubricated fasteners with appropriate torque values for consistency.

Common Mistakes and How to Avoid Them

Understanding frequent errors in fastener selection and installation helps you avoid problems that compromise joint performance, create safety hazards, or necessitate costly repairs and rework.

Mixing Thread Standards

Attempting to thread metric nuts onto imperial bolts or vice versa damages threads even when sizes seem close. A 1/4-20 bolt measures 0.250 inches diameter while an M6 bolt is 6mm (0.236 inches)—close enough to partially engage but different enough to destroy threads. Similarly, thread pitch differences prevent proper mating even when diameters match. Always verify thread compatibility before assembly and never force fasteners that don't thread smoothly by hand for the first several turns.

Over-Tightening and Fastener Failure

Excessive tightening torque stretches bolts beyond their elastic limit, causing permanent deformation that reduces strength and can lead to immediate or delayed failure. Signs of over-tightening include elongated bolt shanks, necking near the head or threads, cracked nuts, or crushed material under fastener heads. Small fasteners in soft materials are particularly vulnerable—an M6 bolt in aluminum can strip threads or pull through the material with surprisingly little force.

Develop a feel for appropriate tightness by practicing on scrap materials and paying attention to resistance feedback. Remember that longer wrenches provide more leverage, making it easier to over-tighten inadvertently. When using power tools, set clutches to appropriate levels and finish with hand tools for final tightening in precision applications.

Inadequate Load Distribution

Omitting washers when fastening to soft materials like wood, plastic, or soft aluminum allows bolt heads and nuts to dig into the surface, reducing clamping force and potentially pulling through under load. Oversized washers or fender washers distribute force over a larger area, preventing this problem. Similarly, using too few fasteners for the load or spacing them inadequately concentrates stress and increases the likelihood of joint failure.

Ignoring Material Compatibility

Galvanic corrosion occurs when dissimilar metals contact each other in the presence of moisture or electrolytes, with the more reactive metal corroding preferentially. Common problematic combinations include aluminum fasteners in steel assemblies, steel fasteners in aluminum structures exposed to weather, and brass components with steel in marine environments. Use fasteners made from the same material as the base components, or isolate dissimilar metals with non-conductive washers and coatings. When material matching isn't possible, make fasteners from the more noble material—stainless steel bolts in aluminum are preferable to aluminum bolts in steel.

Reusing Lock Nuts and Single-Use Fasteners

Nylon insert lock nuts lose effectiveness after several uses as the nylon deforms, reducing prevailing torque. Deformed thread lock nuts similarly lose their locking capability with repeated use. Critical applications should use new lock nuts for each assembly cycle. Thread-locking compounds can only be reused after thorough cleaning to remove old compound residue. Some fasteners, particularly those used in automotive safety systems, are designed for single use only and must be replaced rather than reinstalled—check manufacturer specifications and replacement intervals for such components.