Mechanical Joint and Fastener Selection: Bolts, Screws, Nuts, Washers, Nails, Spikes, and Wood Screws

Bolts, Screws, Nuts, Washers, Nails, Spikes, and Wood Screws — From First Principles to Shop-Floor Mastery

The Bridge That Shouldn't Have Failed

In 1983, an illustrative engineering practitioner stood at the edge of a collapsed pedestrian overpass in a developing industrial zone, staring at a pile of twisted steel and shattered concrete. Three workers were in the hospital. The steel was fine. The welds were fine. The concrete was fine.

The bolts had failed.

Not because they were the wrong size. Not because there weren't enough of them. But because someone — a well-meaning purchasing agent trying to save 12% on a procurement order — had substituted Grade 2 bolts where the specification called for Grade 5.

The substitution looked identical on the loading dock. Same diameter. Same thread pitch. Same zinc plating. But under cyclic loading and wind shear, those bolts stretched past their yield point in less than eight months. By month ten, the joint had relaxed enough to allow micro-movement. By month twelve, the overpass dropped four feet and came to rest on a delivery truck.

the practitioner spent the next thirty years of his career with a single obsession: every engineer, every machinist, every fabricator, and every purchasing agent on the planet should understand fasteners the way a surgeon understands a scalpel.

This guide is that education.

You're about to walk through the most comprehensive fastener reference ever assembled in a single document — from the penny nail in your toolbox to the heavy hex structural bolt holding up a skyscraper. Every specification. Every formula. Every decision matrix you'll ever need.

Bookmark this page. You'll come back to it for years.

What You'll Master in This Guide

• The fundamental difference between a bolt and a screw (it's not what you think)
• Torque, tension, and preload — the physics that keep assemblies alive
• Grade marks, mechanical properties, and how to detect counterfeit fasteners
• Complete dimensional data for inch and metric bolts, screws, nuts, and washers
• Riveted joints — design, failure modes, and strength analysis
• Nails, spikes, and wood screws — types, sizes, and selection
• Self-threading screws — when and why to use each type
• Pins, studs, and retaining rings — the unsung heroes of alignment
• Wing nuts, wing screws, and thumb screws — manual fastening done right
• T-slots, T-bolts, and T-nuts — fixturing essentials
• British and metric fastener systems — cross-referencing international standards
• Decision frameworks for choosing the right fastener every time

Foundations — What Is a Fastener, Really?

Before the practitioner's catastrophe, he — like most engineers — thought of fasteners as commodities. Bolts were bolts. Screws were screws. You picked the right diameter, tightened them until they felt right, and moved on.

That thinking nearly killed three people.

A fastener is a precision-engineered mechanical component designed to create a specific clamping force between assembled parts. Every fastener has a defined tensile strength, a proof load, a yield point, and a fatigue life. Ignore any one of these, and you're building a time bomb.

Bolt vs. Screw: The Real Definition

The industry-standard definitions from ANSI/ASME B18.2.1-1996 are precise and surprisingly counter-intuitive:

• A bolt is an externally threaded fastener designed for insertion through holes in assembled parts and is normally intended to be tightened or released by torquing a nut.
• A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head.

The key distinctions:

Shop-Floor Rule: If you hold the head and turn the nut, it's a bolt. If you turn the head into a tapped hole, it's a screw. If you're arguing about it at lunch, it's probably a cap screw — which can function as either.

Bolts, Screws, Nuts, and Washers — The Core Arsenal

the practitioner's first lesson to every new hire was always the same: "Master these four components and you control the integrity of every assembly in this building."

The American Square and Hexagon System

The modern standardization of bolts, screws, and nuts began with ASA B18.2 in 1941, which covered only head dimensions. Through successive revisions in 1952, 1955, and 1965, the standards expanded to cover the complete product, and several classifications were simplified through agreements with Britain and Canada.

