Fastener FAQ

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What is a fastener?

A fastener is any hardware used to join two or more objects together. This includes bolts, screws, nuts, rivets, pins, washers, and anchors. Fasteners can create permanent joints (like rivets) or allow parts to be assembled and disassembled (like bolts and screws).

What are the main types of fasteners?

The main fastener types are: Bolts (used with nuts), Screws (thread into material or tapped holes), Nuts (mate with bolts/studs), Washers (distribute load), Rivets (permanent fastening), Anchors (attach to concrete/drywall), and Pins (alignment and retention).

What do the numbers on a bolt mean?

Bolt numbers indicate size and thread pitch. For inch bolts like "1/4-20 x 1", that's 1/4" diameter, 20 threads per inch, and 1" long. For metric bolts like "M8-1.25 x 20", it's 8mm diameter, 1.25mm thread pitch, and 20mm long. Head markings indicate strength grade.

What is the strongest type of fastener?

For common applications, Grade 8 bolts (150,000 PSI tensile strength) or Class 12.9 metric bolts are the strongest readily available. For extreme applications, alloy steel socket head cap screws and aerospace-grade fasteners offer even higher strengths.

Why are there so many types of screw heads?

Different head types serve different purposes: Flat heads sit flush with surfaces, pan heads are versatile for general use, hex heads allow high torque, and socket heads work in tight spaces. Drive types (Phillips, Torx, hex) vary in torque capacity and cam-out resistance.

What is thread pitch?

Thread pitch is the distance between threads. For inch fasteners, it's expressed as threads per inch (TPI) - like "20 TPI." For metric, it's the distance in millimeters between threads - like "1.25mm pitch." Coarse threads have fewer threads per inch; fine threads have more.

What is a machine screw?

A machine screw is a fastener with uniform machine-cut threads (not tapered like wood screws) designed to go into a nut or a tapped (pre-threaded) hole. They typically range from #0 to 1/2" diameter and come in various head styles.

What is a cap screw?

A cap screw is a finished fastener intended for use in a tapped hole without a nut. It has a larger, more finished head than a machine screw. Common types include hex cap screws, socket head cap screws, and button head cap screws.

What is a set screw?

A set screw is a headless screw threaded all the way to the end. It's used to secure a pulley, gear, or collar to a shaft. The screw threads into a tapped hole and the point presses against the shaft to prevent movement. Common point types include cup, cone, flat, and dog point.

What is a stud bolt?

A stud bolt (or stud) is a threaded rod without a head, threaded on both ends or the entire length. One end screws into a tapped hole semi-permanently, and a nut tightens on the other end. Studs are common in engines, flanges, and equipment that needs frequent disassembly.

What does "fully threaded" mean?

A fully threaded bolt has threads running from just under the head all the way to the tip, with no smooth shank section. This is useful when you need threads to engage through the entire grip length. Bolts with a smooth shank portion are called "partially threaded."

What is the shank of a bolt?

The shank is the smooth, unthreaded portion of a bolt between the head and the threads. The shank provides shear strength without the stress concentration of threads. In precision applications, the shank fits snugly in a hole for alignment.

What is a hex bolt?

A hex bolt has a six-sided (hexagonal) head that's driven with a wrench or socket. It's the most common bolt type for structural and mechanical applications. Hex bolts come in various grades, materials, and with either partial or full threading.

What is a carriage bolt?

A carriage bolt has a round, dome-shaped head with a square neck underneath. When installed in a square or round hole in wood, the square neck pulls into the material and prevents the bolt from spinning. You only need a wrench on the nut side, making it great for through-bolting wood.

What is a lag bolt?

A lag bolt (or lag screw) is a heavy-duty wood screw with a hex or square head. It has coarse, widely-spaced threads and a pointed tip designed to cut into wood. Lag bolts are used for heavy structural wood connections like attaching ledger boards or deck posts.

What is the difference between a bolt and a screw?

The traditional distinction: a bolt passes through a hole and fastens with a nut, while a screw threads directly into material or a tapped hole. However, the terms often overlap. Generally, bolts are tightened by turning the nut, and screws are tightened by turning the head.

Can you use a bolt without a nut?

Yes! When a bolt threads into a tapped (pre-threaded) hole, no nut is needed. This is common in machinery, engines, and anywhere you're fastening to a threaded component. When used this way, the bolt is technically functioning as a cap screw.

When should I use a bolt vs a screw?

Use a bolt with nut when joining two pieces that don't have threaded holes, when you need to clamp pieces together, or when disassembly might be needed. Use a screw when fastening to wood, plastic, or into tapped metal holes, or when you only have access to one side.

Which is stronger: a bolt or a screw?

A bolt with a properly matched nut is typically stronger because the load is distributed across more engaged threads. However, strength depends on the grade, material, and application. A Grade 8 bolt is stronger than a Grade 2 bolt, regardless of whether it uses a nut or threads into a hole.

What's the difference between a wood screw and a machine screw?

Wood screws have tapered shanks and coarse, aggressive threads designed to cut into wood. They're pointed and self-starting. Machine screws have straight shanks with uniform threads designed for nuts or tapped holes. They won't cut their own threads into wood.

What is a self-tapping screw?

A self-tapping screw creates its own mating threads as it's driven into a pre-drilled hole. Thread-forming types push material aside (for soft metals and plastics). Thread-cutting types have a cutting edge that removes material like a tap. Both eliminate the need for tapping holes.

What is a self-drilling screw?

A self-drilling screw (often called a TEK screw) has a drill-point tip that drills its own hole and then taps threads in one operation. No pre-drilling needed! They're common for attaching sheet metal to metal framing. The drill point size is matched to the screw size.

What is the difference between a lag screw and a lag bolt?

They're the same thing! "Lag screw" and "lag bolt" are used interchangeably. It's a large wood screw with a hex or square head. Technically "lag screw" is more accurate since it threads into wood rather than using a nut, but "lag bolt" is equally common.

Can I use a screw in place of a bolt?

Sometimes, but be careful. If replacing a bolt in a tapped hole, use a cap screw of the same size, grade, and length. Don't substitute a wood screw for a machine screw/bolt - the threads are completely different. Always match the strength grade for structural applications.

How do you measure bolt length?

For most bolts (hex, socket cap, pan head), measure from under the head to the tip. For countersunk heads (flat head, oval head), measure the total length including the head since the head sits below the surface. When in doubt, measure the part that goes into the hole.

How do you measure bolt diameter?

Measure the major diameter - the widest point across the threads (from thread peak to thread peak). Use calipers for accuracy. For inch bolts, this gives the nominal size (1/4", 3/8", etc.). For metric, it's the "M" number (M6, M8, etc.).

How do I determine what size bolt I need?

Consider: 1) Hole size - bolt diameter should match the hole or be slightly smaller, 2) Grip length - total thickness of material plus room for nut/threads, 3) Load requirements - heavier loads need larger diameter or higher grade. Add at least one full diameter of thread engagement.

What size drill bit for a 1/4" bolt?

For a clearance hole (bolt passes through freely): 9/32" or 17/64" drill. For a tap drill (to thread the hole for a 1/4-20 bolt): #7 drill (0.201"). For 1/4-28 fine thread: #3 drill (0.213"). Always verify tap drill sizes for your specific thread.

How do I measure thread pitch?

Use a thread pitch gauge - match the gauge teeth to the threads until you find a perfect fit. Alternatively, count the threads in one inch (for inch fasteners) or measure the distance between threads with calipers (for metric). Thread pitch gauges are inexpensive and very helpful.

