Explore all major thread screw types with charts, comparisons, and expert tips to choose the right fastener for any project or material.

Thread Basics: Terminology You Need to Know First

Before you dive into screw thread types, understanding the basics is a must. Here’s a quick rundown of the key terms you’ll see often:

• Major Diameter: The largest diameter of the screw thread. It’s basically the outside edge from peak to peak.
• Minor Diameter: The smallest diameter measured at the root (the groove between threads).
• Pitch: Distance between one thread crest to the next. In metric threads, this is measured in millimeters.
• Thread Angle: The angle formed between the flanks (sides) of the thread, commonly 60° for most screws.
• Lead: The distance a screw moves forward in one complete turn. For single-start threads, lead = pitch. For multi-start, lead = pitch × number of starts.
• TPI vs Pitch: TPI (Threads Per Inch) is used for imperial screws, while pitch (distance between threads) is favored in metric systems.

Thread Profile Parts

• Crest: The top surface of the thread.
• Root: The bottom of the thread groove.
• Flank: The sides connecting the crest and root.

Right-Hand vs Left-Hand Threads

Most screws use right-hand threads—turn clockwise to tighten. Left-hand threads turn counterclockwise and are used where rotation or vibration might loosen a right-hand screw (think: gas fittings or some machinery).

Single-Start vs Multi-Start Threads

• Single-Start: One continuous helical thread—standard on most screws.
• Multi-Start: Multiple threads start evenly spaced around the screw, allowing faster linear movement per turn (found in power screws and some fasteners needing quick engagement).

Master these terms, and you’ll never confuse TPI with pitch or major diameter with minor diameter again. This foundation keeps you sharp when choosing the right thread for your project or decoding specs on that mystery fastener you just pulled out of the toolbox.

The 8 Most Common Screw Thread Types

When it comes to thread screw types, understanding the most common varieties is key to picking the right fastener for your project. Here’s a breakdown of the top eight thread types you’ll encounter in the U.S. market, covering everything from heavy-duty bolts to delicate machine screws.

Unified Thread Standard (UTS) – UNC & UNF (Imperial)

The UTS system is the backbone of most American screw threads. It splits into two main types: UNC (Unified National Coarse) and UNF (Unified National Fine). UNC threads are chunkier and better for quick assembly or softer materials, while UNF threads are finer, offering better strength and precision in tight fits. These are the go-to thread forms for machine screws and bolts used in everything from automotive to construction.

Metric ISO Threads (Coarse & Fine)

Metric threads follow the ISO standard and are widely used for international and increasingly U.S. applications. Like UNC and UNF, metric threads come in coarse and fine pitches. Coarse threads are easier to assemble and resist cross-threading, while fine threads provide stronger joints with better vibration resistance. Metric fasteners are measured by millimeters (pitch and diameter), so keep a pitch gauge handy for identification.

Wood Screw Threads (Traditional vs Modern High-Low)

Wood screws are specially designed for gripping wood fibers. Traditional wood screws have a full, continuous thread with a sharp point, but modern versions often feature a high-low thread design. High-low threads alternate between tall and short ridges, reducing splitting while doubling holding power. These are perfect for cabinetry, furniture, and general woodworking projects.

Self-Tapping Screws (Type A, B, AB, F, 17, 23, 25, etc.)

Self-tapping screws cut their own threads into materials, saving time and eliminating the need for pre-tapped holes. They come in various types—Type A and AB are common for plastic, while Type B is better for metal. Specialized points like Type 17 and Type 23 improve cutting performance in tough materials. Knowing each type helps you avoid stripped threads or weak joints.

Sheet Metal Screws (Type A, B, C, D/F, U-Drive)

Sheet metal screws are designed to fasten thin metal sheets and light gauges. They vary in thread style; Type A is coarse and best for plastics or thin metals, while Type B is sharper and for harder metals. Type C and D/F offer threads that prevent damage to soft metals, and U-Drive screws are built for high-strength applications with reduced strip-out risk.