By 1996 (ANSI/ASME B18.2.1-1996 for bolts and screws) and 1999 (ANSI/ASME B18.2.2-1987, R1999 for nuts), the unified system covered:

Bolts and Screws:

• Square Bolts
• Hex Bolts and Heavy Hex Bolts
• Hex Cap Screws and Heavy Hex Screws
• Heavy Hex Structural Bolts
• Square and Hex Lag Screws
• Round Head Square Neck Bolts (Carriage Bolts)
• T-Head Bolts
• Countersunk Bolts

Nuts:

• Hex Nuts and Hex Jam Nuts
• Heavy Hex Nuts and Heavy Hex Jam Nuts
• Heavy Hex Slotted Nuts
• Square Nuts
• Hex Flat Nuts and Hex Flat Jam Nuts
• Low and High Crown (Acorn) Nuts

Designation: How to Specify a Fastener Correctly

A properly designated fastener includes the following data in this exact sequence:

1. Nominal size (fractional and decimal equivalent)
2. Threads per inch (omit for lag screws)
3. Product length (fractional or two-place decimal equivalent)
4. Product name
5. Material, including specification where necessary
6. Protective finish, if required

Examples:

• 3/8-16 × 1-1/2 Square Bolt, Steel, Zinc Plated
• 1/2-13 × 3 Hex Cap Screw, SAE Grade 8 Steel
• .75 × 5.00 Hex Lag Screw, Steel
• 1/2-13 Square Nut, Steel, Zinc Plated
• 3/4-16 Heavy Hex Nut, SAE J995 Grade 5 Steel

Pro Tip: Items recognized as "unified" dimensionally with British and Canadian standards are specifically identified in ANSI standards — this matters when you're sourcing from international suppliers.

Square Bolts

Square bolts are the workhorses of rough structural and agricultural applications. Their four-sided head provides maximum wrench engagement and resistance to rounding.

Key Specifications (ANSI/ASME B18.2.1-1996):

Threads are Unified Coarse, Fine, or 8-thread series, Class 2A.

Hex Bolts and Heavy Hex Bolts

Hex bolts are the most widely used structural fastener in the world. The six-sided head provides excellent wrench engagement, and the hex geometry allows operation in tighter spaces than square bolts.

Heavy hex bolts have the same body diameter but larger heads (wider across flats), providing a greater bearing surface area — critical for structural applications where embedment into softer materials must be minimized.

Hex Cap Screws vs. Hex Bolts

Here's where many engineers get confused. A hex cap screw and a hex bolt share the same head geometry, but:

• Hex cap screws have a washer face (bearing surface) under the head, tighter tolerances, and are designed to be torqued by the head into a tapped hole
• Hex bolts are designed to be used with a nut

In 1965, the standards consolidated hexagon head cap screws and finished hexagon bolts into a single product, and heavy semifinished hexagon bolts with heavy finished hexagon bolts into another. This simplification eliminated considerable market confusion.

Lag Screws: The Heavy-Duty Wood Fastener

Lag screws (both square and hex head) are designed for heavy-duty fastening into wood or other soft materials. Unlike machine screws, lag screws have coarse, widely-spaced threads designed to cut their own mating thread in wood.

Key difference from wood screws: Lag screws require a wrench for installation (they have hex or square heads), while wood screws are driven with a screwdriver.

Nuts — The Other Half of the Equation

the practitioner's second-favorite saying: "A bolt without the right nut is just a piece of decorated steel."

Hex Nuts (Style 1 and Style 2)

The standard hex nut is the universal mating component for hex bolts. Two "styles" exist:

• Style 1: Standard thickness, adequate for most applications
• Style 2: Slightly thicker, provides greater stripping resistance for high-strength applications

Hex Jam Nuts

Jam nuts are thinner than standard hex nuts and are primarily used as locknuts. They're typically installed beneath a standard nut; the two nuts are then tightened against each other, creating a locking action through thread friction.