What does M8 mean?

M8 is a metric bolt with an 8mm major (outside) diameter. The "M" stands for metric. If you see "M8-1.25", that's 8mm diameter with 1.25mm thread pitch (standard/coarse). "M8-1.0" would be 8mm with fine 1.0mm pitch.

What is the metric equivalent of a 1/4" bolt?

1/4" (0.250") is closest to M6 (6mm = 0.236"). M6 is slightly smaller. For a closer match, M7 exists but is rare. In practice, you can't interchange them - a 1/4" bolt won't fit an M6 nut because the threads are different (even if diameter were exact).

What do screw numbers like #6, #8, #10 mean?

These are gauge numbers for small inch-series screws. The formula is roughly: Diameter = (Gauge × 0.013) + 0.060 inches. So #6 = 0.138", #8 = 0.164", #10 = 0.190". The numbers are a legacy sizing system - higher numbers = larger diameter.

How do I know what size nut fits my bolt?

The nut must match both the diameter and thread pitch of the bolt. A 1/4-20 bolt needs a 1/4-20 nut. An M8-1.25 bolt needs an M8-1.25 nut. If you're unsure, try threading them together by hand - they should turn freely without forcing.

What size wrench for a 3/8" bolt?

A standard 3/8" hex bolt uses a 9/16" wrench. The wrench size is the measurement across the flats of the hex head - not the bolt diameter. Here's a quick reference: 1/4" bolt = 7/16" wrench, 5/16" = 1/2", 3/8" = 9/16", 1/2" = 3/4", 5/8" = 15/16".

How long should a bolt be?

Add up: material thickness + washer(s) + nut height + 1-3 threads extra. For tapped holes, the bolt should engage threads equal to at least 1× the bolt diameter (1.5× in aluminum or soft materials). Too short = weak connection. Too long = threads bottoming out.

What size hole for an M10 bolt?

For a clearance hole: 10.5mm (normal fit) or 11mm (loose fit). For a tap drill (M10-1.5 coarse): 8.5mm. For M10-1.25 fine thread: 8.8mm. Always check tap drill charts for the specific thread pitch you're using.

How do I identify an unknown bolt?

Measure: 1) Diameter with calipers, 2) Thread pitch with a gauge or by counting TPI, 3) Length, 4) Head style, and 5) Head markings for grade. Determine if it's inch or metric by trying standard thread gauges. Most hardware stores can help identify mystery fasteners.

What is the difference between SAE and metric?

SAE (Society of Automotive Engineers) refers to inch-based fasteners - measured in fractions of inches with threads per inch. Metric fasteners use millimeters for diameter and thread pitch. They're NOT interchangeable even when sizes seem close - the thread angles and pitches differ.

How much thread engagement is enough?

The rule of thumb is 1× bolt diameter minimum in steel (a 1/2" bolt needs at least 1/2" of thread engagement). Use 1.5× diameter in aluminum or cast iron, and 2× in plastics. More engagement is always better, up to about 1.5× diameter in steel.

What is the difference between coarse and fine thread?

Coarse thread (UNC) has fewer, larger threads per inch. It's the standard for most applications - easier to assemble, more tolerant of damage, and less likely to cross-thread. Fine thread (UNF) has more threads per inch, providing slightly higher strength and better resistance to vibration loosening, but is easier to strip.

When should I use fine thread vs coarse thread?

Use coarse thread for: general purpose, soft materials, quick assembly, and dirty/damaged conditions. Use fine thread for: thin-wall tubing, high-vibration applications, fine adjustment, and where maximum strength is critical. When in doubt, coarse is usually the better choice.

What does UNC mean on a bolt?

UNC stands for Unified National Coarse - the standard coarse thread series for inch fasteners in North America. A 3/8-16 UNC bolt has 16 threads per inch. UNF is Unified National Fine (finer pitch), and UNEF is extra fine.

What is the standard thread pitch for a 1/2" bolt?

For a 1/2" bolt: Coarse (UNC) = 13 TPI (1/2-13). Fine (UNF) = 20 TPI (1/2-20). Coarse is far more common. If a parts list just says "1/2 inch bolt" without specifying, it's almost always 1/2-13 coarse thread.

Can coarse and fine thread nuts interchange?

No! A coarse thread nut will not properly engage a fine thread bolt of the same diameter (and vice versa). They might start but will cross-thread and seize. Always match the exact thread pitch. Check markings or test by hand-threading - it should turn freely.

What is a left-hand thread?

Left-hand threads tighten counterclockwise (opposite of normal). They're used where rotation might loosen a standard right-hand thread - like left bicycle pedals, some gas fittings, and certain machinery. Left-hand fasteners are usually marked "LH" or have a groove cut around the hex head.

How do I know if a thread is metric or standard?

Try a thread pitch gauge - metric and inch pitches are different. Metric threads usually have pitch in even millimeters (1.0, 1.25, 1.5mm). Inch threads are often in TPI like 13, 16, 18, 20, 24, 32. Metric bolts often have "M" marked on the head. When in doubt, try threading a known nut.

What is thread class?

Thread class specifies the fit tolerance between mating threads. Classes 1A/1B are loose (easy assembly), 2A/2B are standard (most common), and 3A/3B are tight (precision). The "A" is for external threads (bolts), "B" for internal (nuts). Class 2A/2B is used for the vast majority of fasteners.

What is the difference between rolled and cut threads?

Rolled threads are formed by pressing the material between dies - no material is removed. This creates stronger threads due to unbroken grain flow and work hardening. Cut threads are machined, which interrupts the grain. Rolled threads are standard for production fasteners; cut threads are used for special sizes or materials.

What is NPT thread?

NPT (National Pipe Tapered) is a tapered thread used for pipe fittings. The threads get tighter as you screw in, creating a seal. NPT requires thread sealant tape or compound. NPTF (dryseal) threads seal without sealant. Don't confuse NPT with straight pipe threads (NPS) which don't seal by themselves.

What is thread engagement?

Thread engagement is the length of thread actually in contact between a bolt and nut (or tapped hole). Sufficient engagement is critical for strength - too little and threads can strip. Minimum engagement equals one bolt diameter in steel; 1.5× in softer metals.

What is the thread angle for standard bolts?

Both Unified (inch) and ISO metric threads use a 60° included angle between flanks. This is one reason metric and inch fasteners aren't interchangeable even when sizes are close - but thread pitch and diameter differences matter more. British Whitworth threads (now rare) use 55°.

What is an ACME thread?

ACME threads have a trapezoidal shape (29° angle) designed for power transmission and linear motion - like lead screws on lathes, vises, and jacks. They're stronger than square threads and easier to manufacture. ACME threads are for motion, not fastening.

What do bolt grades mean?

Bolt grade indicates strength. For inch bolts: Grade 2 = low-carbon steel (basic hardware), Grade 5 = medium-carbon, quenched & tempered (automotive, machinery), Grade 8 = alloy steel, higher strength (heavy equipment, critical applications). Higher grade = higher tensile and yield strength.

What is the difference between Grade 5 and Grade 8 bolts?

Grade 5: 120,000 PSI tensile, 92,000 PSI yield. Marked with 3 radial lines. Good for most automotive and mechanical work. Grade 8: 150,000 PSI tensile, 130,000 PSI yield. Marked with 6 radial lines. About 25% stronger, but also more brittle. Use Grade 8 only when needed.