Machine Screw Threads

Machine screws typically use UNC or UNF threads, but you might also find remnants of BA (British Association) and Whitworth threads in older equipment or specialty tools. These screws are used with nuts or tapped holes in metal, requiring precise threading for secure assemblies.

Drywall Screws (Coarse vs Fine Thread)

Drywall screws come in both coarse and fine thread profiles. Coarse threads grab better in wood studs, speeding installation, while fine threads are designed for metal studs, providing cleaner penetration and stronger hold. Choosing the right drywall screw thread type ensures your walls stay strong and crack-free.

Lag Screws / Lag Bolts

Lag screws, or lag bolts, are heavy-duty wood fasteners with thick, deep threads. They’re used for wood-to-wood joints where strong holding power is necessary, like beams and heavy framing. Their large major diameter and coarse thread make them ideal for load-bearing applications.

Understanding these eight common thread screw types ensures you pick the best fastener for your material and project demands. For precision parts involving tight tolerances and flatness, check our guide on mastering flatness tolerance in GDT for precision manufacturing.

Specialty & High-Performance Thread Types

When standard threads just won’t cut it, specialty and high-performance thread types come into play. These screws are designed for specific materials, loads, or environments, offering improved hold, strength, and durability.

Hi-Lo & Twin-Thread (Furniture & Particleboard)

Hi-Lo threads are common in furniture and particleboard screws. They combine high and low threads on the same screw, providing excellent grip without damaging the softer wood fibers. Twin-thread screws speed up insertion by having two thread starts, making fastening quicker and reducing wear on materials.

Thread-Forming vs Thread-Cutting (Tri-lobular, PT, Delta PT)

Thread-forming screws actually displace material to create their own threads without removing it, ideal for plastics and soft metals. Tri-lobular and PT (Pan-Head Thread) designs reduce torque and prevent stripping. Delta PT screws add a special thread profile for even better grip and reduced installation torque. These are preferred when you need a strong hold but minimal material damage.

Pipe Threads (NPT, NPTF, BSPT, BSPP)

Pipe threads handle plumbing and fluid sealing jobs. NPT (National Pipe Thread) and NPTF (Fuel) are tapered U.S. standards that seal by wedging threads tighter as you screw them in. BSPT (British Standard Pipe Taper) and BSPP (Parallel) are common in European plumbing. These threads aren’t interchangeable, so matching the right standard to your fittings is key.

ACME & Trapezoidal (Lead Screws & Motion Control)

For motion control and heavy-duty lead screws, ACME and trapezoidal threads are the go-to. Their broad, angled thread profile offers high strength and smooth travel, making them perfect for machinery requiring precise movement and load-bearing capability.

Buttress Threads (One-Direction High Load)

Buttress threads handle high loads traveling in one direction. Their asymmetric design features a strong load-bearing flank and a shallow trailing flank. This makes them ideal for applications like jackscrews or clamps, where the force is mostly pushing in one direction.

For more on selecting threads based on materials and precise machining details, check out our guide on material metal types and properties. Also, to understand how thread cutting and forming processes differ, our machining coolant tips can be a useful resource.

Side-by-Side Comparison Tables & Charts

To make choosing the right thread screw types easy, here’s a quick reference with key comparisons between imperial and metric threads, thread profiles, holding power by material, and typical drive types used.

Imperial vs Metric: TPI and Pitch Quick Reference

Measurement Type	Imperial (UNC/UNF)	Metric (ISO)	Notas
Thread Pitch	Threads Per Inch (TPI)	Millimeters (mm)	TPI is inverse of pitch
Common Pitch	UNC: 13 TPI, UNF: 20 TPI	Coarse: 1.25 mm, Fine: 0.8 mm	Metric pitch standard varies by diameter
Conversion	1 TPI = 25.4 mm pitch (approx.)	Pitch in mm directly	Use a thread pitch gauge for exact measures

Thread Profile Visual Chart

Thread Type	Angle	Shape	Typical Uses
Unified (UNC/UNF)	60° V	Rounded crest & root	General machinery, automotive
Metric ISO	60° V	Similar to UTS	Global standard for most fasteners
Hi-Lo (Wood)	30°-60°	Flat crest, deeper groove	Particleboard, furniture
Square & ACME	90°	Flat sides (square)	Lead screws, motion control