Heavy Hex Nuts

Heavy hex nuts have a larger width across flats than standard hex nuts for the same thread size. They're specified for:

• Structural connections requiring greater wrench engagement
• Applications where larger bearing surfaces reduce embedment
• Mating with heavy hex bolts and heavy hex structural bolts

Heavy Hex Slotted Nuts

These nuts have slots cut across the flats to accommodate a cotter pin, providing a positive locking mechanism for safety-critical applications. Common in automotive suspension, agricultural equipment, and any assembly subject to severe vibration.

Metric Nut Reference Data

Mechanical Properties of Nuts

Nuts must be matched to bolts by property class. Using a nut with insufficient proof load negates the bolt's strength capacity. The nut must be at least as strong as the bolt in the joint — otherwise, thread stripping in the nut becomes the failure mode.

Matching Rule: Always match nut property class to bolt property class. When in doubt, go one class higher on the nut.

Washers — Small Components, Massive Impact

Washers seem simple. They're flat. They go under bolt heads and nuts. How complicated can they be?

the practitioner would tell you about the time a missing washer on a single Grade 8 bolt in a crane assembly allowed the bolt head to embed 0.003 inches into the flange surface. That embedment caused a 15% preload loss. Under cyclic loading, the bolt backed off over six months, and the boom hinge developed play that cascaded into a catastrophic bearing failure.

Three thousandths of an inch. One washer. One crane.

Plain Washers — Type A and Type B

Type A Plain Washers (ANSI B18.22.1-1965, R1998):

• Broad tolerance, preferred for general applications
• Available in narrow, regular, and wide series

Type B Plain Washers (ANSI B18.22.1-1965, R1998):

• Tighter tolerance
• Available in narrow (N), regular (R), and wide (W) series
• Sizes from No. 0 (0.060 in) through 3 inches

Helical Spring Lock Washers

These washers have a single helical coil with a squared-off cross section. When compressed under bolt head or nut, they exert a spring force that maintains tension in the joint and resist loosening.

Three classes based on duty:

• Regular: Standard applications
• Heavy: High-vibration environments
• Extra Duty: Severe vibration or critical safety applications

Important: When carbon steel helical spring lock washers are to be hot-dipped galvanized for use with hot-dipped galvanized bolts, they must be coiled to limits 1/16 inch in excess of the standard minimum inside diameter and maximum outside diameter. Galvanizing washers under 1/4-inch nominal size is not recommended.

Tooth Lock Washers

Tooth lock washers serve to lock fasteners to the component parts of an assembly or increase the friction between the fasteners and the assembly. They come in two primary configurations:

• Internal tooth: Teeth point inward — provides a clean external appearance
• External tooth: Teeth point outward — maximum grip, especially on oversized holes
• Internal-external tooth: Teeth on both sides — maximum locking across the widest range of hole sizes

Metric Plain Washers (ANSI B18.22M-1981, R1990)

Metric washers are available in three series across nominal sizes from 1.6 mm to 36 mm:

Types of Metric Plain Washers:

• Soft (as fabricated): Available in 1.6 mm through 36 mm in various materials. Used in low-strength applications to distribute bearing load, provide a uniform bearing surface, and prevent marring of the work surface.
• Hardened: Available in 6 mm through 36 mm in narrow and regular series. Intended for high-strength joints to minimize embedment, provide a uniform bearing surface, and bridge large clearance holes and slots. Tempered to 38–45 HRC.

Designation Example: Plain washer, 10 mm, regular, hardened steel

Note: Metric washer outside diameters of 18.80/18.37 mm and 25.40/24.48 mm were specifically chosen to avoid washers that could be used in coin-operated devices. Standards engineers think of everything.