What does 8.8 mean on a metric bolt?

The numbers indicate strength class. First number × 100 = minimum tensile strength in MPa. First × second × 10 = minimum yield strength in MPa. So 8.8 means: 800 MPa tensile, 640 MPa yield. Class 8.8 is roughly equivalent to SAE Grade 5. Class 10.9 ≈ Grade 8.

What grade bolt should I use?

Grade 2/Class 4.6: Light duty, non-critical. Grade 5/Class 8.8: Most automotive, machinery, structural. Grade 8/Class 10.9: Heavy equipment, suspension, high-stress joints. Class 12.9: Socket head cap screws, extreme loads. Match the grade to what you're replacing or use engineering guidelines.

What are the markings on bolt heads?

Inch bolts: No marks = Grade 2, 3 radial lines = Grade 5, 6 radial lines = Grade 8. Metric bolts: Number stamped on head (8.8, 10.9, 12.9). Stainless: Often marked "18-8", "316", "A2-70", or "A4-80". Manufacturer marks may also appear.

What is tensile strength?

Tensile strength is the maximum stress a bolt can withstand while being pulled before it breaks. It's measured in PSI (pounds per square inch) or MPa (megapascals). A Grade 8 bolt has 150,000 PSI tensile strength - meaning it takes 150,000 pounds of force per square inch of cross-section to break it.

What is yield strength?

Yield strength is the stress at which a bolt starts to permanently deform (stretch and not return). It's always lower than tensile strength. Bolts should never be loaded past yield strength - this is the practical limit for design. A Grade 5 bolt yields at about 92,000 PSI.

What is proof load?

Proof load is the maximum load a bolt can handle without permanent deformation - typically about 85-93% of yield strength. It's a quality test: fasteners must survive proof load testing without stretching. Manufacturers test samples from each lot to verify quality.

What is shear strength?

Shear strength is resistance to forces pushing sideways across the bolt (perpendicular to its length). For steel bolts, shear strength is roughly 60% of tensile strength. A joint where bolts carry shear loads (like a clevis or lap joint) needs this calculation - not tensile strength.

Is a higher grade bolt always better?

Not always! Higher grade bolts are stronger but also more brittle - they can snap without warning under shock loads. They're more susceptible to hydrogen embrittlement when plated. Grade 8 bolts should only be used where truly needed. Grade 5 is often the better all-around choice.

Can I replace a Grade 5 bolt with Grade 8?

Usually yes for strength purposes, but consider: Grade 8 is more brittle and costs more. In some cases (like suspension), engineers specifically chose Grade 5 because it will stretch before breaking, giving warning. Never go DOWN in grade for safety-critical applications.

What is Class 12.9?

Class 12.9 is the highest common metric strength grade - 1200 MPa (174,000 PSI) tensile, 1080 MPa yield. It's primarily used for socket head cap screws where maximum strength in a compact package is needed. Equivalent to (slightly stronger than) SAE Grade 8.

What is A2-70 stainless?

A2 designates 304/18-8 stainless steel (the alloy type). 70 indicates 700 MPa (~100,000 PSI) tensile strength (the property class). A2-70 is equivalent to "18-8" in the US. A2-80 is the same alloy cold-worked to higher strength (800 MPa).

What is A4-80 stainless?

A4 designates 316 stainless steel (better marine corrosion resistance than A2). 80 means 800 MPa tensile strength. A4-80 is the go-to for marine applications where you need both corrosion resistance and decent strength.

What is a structural bolt?

Structural bolts are heavy-duty bolts designed for steel construction. They're larger diameter (usually 3/4" and up), have short thread lengths, and use heavy hex heads and nuts. Common grades: A325 (equivalent to Grade 5) and A490 (higher strength). They're governed by AISC and RCSC specifications.

Can I use Grade 8 bolts on my car?

It depends on the application. Grade 8 is fine for many automotive uses, but some components (like suspension) may specify Grade 5 intentionally - it's more ductile and gives warning before failure. Also, Grade 8 shouldn't be used where galvanized bolts are specified (can't hot-dip galvanize Grade 8). Match the OEM spec when possible.

What materials are bolts made from?

Most common: Carbon steel (various grades), stainless steel (304, 316), alloy steel (Grade 8, socket caps). Also available: brass, bronze, aluminum, titanium, nylon, and specialty alloys like Monel and Inconel. Material choice depends on strength needs, corrosion environment, and cost.

What is the difference between stainless steel and zinc plated bolts?

Stainless steel is inherently corrosion-resistant throughout the material - it can't "wear off." Zinc plating is a thin coating on carbon steel - once scratched or worn through, the steel underneath will rust. Stainless costs more but lasts longer in corrosive environments. Zinc plating is economical for indoor or mild outdoor use.

What is the best material for outdoor fasteners?

For general outdoor use: hot-dip galvanized or 304 stainless steel. For coastal/marine: 316 stainless or silicon bronze. For pressure-treated wood: hot-dip galvanized (G185) or stainless. Plain zinc plating is NOT adequate for outdoor exposure - it's too thin.

What fasteners should I use with pressure-treated wood?

Pressure-treated wood (especially ACQ treatment) is highly corrosive to regular fasteners. Use hot-dip galvanized (G185 coating minimum), stainless steel, or fasteners labeled "ACQ approved." Standard zinc plating will corrode quickly. This is a code requirement for deck construction.

How strong is brass compared to steel?

Brass is significantly weaker than steel - roughly 1/3 to 1/2 the tensile strength. Brass fasteners are used for corrosion resistance, electrical conductivity, non-sparking properties, and appearance - not strength. Never substitute brass for steel in structural applications.

Can I use aluminum bolts?

Yes, but carefully. Aluminum bolts are lightweight and corrosion-resistant, but much weaker than steel (about 1/3 the strength). They're used in aerospace, marine, and weight-critical applications. Watch for galvanic corrosion when used with dissimilar metals. Never use for high-stress or high-temperature applications.

What is silicon bronze?

Silicon bronze is a copper alloy (about 96% copper) with excellent corrosion resistance, especially in saltwater. It's the traditional choice for wooden boat building and marine hardware. Silicon bronze is stronger than brass but still weaker than steel. It's expensive but very durable in marine environments.

What is galvanic corrosion?

When two dissimilar metals touch in the presence of moisture, an electrochemical reaction causes the more "active" metal to corrode faster. For example, aluminum corrodes rapidly when in contact with steel. Use compatible metals, insulating washers, or coatings to prevent this. Stainless and aluminum together is particularly bad.

Can I use stainless steel fasteners with aluminum?

Use caution. Stainless and aluminum have very different positions on the galvanic scale - the aluminum will corrode, especially in wet conditions. If you must combine them: use a barrier (paint, plastic washer), keep the joint dry, or use aluminum fasteners instead. In marine environments, avoid this combination entirely.

What is alloy steel?

Alloy steel contains additional elements (chromium, molybdenum, nickel, etc.) beyond basic carbon steel to improve properties like strength, hardness, or toughness. Grade 8 bolts and socket head cap screws are alloy steel. Alloy steel fasteners are heat-treated to achieve high strength but usually require plating for corrosion resistance.

What fasteners won't rust?

Truly rust-proof options: stainless steel (304, 316), brass, bronze, aluminum, titanium, and plastic/nylon. Coated fasteners (galvanized, zinc plated) resist rust but will eventually corrode when the coating wears. For permanent outdoor exposure, stainless is the most practical choice.

