A screw thread is a ridge of uniform shape that wraps in a spiral around the surface of a cylinder. That spiral, called a helix, is what gives screws, bolts, and nuts the ability to fasten things together or convert rotational motion into straight-line movement. Whether on the outside of a bolt or the inside of a nut, the thread is what makes the connection work.
How a Thread Actually Works
A screw thread is really just an inclined plane wrapped around a shaft. Picture a ramp spiraling around a pole, and you have the basic idea. When you turn a screw, you’re pushing material along that ramp, which converts the twisting force from your hand or a drill into a much stronger force pushing the screw forward. This is why a relatively small amount of effort with a screwdriver can pull two pieces of wood tightly together.
The mechanical advantage of a screw depends on how tightly the threads are spaced. You calculate it by dividing the circumference of the screw by the distance between threads (the pitch). A finely threaded screw with threads packed close together gives you more mechanical advantage than a coarse one, meaning it takes less effort per turn but more turns to drive it in.
Parts of a Thread
Every screw thread has the same basic anatomy, regardless of size or type:
- Crest: The outermost edge of the thread, the peak of the ridge. On a bolt, this is the part your fingertip catches when you run it across the threads.
- Root: The lowest point in the groove between threads. This is where the material is thinnest and where stress concentrates under heavy loads.
- Flank: The sloped surface connecting the crest to the root. The angle of the flank determines much of how the thread behaves under load.
- Pitch: The distance from one thread to the next, measured parallel to the screw’s axis. A smaller pitch means finer threads packed more closely together.
- Major diameter: The largest diameter of the thread, measured across the crests on a bolt or across the roots inside a nut.
- Minor diameter: The smallest diameter, measured across the roots on a bolt. This represents the narrowest cross-section of the shaft and is the weakest point structurally.
Pitch vs. Lead
These two terms often get confused, and on most everyday screws they’re identical. Pitch is the distance between adjacent thread grooves. Lead is how far the screw advances in one full turn. On a standard single-start screw (one continuous spiral), they’re the same number.
The distinction matters with multi-start screws, which have two or more spirals running side by side. A double-start screw advances twice the pitch in a single revolution because it has two separate helical ridges. The lead equals the pitch multiplied by the number of starts. Multi-start designs are common in applications where you need fast linear travel per turn, like the cap on a water bottle or the lead screw on a 3D printer.
Common Thread Profiles
If you sliced a thread in cross-section, you’d see its profile, the shape of that ridge. Different shapes serve different purposes.
V-threads are by far the most common. They have a triangular cross-section and are what you’ll find on nearly every standard bolt, screw, and nut. The angled flanks create natural self-locking properties, meaning the thread resists loosening on its own. This makes V-threads ideal for general fastening.
Square threads have a flat-topped, rectangular cross-section. They transfer force more efficiently along the screw’s axis because the load pushes straight against the flat flanks rather than at an angle. This makes them a go-to choice for heavy-duty applications like vises and hydraulic presses, though they’re harder to manufacture than V-threads.
Acme threads are a practical compromise. Their trapezoidal shape (like a slightly angled square) handles heavy loads well while being easier to machine than true square threads. You’ll find them on lathe lead screws and clamping mechanisms.
Buttress threads have an asymmetrical profile, with one steep flank and one shallow one. They’re designed to handle strong forces in a single direction, which makes them useful in applications like artillery breech locks or jack screws that only need to push one way.
Right-Hand and Left-Hand Threads
The vast majority of threads are right-handed, meaning you turn them clockwise to tighten. This is so universal that most people never think about it.
Left-hand threads exist for specific situations where a right-hand thread would work itself loose. Rotating machinery is the classic example: if a shaft spins counterclockwise, a standard right-hand thread on that shaft could gradually unscrew itself. A left-hand thread resists that loosening. You’ll also find left-hand threads on the left pedal of a bicycle, on some gas fittings (as a safety measure to prevent connecting the wrong gas line), and on turnbuckles where one end needs to tighten while the other loosens.
Thread Standards and Sizing Systems
Two major systems dominate the world of fastener threads, and they are not interchangeable.
ISO metric threads are the global standard. They’re designated with an “M” followed by the diameter in millimeters and the pitch. An M8×1.25 thread, for example, has an 8 mm major diameter with 1.25 mm between threads. Metric threads use a 60° V-profile.
Unified Thread Standard (UTS) is the inch-based system common in North America. It also uses a 60° profile, but diameters and pitches are specified in inches. A ¼-20 UNC thread means a quarter-inch diameter with 20 threads per inch. UNC designates coarse threads, while UNF designates fine threads with more threads packed into the same length.
Because both systems use the same 60° angle, some sizes look deceptively similar. An M6×1.0 metric thread and a ¼-20 UNC thread have nearly the same diameter, but they will not fit together. Forcing one into the other strips the threads.
Pipe Threads
Plumbing and fluid systems use their own thread families. The two main ones are NPT (National Pipe Taper), used primarily in North America, and BSP (British Standard Pipe), used in much of the rest of the world. They differ in three ways: threads per inch, thread shape, and cut angle. NPT threads are cut at 60° with flat, sharp peaks and valleys. BSP threads are cut at 55° with rounded peaks and valleys. You can often tell them apart by touch alone, since NPT threads feel noticeably sharper under your finger.
How to Identify an Unknown Thread
If you’re trying to figure out what thread you’re dealing with, you need two measurements: the major diameter and the pitch.
A caliper is the best tool for measuring the major diameter. Place the bolt between the jaws (or the jaws inside a nut) and read the measurement. If you suspect the thread might be tapered, measure at the first, fourth, and last threads. If the diameter changes, it’s tapered. If it stays constant, the thread is straight.
For pitch, a thread pitch gauge is the fastest option. It’s a small fan of metal leaves, each cut with a different thread spacing. You press leaves against the threads until you find one that fits perfectly into every groove. One important tip: have both metric and imperial gauges on hand. Some metric pitches are close enough to certain imperial pitches that you can get a false match. Checking with both gauge sets confirms which system you’re actually dealing with.
If you don’t have a pitch gauge, you can count threads over a measured length using a ruler. Count the number of thread crests in one inch for imperial threads, or measure the distance between crests in millimeters for metric.