British Standard Metal Washers (BS 4320:1968)

British washers come in two categories:

Bright Metal Washers:

• Normal and large diameter series
• Normal thickness (Form A or C) and light thickness (Form B or D)
• Light-range washers are 1/2 to 2/3 the thickness of normal-range washers
• Reasonably flat and free from burrs, normally unchamfered

Black Metal Washers:

• Three size categories: normal (Form E), large (Form F), extra large (Form G)
• Made from mild steel
• Normal series: M5 to M68
• Large series: M8 to M39
• Extra large series: M5 to M39

Torque, Tension, and Preload — The Physics That Hold Everything Together

This is where the practitioner's bridge collapsed. This is the section that separates engineers who build things that last from engineers who build things that fail.

Why Preload Matters

Bolts are often tightened by applying torque to the head or nut, which causes the bolt to stretch. That stretching results in bolt tension or preload — the force that actually holds a joint together.

High preload tension does four critical things:

1. Keeps bolts tight — prevents loosening under vibration
2. Increases joint strength — keeps clamped members in compression
3. Creates friction between parts to resist shear
4. Improves fatigue resistance of bolted connections

The Preload Formula

The recommended preload $F_i$ for reusable and permanent connections:

$$F_i = 0.75 \times A_t \times S_p \quad \text{(reusable connections)}$$

$$F_i = 0.9 \times A_t \times S_p \quad \text{(permanent connections)}$$

Where:

• $F_i$ = bolt preload (lbf or N)
• $A_t$ = tensile stress area of the bolt
• $S_p$ = proof strength of the bolt material

For materials where proof strength is unknown, use:

$$S_p \approx 0.85 \times S_y$$

Where $S_y$ is the yield strength. Soft materials should never be used for threaded fasteners.

Measuring Preload: From Best to Worst

the practitioner's Rule: "If you're tightening structural bolts by feel, you're gambling with human lives at ±35%. That's not engineering — that's hope."

Bolt Elongation Formulas

The most accurate way to ensure correct preload (short of strain gages) is to measure bolt elongation directly:

Detailed formula (for bolts with both threaded and unthreaded sections in the grip):

$$\delta = \frac{F_i \times (A_d \times l_t + A_t \times l_d)}{A_d \times A_t \times E}$$

Simplified formula (for constant-area fasteners):

$$\delta = \frac{F_i \times l}{A \times E}$$

Where:

• $\delta$ = change in bolt length
• $F_i$ = bolt preload
• $A_d$ = major-diameter area of the bolt
• $A_t$ = tensile-stress area of the bolt
• $E$ = bolt modulus of elasticity
• $l_t$ = length of threaded portion within grip
• $l_d$ = length of unthreaded portion within grip
• $l$ = total bolt length
• $A$ = bolt cross-sectional area

The Torque-Tension Relationship

If measuring bolt elongation isn't possible, you must estimate the torque required:

$$T = K \times F_i \times d$$

Where:

• $T$ = wrench torque
• $K$ = torque coefficient (dimensionless)
• $F_i$ = desired preload
• $d$ = nominal bolt diameter

Values of K for steel bolts (1/4 to 1 inch range):

The Complete Torque Equation

For those who need precision, the total torque $T$ required to develop an axial bolt load $P_B$ is:

$$T = P_B \left[ \frac{l}{2\pi} + \frac{d_2 \mu_1}{2 \cos \alpha} + \frac{\mu_2 (d + b)}{4} \right]$$

Where:

• $l$ = thread lead (= pitch for single-start threads)
• $d_2$ = bolt pitch diameter
• $\mu_1$ = coefficient of friction between threads
• $\mu_2$ = coefficient of friction under nut/bolt head
• $\alpha$ = thread half-angle
• $d$ = nominal bolt diameter
• $b$ = pressure face diameter of nut or bolt head

For a fastener system with 60° threads ($\alpha = 30°$) where $d_2 \approx 0.92d$ and no loose washer is used ($b \approx 1.5d$):

$$T = P_B \left[ 0.159l + 0.531\mu_1 d + 0.625\mu_2 d \right]$$

And if thread and bearing friction coefficients are equal ($\mu_1 = \mu_2 = \mu$):

$$T = P_B (0.159l + 1.156\mu d)$$

Worked Example: Torque Calculation

Problem: Estimate the torque required to tighten a UNC 1/2-13 Grade 8 steel bolt to a preload equivalent to 55% of minimum tensile bolt strength. Assume unplated bolt with $\mu = 0.15$ for both thread and bearing friction.