What is Monel?

Monel is a nickel-copper alloy (about 67% nickel, 30% copper) with exceptional corrosion resistance, especially to saltwater and acids. It's stronger than stainless steel and maintains strength at high temperatures. Monel fasteners are expensive but used in marine, chemical processing, and aerospace applications where nothing else will survive.

Are nylon screws strong?

Nylon screws are much weaker than metal - about 10,000-12,000 PSI tensile vs. 60,000+ for steel. They're used where strength isn't critical but properties like electrical insulation, chemical resistance, non-marring, or non-magnetic are needed. Common in electronics, plastics assembly, and chemical equipment.

What is 18-8 stainless steel?

18-8 refers to the approximate composition: 18% chromium, 8% nickel. It's the same as 304 stainless or A2 (European designation). This is the most common stainless steel for fasteners - good corrosion resistance, non-magnetic (usually), and economical. Fine for most applications except marine.

What is the difference between 304 and 316 stainless steel?

316 stainless contains 2-3% molybdenum, giving it superior resistance to chlorides (saltwater, road salt, bleach). Use 304/18-8 for general corrosion resistance; use 316 for marine, coastal, pool/spa, and chemical environments. 316 costs about 20-30% more than 304.

Can stainless steel rust?

Yes, stainless steel can corrode under certain conditions - it's stain-LESS, not stain-PROOF. Chlorides (salt), acids, lack of oxygen (crevice corrosion), and contact with carbon steel can cause corrosion. 316 resists these better than 304. Proper grade selection for the environment is key.

Is stainless steel stronger than regular steel?

Generally no. Common 18-8 stainless (A2-70) has about 100,000 PSI tensile - similar to Grade 2 or low Grade 5 steel. A4-80 stainless reaches about 116,000 PSI. Grade 8 steel is 150,000 PSI. Stainless is chosen for corrosion resistance, not strength. For high-strength stainless, specialty grades exist but are expensive.

Why do stainless steel bolts gall and seize?

Stainless steel is prone to galling - microscopic welding between threads from friction and heat during tightening. The oxide layer that prevents corrosion also contributes to galling. Prevent it by: using anti-seize lubricant, tightening slowly, using different alloys for bolt vs. nut, or using coated stainless fasteners.

Should I use anti-seize on stainless steel?

Yes! Anti-seize is highly recommended for stainless steel fasteners to prevent galling and make future disassembly possible. Use nickel-based or copper-based anti-seize. Apply sparingly to threads. Note: anti-seize reduces friction, so reduce torque values by about 20-25% when using it.

Is stainless steel magnetic?

It depends on the type. 304 and 316 (austenitic) are generally non-magnetic or only slightly magnetic. Cold working can make them somewhat magnetic. 410 stainless (martensitic) IS magnetic. You can't reliably identify stainless by magnet test alone - some stainless is magnetic, and some plated steel isn't strongly magnetic.

What stainless steel should I use for saltwater/marine?

316 stainless steel minimum for marine applications. The molybdenum content resists chloride corrosion. For underwater or extreme conditions, consider 316L (low carbon, better weld corrosion resistance) or specialty grades like 317L, duplex stainless, or even Monel. 304/18-8 will eventually corrode in saltwater.

What is 410 stainless steel?

410 is a martensitic stainless steel - it can be hardened by heat treatment, unlike 304/316. It's magnetic and has less corrosion resistance than 304 but much higher strength and hardness. 410 is used for self-drilling screws, knife blades, and applications needing hardness. Not suitable for marine use.

What is passivation?

Passivation is a chemical treatment that removes surface iron contamination and enhances the protective chromium oxide layer on stainless steel. This improves corrosion resistance. Stainless naturally forms this layer, but passivation creates a more uniform, complete layer. It's recommended after machining or welding.

Can I use stainless steel with galvanized steel?

Yes, this is usually fine. Stainless and zinc (galvanizing) are relatively close on the galvanic scale, so corrosion is minimal. The zinc may sacrifice slowly to protect the stainless, but the effect is mild. This combination is common and generally acceptable, especially in non-marine environments.

What is the difference between zinc plated and galvanized?

Both use zinc for corrosion protection, but thickness differs greatly. Zinc plating (electroplated) is thin (0.0002"-0.0005") - good for indoor/mild outdoor use. Hot-dip galvanized is thick (0.002"-0.005") - 10× more zinc provides much longer outdoor protection.

What is zinc plating?

Zinc plating is an electrochemical process that deposits a thin layer of zinc onto steel. It provides basic corrosion resistance and improved appearance. Available in clear (bluish), yellow (gold iridescent), and black finishes. Good for indoor use and mild outdoor exposure. Also called electro-galvanized.

What is hot-dip galvanizing?

Hot-dip galvanizing involves dipping steel into molten zinc (about 840°F), creating a thick, metallurgically-bonded coating. The result is a dull gray finish with much better corrosion protection than zinc plating. Required for outdoor structural work, utility poles, and anywhere long-term outdoor exposure is expected.

What is black oxide finish?

Black oxide is a chemical conversion coating that turns steel surfaces black. It provides minimal corrosion protection on its own - usually requires oil for rust resistance. Benefits: attractive appearance, no dimensional change, reduced glare, and mild abrasion resistance. Common on socket head cap screws and tools.

What is the difference between yellow zinc and clear zinc?

Both are zinc electroplating with a chromate conversion coating. Clear zinc has a bright, bluish-silver appearance. Yellow zinc (gold iridescent) traditionally indicated thicker chromate with better corrosion resistance, though modern clear zinc can match it. The color is mainly cosmetic now - both provide similar protection.

What is phosphate coating?

Phosphate coating (parkerizing) creates a gray or black crystalline surface that absorbs oil and improves paint adhesion. By itself, it offers minimal corrosion protection. Combined with oil, it provides moderate protection. Common on drywall screws, military firearms, and as a primer base. Also aids break-in of threaded fasteners.

What is Dacromet or Geomet coating?

These are high-performance coatings using zinc and aluminum flakes in an inorganic binder. They provide excellent corrosion resistance (often 500-1000+ hours salt spray), no hydrogen embrittlement risk, and consistent torque characteristics. Used in automotive, construction, and where zinc plating isn't enough. More expensive than standard plating.

What is hydrogen embrittlement?

Hydrogen embrittlement occurs when hydrogen atoms enter high-strength steel during plating or cleaning, causing brittleness and sudden failure under load. It mainly affects hardened fasteners (Grade 8, Class 10.9+). Prevented by proper baking after plating. Hot-dip galvanizing and mechanical plating avoid this issue.

What is the most corrosion resistant coating?

For coated steel: hot-dip galvanizing provides the most zinc and longest life. Dacromet/Geomet can exceed it in salt spray tests. Ceramic coatings and fluoropolymer (Xylan) offer extreme protection. For ultimate corrosion resistance, use inherently corrosion-resistant materials like stainless steel, bronze, or titanium instead of coatings.

Does galvanizing affect bolt strength?

The galvanizing process itself doesn't significantly reduce strength. However, hot-dip galvanizing Grade 8 or Class 10.9+ bolts is not recommended - the high heat can temper the steel and reduce hardness. Galvanized structural bolts are typically A325 (similar to Grade 5). For high-strength + corrosion resistance, consider mechanical galvanizing or stainless.

Will galvanized nuts fit on galvanized bolts?