Holding Power by Material

Material	Best Thread Type	Why It Works
Wood	Hi-Lo, Traditional Wood Screws	Deep threads grip coarse fibers
Metal	Machine Screws (UNC, UNF)	Tight thread fit, metal-to-metal grip
Plastic	Thread-Forming Screws (Tri-lobular, PT)	Creates threads without cracking
Drywall	Coarse Drywall Screws	Designed for grip in gypsum board

Drive Type Usually Paired with Each Thread

Thread Type	Common Drive Types
Machine Screw Threads	Phillips, Slotted, Hex, Torx
Wood Screws	Slotted, Phillips
Self-Tapping Screws	Phillips, Pozidriv, Torx
Drywall Screws	Phillips
Sheet Metal Screws	Hex Washer, Pozidriv

Matching the right drive with your thread type improves installation speed and reduces stripping risk.

For more on screw thread standards and exact measurements, check out this detailed sheet metal guide with types and fabrication essentials to understand how thread profiles affect applications.

How to Choose the Right Thread Type – Decision Guide

Picking the right thread screw type isn’t just about size—it’s about matching the thread to your project’s material, load, environment, and speed. Here’s a simple step-by-step flowchart and essential tips to guide your choice.

Step-by-Step Thread Selection Flowchart

Step	Question	Recommendation
Material	Are you fastening wood, metal, plastic, or drywall?	Use wood screws for lumber, machine screws for metal, self-tapping for plastic, drywall screws for drywall.
Load	Heavy, medium, or light load?	Lag screws for heavy wood loads, machine screws for metal under stress, Hi-Lo or twin-threads for particleboard/light loads.
Environment	Will the screw face moisture, corrosion, or heat?	Choose stainless or coated screws (like vast UltraGrip Hi-Lo) for moisture. Use specialty alloys for high heat.
Speed	Is fast installation a priority?	Self-tapping or thread-forming screws speed up assembly without pre-drilling.

Common Mistakes & How to Avoid Them

• Mixing thread standards: Don’t use metric screws in imperial-sized holes or vice versa—this risks loosening and damage.
• Wrong thread pitch: Using coarse instead of fine leads to poor grip on metal or vibration issues.
• Ignoring material compatibility: For example, a standard machine screw in soft wood offers little holding power.

Pro Tips from Machinists and Carpenters

• Use a thread pitch gauge to match screws perfectly to nuts or tapped holes.
• For particleboard and furniture, Hi-Lo and twin-thread screws offer superior holding with less damage.
• When working with metals, consider thread-forming screws like Tri-lobular types for clean installation and strong threads.
• Always check screw quality grades (like Class 2A or 4g6g) for precise fits in critical applications.

Choosing the right thread saves time, improves strength, and prevents costly mistakes, especially if you plan to reuse or tighten fasteners later. For a deeper dive into thread profiles and precision machining, check out our detailed guide on CNC milling machines and processes.

Thread Standards & Quality Grades Explained

When choosing the right thread screw types, understanding thread standards and quality grades is key to getting the proper fit and performance. Threads come with different tolerance classes that define how tightly or loosely they fit together.

• Class 1A, 2A, 3A (UNC/UNF Threads): These are common American National thread fit classes used for external threads.
  • Class 1A: Loose fit for quick assembly, often used in low-stress applications.
  • Class 2A: The most common, general-purpose fit with a balance of ease and strength.
  • Class 3A: Tight fit for high-precision, high-strength applications where vibration resistance matters.
• Metric Quality Grades (e.g., 4g6g, 6g): In metric screw threads (ISO standard), grades like 4g6g or 6g specify tolerance ranges for external threads.
  • 4g6g: Medium tolerance fit, suitable for general use with good strength.
  • 6g: A tighter tolerance preferred when a more precise and durable thread engagement is required.

These classes and grades help machinists, engineers, and DIYers pick screws and bolts that fit with correct tightness, ensuring reliability without forcing or loosening.