Solution:

Minimum tensile strength for SAE Grade 8 = 150,000 psi

Tensile stress area (from thread geometry):

$$A_t = \frac{\pi}{4} \left( \frac{0.4500 + 0.4001}{2} \right)^2 = 0.1419 \text{ in}^2$$

Required preload:

$$P_B = 0.55 \times 150{,}000 \times 0.1419 = 11{,}707 \text{ lbf}$$

Applied torque:

$$T = 11{,}707 \left( \frac{0.159}{13} + 1.156 \times 0.15 \times 0.500 \right) = 1{,}158 \text{ lb·in} = 96.5 \text{ lb·ft}$$

Coefficients of Friction for Bolts and Nuts

Critical Note: "Dry threads" (indicated by "None added") assume residual machine oil from manufacturing. These values are not valid for threads cleaned to remove all lubrication — those threads will have much higher friction unless a plating or film is acting as lubricant.

Torque Coefficients K for Metric Hex Fasteners

For metric coarse-thread fasteners, the torque coefficient $K$ varies with friction conditions. Here is a representative selection:

Preload Relaxation: The Silent Killer

Even when preload is perfectly applied, it will decrease over time due to:

• Local yielding under nuts and bolt heads (high spots, rough surfaces, lack of squareness)
• Thread deformation as load redistributes over the threaded length
• Vibration causing micro-movement
• Temperature cycling including ambient changes
• Creep (significant at high temperatures, but present even at room temperature)

Rule of Thumb: Allow for approximately 10% preload loss when designing a joint.

Design Recommendation: Maintain a joint-length to bolt-diameter ratio of 4:1 or greater to improve resilience against local yielding. For example, a 1/4-inch bolt should clamp at least 1 inch of material. Use through bolts, far-side tapped holes, spacers, and washers to improve this ratio.

Preload for Shear-Loaded Joints

In joints where members slide, the preload must hold joint members in contact while the members transmit shear loads directly to the fasteners. In non-sliding joints, shear loads are transmitted by friction — and that friction comes almost entirely from preload.

Design Principle: Preload must be sufficient that the resulting friction force exceeds the applied shear force with an appropriate safety margin.

Grade Marks — Reading the Head of a Fastener

the practitioner always carried a pocket reference card with bolt grade markings. "The marks on the head tell you everything about what's inside," he'd say. "If you can't read them, you have no business specifying them."

SAE and ASTM Grade Identification

Bolts, screws, and other fasteners are marked on the head with a symbol that identifies the grade. The grade establishes the minimum mechanical properties the fastener must meet. Additionally, industrial fasteners must be stamped with a registered head mark identifying the manufacturer.

Approximate Tightening Torque Formula: $T = 10^{b + m \log d}$ (in ft-lb) where $d$ is the bolt diameter in inches and $b$, $m$ are grade-dependent constants. For SAE Grade 8: $b = 3.095$, $m = 2.983$.

Detecting Counterfeit Fasteners

This is the nightmare that keeps quality engineers awake. Counterfeit fasteners enter the supply chain through gray-market distributors and look identical to genuine products but fail catastrophically under load.

Red flags that suggest counterfeit fasteners:

• Grade marks that are unusually shallow, poorly defined, or asymmetric
• No manufacturer's head mark (required for industrial fasteners)
• Lot pricing dramatically below market
• Inconsistent hardness readings across samples from the same lot
• Plating that chips, flakes, or shows blistering

Verification protocol:

1. Visual inspection of grade marks under magnification
2. Hardness testing (portable Rockwell tester)
3. Tensile testing of sample specimens
4. Dimensional verification against ANSI standards
5. Certificate of compliance from a traceable manufacturer

Working Strength of Bolts

Experiments conducted at the source research institution to determine the initial stress due to tightening nuts showed that experienced mechanics tighten nuts with a pull roughly proportional to bolt diameter. It was also found that the stress from nut tightening was often sufficient to break a 1/2-inch bolt, but not larger sizes.