Hot-dip galvanizing adds significant thickness, so nuts are tapped oversize after galvanizing to fit. A regular nut won't fit a hot-dip galvanized bolt. Always use nuts and bolts from the same source or verify fit. Zinc plated fasteners don't have this issue - the coating is thin enough that standard tolerances work.

What is plain finish?

Plain finish (also called "plain," "self-color," or "black") means no protective coating - just bare steel, possibly with oil. It provides no corrosion protection and will rust quickly if exposed to moisture. Plain finish is used for indoor applications, or where parts will be painted or plated later.

Why do zinc plated bolts come in different colors?

The color comes from the chromate conversion coating applied after zinc plating. Clear/blue, yellow/gold, and black are common. Historically, thicker chromate (yellow) meant better protection. Modern RoHS-compliant chromates can achieve similar protection in any color. The zinc layer underneath is the same regardless of color.

What are the best screws for wood?

For general woodworking: wood screws with coarse threads and sharp points. For decks: deck screws (coated or stainless). For cabinets: cabinet screws or pocket hole screws. For heavy structural: structural screws or lag screws. Match the screw to the application - indoor/outdoor, hardwood/softwood, and load requirements.

Do I need to pre-drill holes for wood screws?

For hardwoods: Yes, always. Oak, maple, and similar woods will split without pilot holes. For softwoods: Often optional for smaller screws, but recommended near edges or ends. For hardwood plywood and MDF: Yes. Pre-drilling prevents splitting and makes driving easier. When in doubt, drill a pilot hole.

What size pilot hole for wood screws?

Pilot hole should be about 70% of the screw's root diameter (the solid center, not counting threads). For softwood, go smaller; for hardwood, closer to root diameter. Quick guide: #6 screw ≈ 3/32" pilot, #8 ≈ 7/64", #10 ≈ 1/8". For the top piece, drill a clearance hole the full screw diameter so it pulls down tight.

What is the difference between wood screws and drywall screws?

Drywall screws are designed for drywall-to-stud attachment. They're hardened (brittle) with fine threads and bugle heads. Don't use them for structural wood projects - they can snap under load. Wood screws have coarser threads optimized for wood, flat or oval heads, and are less brittle. Use the right screw for the job.

What are deck screws?

Deck screws are specially designed for outdoor deck construction. They feature: corrosion-resistant coating (ceramic, ACQ-compatible), bugle or flat head (sits flush), coarse threads with cutting tips, and often star or square drive for better torque. Most are rated for use with pressure-treated lumber.

What is a lag screw used for?

Lag screws (lag bolts) are heavy-duty wood fasteners for structural connections: attaching ledger boards to houses, securing deck posts, mounting heavy equipment, and joining large timbers. They have hex heads for wrench driving and require pilot holes. Always check span tables or engineering requirements for structural applications.

What size pilot hole for lag screws?

Lag screws need two holes: a clearance hole through the top piece (full diameter) and a pilot hole in the receiving piece. Pilot hole should be about 60-75% of lag shank diameter. For a 3/8" lag: clearance hole = 3/8", pilot hole ≈ 1/4" (15/64" to 9/32"). Always verify for specific wood species.

What are structural screws?

Structural screws (like GRK, SPAX, or Simpson Strong-Tie) are engineered wood fasteners that can replace lag screws and bolts in many applications. They feature: high-strength steel, self-drilling tips (no pilot hole needed), code-approved load ratings, and often star drives. Faster to install than lags with comparable or better strength.

What are trim head screws?

Trim head screws have small, low-profile heads that are less visible than standard screw heads. They're used for finish carpentry - trim, molding, cabinetry - where appearance matters. The small head can be set just below the surface and filled. Also called "finish screws" or "trim screws."

What screws work best in plywood?

For plywood, use screws with coarse threads and sharp points. Standard wood screws work well. For edge screwing into plywood (which tends to split), use special plywood screws with finer threads, or pre-drill. Cabinet screws with washer heads distribute load and prevent pull-through. Avoid drywall screws - they're too brittle.

What screws should I use for pressure-treated wood?

Use fasteners rated for ACQ treatment: hot-dip galvanized (G185 minimum), stainless steel, or fasteners specifically labeled "ACQ approved" or "PT compatible." Standard zinc plating WILL corrode and fail. This is a building code requirement for decks. Polymer-coated deck screws are also a good option.

What is a hanger bolt?

A hanger bolt has wood screw threads on one end and machine threads on the other. The wood screw end goes into wood; a nut threads onto the machine end. Used for attaching legs to tables, hanging plants, and anywhere you need a threaded stud projecting from wood. Install by locking two nuts together to drive it.

Why do screws split wood?

Screws split wood when they displace too much material. Causes include: no pilot hole (especially in hardwood), too close to edge, wrong screw type, or driving into end grain. Solutions: always pre-drill in hardwood, stay at least 2× screw diameter from edges, and use properly sized pilot holes.

What are pocket hole screws?

Pocket hole screws are specialty screws designed for pocket hole joinery (like Kreg systems). They have: self-drilling tips, flat-bottomed heads that seat properly in the angled pocket hole, and coarse or fine threads depending on material. They create strong, hidden joints for cabinets, furniture, and face frames.

What is the best anchor for concrete?

It depends on the load and application. Wedge anchors are excellent for heavy loads in solid concrete. Sleeve anchors work in concrete and block. Tapcon screws are great for medium loads with easy installation. Drop-in anchors provide flush female threads. For maximum strength, epoxy anchors are unbeatable.

How does a wedge anchor work?

A wedge anchor has a threaded bolt body with a clip around the bottom. When tightened, the bolt pulls up while the clip stays in place, wedging against the hole walls. This creates tremendous holding power in solid concrete. Wedge anchors are NOT removable and only work in solid concrete (not hollow block).

What is a Tapcon screw?

Tapcon is a brand name that's become generic for concrete screws. They have special threads that cut into a pre-drilled hole in concrete or masonry. Benefits: fast installation, removable and reusable, no expansion force on the concrete. Drill the right size hole (usually marked on the package), blow out dust, and drive the screw.

What size hole for Tapcon screws?

Critical: use the exact drill bit size specified! For 3/16" Tapcons: 5/32" hole. For 1/4" Tapcons: 3/16" hole. Use a carbide-tipped masonry bit and hammer drill. Hole depth should be at least 1/4" deeper than screw penetration. Clean dust from hole before driving.

How deep should concrete anchors be?

General rule: anchor embedment should be at least 4-5 times the anchor diameter. A 1/2" wedge anchor needs at least 2-2.5" embedment. Always follow manufacturer specifications. Also maintain minimum edge distance (typically 5-10× diameter from edge) and spacing (typically 10× diameter between anchors).

Can you use regular bolts in concrete?

Not without anchors. Regular bolts have no way to grip concrete. You need either: 1) Cast-in-place anchors set when concrete is poured, 2) Post-installed anchors (wedge, sleeve, drop-in) that expand in drilled holes, 3) Concrete screws that thread into the hole, or 4) Epoxy anchors with threaded rod.

What is an epoxy anchor?

Epoxy anchors use a two-part adhesive to bond threaded rod or rebar into a drilled hole. The epoxy fills the hole, cures, and creates an extremely strong bond. Benefits: highest load capacity, works in cracked concrete, no expansion stress. Drawbacks: longer cure time, temperature sensitive, more expensive. Used for critical structural connections.

What is a drop-in anchor?