Recognizing Thread Markings on Packaging

Screw and bolt packages often include markings from standards organizations. Knowing what these mean can save you time and frustration:

• ANSI/ASME: U.S. standards for thread profiles and tolerances, commonly seen on imperial (UNC/UNF) screw packaging.
• ISO: The international metric thread standard used worldwide, including designations like M6x1.0 for a 6mm coarse thread with 1.0mm pitch.
• DIN: German industrial standards often used alongside ISO for specialized screws, especially in machinery and automotive parts.
• JIS: Japanese Industrial Standards – seen mostly in imported fasteners, occasionally with unique profiles or finishes.

Understanding this helps when selecting the correct screw for replacement or new projects — especially if you blend metric and imperial parts.

For more detail on machining precision and tolerances, check out our guide on transition fits for precise mechanical assembly.

Where to Buy the Best Screws for Each Thread Type

Finding the right screws for your project is just as important as knowing the thread types. Vast offers a wide selection of top-quality screws tailored for various thread needs in the U.S. market.

• Vast UltraGrip Hi-Lo Threads: Perfect for furniture and particleboard, these screws combine high holding power with easy driving thanks to the Hi-Lo thread design. Ideal for woodworking projects that need strong, lasting joints.
• Vast Titanium Metric Fine Threads: These screws provide excellent corrosion resistance and precision fit for machinery and automotive repairs requiring fine thread engagement.
• Vast Stainless UNC Assortment Kits: Great for heavy-duty, rust-resistant applications, these kits include a range of Unified National Coarse screws suitable for both metal and outdoor use.

Shopping Vast means you get premium screws engineered for specific thread profiles and materials, ensuring your project performs flawlessly. For precision projects, you might also find our detailed resources on engineer drawings and precision design handy to match screws to your exact specifications.

FAQ Section: Common Questions About Thread Screw Types

What’s the difference between coarse and fine threads?

Coarse threads have fewer threads per inch (TPI), making them thicker and deeper. They’re great for quick assembly, better for soft materials like wood, and resist stripping well. Fine threads have more TPI, are shallower, and offer better holding power in hard materials like metal. They also allow more precise tightening.

Can I use metric screws in imperial holes (and vice-versa)?

Generally, it’s best to avoid mixing metric screws in imperial holes or vice-versa. The thread pitch and diameter slightly differ, risking poor fit, loose connections, or stripped threads. When tight tolerances matter, always match the standard. For less critical uses, some people get away with similar sizes, but it’s not recommended for long-term strength.

Are self-tapping screws weaker than machine screws?

Typically, yes. Self-tapping screws are designed to create their own thread as they’re driven into soft material, so their threads aren’t as strong or precise as machine screws, which are made to fit pre-cut threads. Use self-tapping for quick installation in wood or metal sheet, but rely on machine screws for high-strength, reusable connections.

What does “Type 17 point” mean?

The Type 17 point is a special screw tip designed to cut threads cleanly as it drills into wood, reducing splitting and making driving easier. It’s common on self-tapping wood screws and helps speed up installation while improving holding power.

More quick answers to popular questions:

• What’s thread pitch gauge used for? — It helps identify thread size by measuring the distance between threads, vital for matching screws and bolts precisely.
• How to read UNC vs UNF charts? — UNC means Unified National Coarse; UNF is Unified National Fine. Coarse is better for quick assembly and soft materials; fine threads offer tighter, stronger loads on metals.
• Are Hi-Lo threads better for particleboard? — Yes, Hi-Lo threads create strong retention in particleboard and furniture materials without splitting.
• Can dry wall screws be used in metal? — Coarse-thread drywall screws work okay in thin metal sheets but are not ideal for heavy metal loads. Use sheet metal screws instead.
• What’s the difference between thread-forming and thread-cutting screws? — Thread-forming screws displace material to make threads, causing less damage; thread-cutting screws remove material to form threads, suitable for harder surfaces.

Have questions beyond this list? Just ask — understanding thread and screw types helps you pick the right fastener for any job.

For more on selecting the best screws by material and application, check out our detailed guides and comparison charts.