Conclusion: Bolts smaller than 5/8 inch (15.9 mm) should not be used for holding cylinder heads or other parts requiring a tight, packed joint.

The working strength formula for bolts in packed joints (where gasket elasticity exceeds bolt elasticity):

$$W = S_t \left( 0.55d^2 - 0.25d \right)$$

Where:

• $W$ = working strength (permissible load, lbf)
• $S_t$ = allowable working stress in tension (psi)
• $d$ = nominal bolt diameter (inches)

Example: Working strength of a 1-inch bolt in a packed joint with $S_t = 10{,}000$ psi:

$$W = 10{,}000 \times (0.55 \times 1^2 - 0.25 \times 1) = 10{,}000 \times 0.30 = 3{,}000 \text{ lbf}$$

Thread Engagement Length

If failure of a threaded assembly should occur, it's preferable for the screw to break rather than have either thread strip. The length of thread engagement must be sufficient to carry the full breaking load of the screw without stripping.

Stripping area formulas for external threads:

$$A_s = \pi n L_e K_{n,max} \left[ \frac{1}{2n} + 0.57735(E_{s,min} - K_{n,max}) \right]$$

Stripping area formulas for internal threads:

$$A_n = \pi n L_e D_{s,min} \left[ \frac{1}{2n} + 0.57735(D_{s,min} - E_{n,max}) \right]$$

Where:

• $n$ = threads per inch
• $L_e$ = length of engagement
• $K_{n,max}$ = maximum minor diameter of internal thread
• $E_{s,min}$ = minimum pitch diameter of external thread
• $D_{s,min}$ = minimum major diameter of external thread
• $E_{n,max}$ = maximum pitch diameter of internal thread

Lock Wire (Safety Wire) Procedures

For safety-critical applications (aerospace, heavy rotating equipment, etc.), bolts are secured with lock wire to prevent loosening. The rules are precise:

• Maximum 3 bolts may be tied together in a single wire run
• Bolt heads may only be tied when the female thread receiver is captive
• Pre-drilled nuts may be tied similarly, but must be heat-treated and factory drilled
• Lock wire must fill a minimum of 75% of the drilled hole
• Lock wire must be aircraft-quality stainless steel

Wire diameter selection:

Rule: The larger wire may be used in smaller bolts for convenience, but smaller wire must never be used in larger fastener sizes.

Metric Fasteners — The Global Standard

As the practitioner expanded his consulting practice internationally, he quickly learned that the metric fastener system — while based on different standards bodies — achieved remarkable interchangeability with its inch counterparts through careful coordination.

ANSI-ISO Harmonization

American National Standards for metric bolts, screws, and nuts were coordinated with comparable ISO Standards. The dimensional differences are few, relatively minor, and none affect functional interchangeability between ANSI and ISO fasteners.

The following functional characteristics agree between ANSI and ISO:

• Diameters and thread pitches
• Body diameters
• Widths across flats
• Bearing surface diameters
• Head heights
• Thread lengths and dimensions
• Nominal lengths

Metric Property Classes

Metric fasteners use a numerical designation system instead of grade marks:

Reading the designation: The first number × 100 = approximate tensile strength in MPa. The first number × the second number × 10 = approximate yield strength in MPa.

Example: Class 8.8 means:

• Tensile strength ≈ 8 × 100 = 800 MPa (minimum is 830)
• Yield strength ≈ 8 × 8 × 10 = 640 MPa (minimum is 660)

Metric Socket Head Cap Screws (ANSI/ASME B18.3.1M-1986)