A drop-in anchor is a female threaded insert that's set flush with the concrete surface. You drill a hole, drop in the anchor, and set it with a setting tool that expands the bottom. Then any standard bolt can thread into it. Great for overhead applications and where a flush surface is needed.

What anchors work in hollow concrete block?

Sleeve anchors work well - they expand against the block wall. Toggle bolts can grip the back of hollow cells. Tapcon screws work in the solid webs. Wedge anchors do NOT work in hollow block - they need solid material to wedge against. For heavy loads in block, fill the cells with grout and use standard anchors.

What anchors work in brick?

For solid brick: sleeve anchors, Tapcon screws, and lead or plastic anchors all work. Drill into the brick (not mortar) for best strength. Avoid wedge anchors - brick can crack. For hollow brick, use toggle bolts or anchors designed for hollow material. Mortar joints are weaker but may be preferred for aesthetics (easier to repair).

How much weight can a concrete anchor hold?

It varies enormously by anchor type, size, and concrete strength. A 3/8" wedge anchor in 3000 PSI concrete might hold 2,000-3,000 lbs in tension. A 1/4" Tapcon might hold 500-1,000 lbs. Always check manufacturer load tables and apply appropriate safety factors (typically 4:1 for static loads). Concrete strength matters a lot.

Can concrete anchors be removed?

Tapcons: Yes, unscrew and reuse hole (with longer screw). Sleeve anchors: Remove bolt, pound sleeve deeper, patch. Wedge anchors: No - grind flush or drill out. Drop-in anchors: No - drill out or drive deeper. Epoxy anchors: No - cut off and patch. Plan ahead if removal might be needed.

What is a sheet metal screw?

Sheet metal screws have sharp threads that run to the tip, designed to cut into thin metal (and also work in plastic and wood). Unlike wood screws, the threads are consistent along the entire length. They come in various head styles (pan, flat, hex washer, truss) and point types (sharp, self-drilling).

What is a self-drilling screw (TEK screw)?

Self-drilling screws have a drill-point tip that drills its own hole and then taps threads in one operation. No pre-drilling needed! The "TEK" number (TEK 2, TEK 3, TEK 5) indicates drilling capacity - higher numbers drill thicker metal. TEK 3 handles up to about 0.210" (like two layers of 12 gauge). Common for HVAC, roofing, and steel framing.

What is the difference between TEK 3 and TEK 5 screws?

The number indicates drill point length and capacity. TEK 3: shorter point, drills up to ~0.210" (about 12 gauge). TEK 5: longer point, drills up to ~0.500" (1/2" steel). Match the TEK number to your material thickness - using too long a point on thin material causes problems.

Do sheet metal screws need pilot holes?

Self-drilling (TEK) screws: No - that's the point. Standard sharp-point sheet metal screws: Usually yes for metal, but depends on thickness. Very thin sheet metal (26+ gauge) often doesn't need pilots. Thicker material benefits from a pilot hole slightly smaller than the screw diameter. Always pilot for stainless.

What are thread-forming vs thread-cutting screws?

Thread-forming screws displace material to create threads (no chips). Good for soft metals, plastics. Thread-cutting screws have a cutting edge that removes material like a tap. Better for harder materials and blind holes where chips can escape. Both are types of self-tapping screws.

What screws should I use for aluminum sheet?

Use thread-forming screws - aluminum is soft enough that threads form well without cutting. For corrosion compatibility, use aluminum, stainless steel, or coated screws. Avoid plain steel (galvanic corrosion). Self-drilling screws work but thread-forming usually provides better holding power in aluminum.

What is a rivet nut (nutsert)?

A rivet nut (also called nutsert, rivnut) is a blind threaded insert. It's installed from one side - insert into a hole, then a tool pulls the mandrel, collapsing the body against the back of the sheet. This creates permanent female threads in thin material. Great for adding threads to sheet metal, plastic, or fiberglass.

What is a PEM nut (self-clinching fastener)?

PEM is a brand name for self-clinching fasteners - nuts, studs, and standoffs that are permanently pressed into sheet metal. A hydraulic or arbor press forces the fastener into a prepared hole; a knurled collar locks it in place. Used extensively in electronics, enclosures, and anywhere permanent threads in sheet metal are needed.

What is a pop rivet?

A pop rivet (blind rivet) is a permanent fastener installed from one side. Insert the rivet, squeeze the tool, and the mandrel pulls through, expanding the back of the rivet body. The mandrel breaks off, leaving a secure joint. Fast, no access to backside needed. Common for sheet metal, plastic, and composite assembly.

How do I choose the right rivet size?

Diameter: Match to hole size and strength needs. Grip range: Critical - must accommodate total material thickness. Rivet is too short if it can't form properly, too long if it mushrooms excessively. Check manufacturer grip range charts. Material: Aluminum rivets for aluminum, steel for steel (watch for galvanic issues).

What are roofing screws?

Roofing screws are self-drilling screws with built-in rubber/EPDM washers that seal the hole against water. They're designed for attaching metal roofing and siding to wood or steel purlins. Features: hex washer head, self-drilling point, and a bonded washer that compresses to create a watertight seal.

What are the different types of nuts?

Common types: Hex nuts (standard), lock nuts (resist loosening), flange nuts (built-in washer), cap/acorn nuts (decorative cover), wing nuts (hand tightening), coupling nuts (join threaded rods), T-nuts (for wood), and square nuts (anti-rotation in channels).

What is a lock nut?

Lock nuts resist loosening from vibration. Types include: Nylon insert (Nylock) - plastic ring creates friction; all-metal prevailing torque - deformed threads create friction; serrated flange - teeth bite into surface; and castle nut with cotter pin - positive mechanical lock. Each has different reuse and temperature limits.

Can nylon lock nuts be reused?

Limited reuse. The nylon insert wears with each installation. For critical applications: use once only. For non-critical: maybe 2-3 times if prevailing torque is still present. Test by hand - if it spins freely on the threads, it's worn out. When in doubt, use a new one. All-metal lock nuts can be reused more often.

What is a flange nut?

A flange nut has a wide, built-in washer (flange) at the base. Benefits: distributes load over larger area, can't lose the washer, faster assembly. Serrated flange nuts have teeth on the flange that bite into the surface to resist loosening. Common in automotive and machinery applications.

What is a jam nut?

A jam nut is a thin hex nut (about half the height of a standard nut) used in combination with a regular nut to lock the assembly. Tighten the jam nut against the regular nut, and they lock against each other. Also used for adjustment where full nut height isn't needed.

Do I need to use a washer?

Use washers when: fastening to soft material (prevent pull-through), hole is oversized, you need to protect the surface, head/nut doesn't sit flat, or using lock washers for vibration resistance. In many structural applications, hardened washers are required under both head and nut to prevent embedment.

What is the difference between SAE and USS washers?

SAE washers have smaller outside diameter and are thinner - used where space is limited or appearance matters. USS washers are larger diameter and thicker - better load distribution, used for larger clearance holes. USS is more common for structural work; SAE for machinery and automotive.

Do lock washers actually work?

Split lock washers are largely ineffective - studies show they don't prevent loosening better than a plain washer with proper torque. Nord-Lock wedge-locking washers and serrated/tooth washers do work well. For critical applications, use proper torque, thread locker, or positive locking (castle nut + cotter pin).

What is a fender washer?

A fender washer has a small hole and large outside diameter - the OD is typically 3× the ID or more. This spreads load over a wide area, making them useful for soft materials, oversized holes, or anywhere you need maximum bearing surface. Originally used for automotive fender attachment (hence the name).

When do I need hardened washers?

Use hardened washers with high-strength bolts (Grade 8, Class 10.9+) to prevent the washer from dishing or the head/nut from embedding into soft material. Also required in structural steel connections (F436 washers). Standard washers will deform under high preload, reducing clamping force.

What are spring washers (Belleville washers)?

Belleville washers are conical spring washers that flatten under load and maintain tension. They compensate for thermal expansion, settling, and relaxation. Can be stacked in series (more deflection) or parallel (more force). Used in precision applications, valves, and thermal cycling environments. Much more effective than split lock washers.

Does the washer go under the nut or bolt head?

Put the washer under the part that turns - usually the nut. The washer protects the surface from rotation damage. For critical applications, use washers on BOTH sides. If only one washer, place it where surface protection is most needed or where you'll be making adjustments.

What is a T-nut?

A T-nut is a threaded insert with prongs that hammer into wood from the back side. The prongs prevent spinning; a machine screw or bolt threads in from the front. Used for jigs, fixtures, furniture assembly, and anywhere you need strong, removable bolt threads in wood. Also called tee nuts or blind nuts.

What is torque?

Torque is rotational force - how hard you're twisting the fastener. It's measured in foot-pounds (ft-lbs), inch-pounds (in-lbs), or Newton-meters (Nm). Proper torque creates the right clamping force (preload) without overstressing the bolt. Too little = loose joint. Too much = stretched/broken bolt.

Do I need a torque wrench?

For critical applications: Yes. Cylinder heads, wheel lugs, suspension components, and anything where proper preload matters needs measured torque. For general assembly and non-critical work, "feel" and experience are often adequate. When in doubt, use a torque wrench - they're inexpensive insurance against problems.

What happens if you over-torque a bolt?

Over-torquing can: stretch the bolt past yield (permanent elongation), strip threads, break the bolt during or after installation, crack the material being fastened, or create stress concentrations that lead to fatigue failure. High-strength bolts are designed to stretch - but only within limits.

What is the torque specification for bolts?

Torque varies by bolt size, grade, and lubrication. Example: a 3/8-16 Grade 5 bolt typically uses about 30-35 ft-lbs dry, or 22-26 ft-lbs lubricated. Always check specifications for your specific application. Use published torque charts as starting points, but OEM specs override general charts.

Does lubrication affect torque?

Yes, significantly! Lubrication reduces friction, so less torque is needed to achieve the same preload. If you use anti-seize, oil, or any lubricant, reduce torque by 20-30%. Using "dry" torque values on lubricated fasteners WILL result in over-tightening and potential failure.

What is preload?

Preload is the tension (clamping force) in a tightened bolt. When you torque a bolt, it stretches slightly like a spring, pulling the joint together. Proper preload keeps the joint tight under service loads. Most of the torque you apply goes into overcoming friction - only 10-15% actually creates preload.

Should I tighten the bolt or the nut?

Generally, turn whichever component rotates more easily against the work - usually the nut. If the bolt head is captured (like a carriage bolt), you must turn the nut. For some applications, the torque spec assumes a specific component is turned. Hold the other side to prevent rotation and surface damage.

What is the correct tightening sequence for multiple bolts?

Tighten in a star or cross pattern, working from the center outward. This distributes clamping force evenly and prevents warping. Usually done in stages: snug all bolts first, then 50% torque, then 75%, then final torque - each time following the pattern. Critical for cylinder heads, flanges, and any multi-bolt joint.

Can bolts be reused?

Most standard bolts: Yes, if undamaged. Inspect threads and check for stretching (compare to new bolt length). Torque-to-yield bolts: Usually no - they're designed to stretch permanently. Critical fasteners: Follow manufacturer guidance. When in doubt or if safety is involved, replace with new.

What is a torque-to-yield bolt?

TTY bolts are tightened past their yield point intentionally - they permanently stretch to achieve precise, consistent preload. Common in modern engines (head bolts, rod bolts). Installation involves torquing to a spec, then turning an additional angle. TTY bolts should not be reused - they've already stretched once.

What does "snug tight" mean?

Snug tight is the tightness from a few impacts of an impact wrench or full effort with a standard wrench. It's enough to bring joint components into firm contact with all plies compressed. Often the first step before final torquing. In some structural connections, snug tight is the final condition (no torque wrench needed).

When should I use thread locker?

Use thread locker (Loctite) for: vibration-prone assemblies, fasteners that must not loosen, and where lock nuts/washers aren't practical. Blue (medium strength): Removable with hand tools. Red (high strength): Requires heat to remove. Apply to clean, dry threads. Don't use on fasteners that will be re-torqued frequently.

How do I prevent bolts from loosening?

Options from most to least effective: 1) Proper torque (most important!), 2) Thread locker, 3) Prevailing torque lock nuts (nylon insert or all-metal), 4) Nord-Lock wedge washers, 5) Safety wire or cotter pins, 6) Serrated flange nuts. Split lock washers are largely ineffective.

How do I remove a stripped screw?

Try these in order: 1) Larger screwdriver with downward pressure, 2) Rubber band between driver and screw for grip, 3) Screw extractor bit, 4) Cut a slot with rotary tool and use flathead, 5) Left-hand drill bit (often grabs and backs out), 6) Screw extractor kit with drilled pilot hole, 7) Weld a nut to the screw head.

How do I loosen a stuck or rusted bolt?

Penetrating oil (PB Blaster, Kroil) - apply and wait, repeat. Heat the nut with a torch, then cool quickly - thermal shock breaks rust bonds. Impact - sharp blows with hammer while turning. Vibration - impact wrench. For severely rusted bolts, combination of heat and penetrant works best. Patience is key.

How do I fix stripped threads in a hole?

Options: 1) Larger bolt - drill and tap for next size up, 2) Helicoil or thread insert - install coiled wire insert for original size threads, 3) Time-Sert - solid threaded insert, 4) Thread repair kit (various brands), 5) Epoxy + new bolt for non-critical, 6) Weld and re-drill/tap for steel.

What is a Helicoil?

Helicoil is a brand of wire thread insert. The damaged hole is drilled larger, tapped with a special tap, and a coiled stainless steel insert is threaded in. The result is original-size threads that are actually stronger than the original. Standard repair for stripped threads in aluminum, cast iron, and soft metals.

How do I remove a broken bolt?

If bolt protrudes: locking pliers or weld a nut to it. If flush or below: center punch, drill a pilot hole, then use screw extractor (easy-out). If extractor breaks: EDM (electrical discharge machining) or careful drilling around the bolt. Left-hand drill bits sometimes grab and remove the bolt during drilling.

Why do screws strip?

Common causes: Wrong size driver (most common!), worn driver bit, poor quality screw, not enough downward pressure, wrong driver type (Phillips vs Pozidriv), over-torquing, or angle not straight. Prevention: correct driver size, new bits, quality screws, and steady pressure.

My bolt spins but won't tighten - what's wrong?

The threads are stripped - either in the hole, on the bolt, or both. The bolt is spinning without engaging. Solutions: Try a longer bolt to engage fresh threads, use a thread repair insert, drill and tap for larger size, or use a bolt with nut through the material if possible.

How do I prevent cross-threading?

Start by hand - never use a power tool to start threads. Turn backwards first until you feel a click (threads engaging), then turn forward. Keep straight alignment - angle causes cross-threading. Don't force it - if it's hard to turn, stop and restart. Clean threads before assembly.

Why did my bolt break?

Common causes: Over-torquing, fatigue from cyclic loading (vibration), corrosion weakening, hydrogen embrittlement (in high-strength plated bolts), wrong grade for application, improper heat treatment, or stress concentration from damage or sharp transitions. Analyze the break surface - brittle vs ductile failure tells a lot.

Can I drill through hardened bolts?

Yes, but it's difficult. Use cobalt drill bits (not standard HSS), slow speed, steady pressure, and cutting fluid. Carbide bits work but are brittle. You can anneal the bolt first with a torch (heat to red, slow cool) to soften it. A broken extractor in a hardened bolt usually requires EDM.

How do I prevent rust on fasteners?

Use appropriate materials: stainless steel, galvanized, or coated fasteners for exposure conditions. Apply protective coating after installation (paint, wax, oil). Prevent galvanic corrosion by matching materials. Keep dry when possible. For steel bolts that must stay bare, periodic light oil application prevents rust.

My stainless bolts are seized together - help!

This is galling - the threads have cold-welded together. For mild cases: penetrating oil and patience. Severe cases: heat may help, but often the fastener must be cut off or drilled out. Prevention is key: always use anti-seize on stainless threads, tighten slowly, and consider using different alloys for bolt and nut.

How do I remove security screws?

Get the right bit - security bit sets are inexpensive and include Torx with center pin, spanner (two-hole), tri-wing, and others. For one-way screws, cut a slot with a rotary tool. For shear bolts (heads designed to break off), drill out or use extractor. Don't damage the fastener if you might need to reinstall it.

What is a shoulder bolt?

A shoulder bolt (shoulder screw, stripper bolt) has a precision-ground unthreaded shoulder between the head and threads. The shoulder acts as a shaft or pivot point. Used for linkages, guides, and anywhere a shaft with threaded retention is needed. Shoulder diameter and length are specified separately from thread size.

What is an eye bolt?

An eye bolt has a loop (eye) at the head for attaching rope, cable, or hooks. Types include plain eye (light duty, angled loads NOT safe), shoulder eye (safe for angled loads), and welded eye. Always check load ratings - eye bolts have specific angular load limitations. Critical for lifting applications.

What is a U-bolt?

A U-bolt is shaped like the letter U, threaded on both ends. Used to clamp pipes, tubes, or round objects to a surface. Specify by pipe size (fits around the pipe) or inside dimensions. Common applications: exhaust pipes, leaf springs, conduit mounting. Square U-bolts are available for rectangular applications.

What is a toggle bolt?

A toggle bolt has spring-loaded wings that fold to pass through a hole in drywall or hollow material, then spring open behind it. Much higher load capacity than plastic anchors in drywall. Drawbacks: requires a large hole, wings fall if bolt is removed. Used for hanging heavy items on hollow walls.

What is a Molly bolt?

A Molly bolt (hollow wall anchor) has a sleeve that expands behind drywall when tightened. Unlike toggle bolts, the anchor stays in place if the screw is removed. Pointed Mollys can be tapped into drywall; drill-type Mollys need a pre-drilled hole. Lower capacity than toggles but more convenient for repeated access.

What is a threaded insert?

Threaded inserts add machine threads to soft materials like wood, plastic, or composite. Types include: press-in (for plastic), heat-set (melted into plastic), knife-thread (screwed into wood), and expansion (spread when bolt tightens). They allow repeated assembly/disassembly without wearing out the base material.

What is a sex bolt (Chicago screw)?

A sex bolt consists of a female barrel (internally threaded post) and a male screw. They fasten from both sides of material, creating a smooth, finished appearance on both faces. Used for leather goods, signage, binders, and architectural applications. Also called Chicago screws, binding posts, or post and screw.

What is a weld stud?

A weld stud is a threaded fastener welded directly to metal using a stud welding gun. No drilling required - the stud is placed, an arc melts the base, and the stud is plunged into the molten pool. Creates very strong attachment. Common for attaching insulation, electrical grounds, and mounting brackets to steel structures.

What is a clevis pin?

A clevis pin is a headed pin with a cross-hole at the opposite end for a cotter pin or clip. Used to connect a clevis (U-shaped bracket) to a rod end, linkage, or other pivoting connection. The cotter pin prevents the clevis pin from falling out. Common in trailers, agricultural equipment, and machinery linkages.

What is a cotter pin?

A cotter pin (split pin) is a bent wire pin inserted through a hole and bent over to lock a clevis pin, castle nut, or other component in place. It provides positive mechanical locking - the part cannot come loose until the cotter pin is removed. Always use new cotter pins - they're designed for single use.

What is a retaining ring?

Retaining rings (snap rings, circlips) are spring steel rings that snap into grooves to hold components on shafts or in bores. External rings fit in shaft grooves; internal rings fit in bore grooves. They provide axial retention without threads. Require special pliers for installation and removal.

What is a dowel pin?

A dowel pin is a precision-ground cylindrical pin pressed into accurately-sized holes to align and locate parts. They ensure repeatable positioning when parts are disassembled and reassembled. Common in fixtures, molds, machinery, and anywhere precise alignment is needed. Usually hardened steel with tight diameter tolerances.

What is a roll pin (spring pin)?

A roll pin is a hollow, split cylinder that compresses when driven into a hole, then springs outward to grip. Used to retain parts on shafts (like gear shift levers) or as pivot pins. Advantages over solid pins: absorbs shock, conforms to hole variations, easier to remove. Also called spring pins or tension pins.

What is a taper pin?

Taper pins have a slight taper (standard 1:48) and fit into tapered reamed holes. The taper creates a self-locking fit - they won't work loose like straight pins. Used for accurate, secure positioning of collars, pulleys, and other components on shafts. Installed and removed with a taper pin punch.

What is a quick-release pin?

Quick-release pins have a spring-loaded mechanism (button, ball, or T-handle) that allows fast one-handed insertion and removal. Used for equipment that needs frequent reconfiguration - like trailer hitches, exercise equipment, and tooling. Push the button, insert or remove, release to lock.

What is a standoff?

A standoff is a threaded spacer that maintains a precise distance between two parts - commonly used for mounting PCBs (circuit boards) in electronics enclosures. One end threads into the base; a screw goes through the board into the other end. Available in hex (most common), round, and square shapes.

What is a thumbscrew?

A thumbscrew has a wide, flat, or knurled head designed for hand tightening without tools. Used for access panels, adjustment mechanisms, and anywhere frequent hand adjustment is needed. Types include flat head (wing-style) and tall knurled heads. Not for high-torque applications.

What is a coupling nut?

A coupling nut (extension nut, rod coupler) is a long hex nut used to join two threaded rods end-to-end. Each rod threads in from opposite ends. Common for extending all-thread rod, creating adjustable assemblies, and suspension systems. Available in standard hex and reduced-diameter styles.

What are captive screws?

Captive screws stay attached to the panel even when fully loosened - they can't fall out or get lost. A shoulder or spring clip retains the screw. Used on access panels, electronic enclosures, and anywhere screws might be lost (aerospace, medical equipment). The screw loosens completely but remains captured.

What is a Torx screw?

Torx screws have a 6-point star-shaped recess (not hex). The design provides excellent torque transfer and virtually eliminates cam-out (driver slipping). Common sizes: T10, T15, T20, T25, T30. Tamper-resistant versions have a pin in the center. Increasingly popular in automotive, electronics, and construction.