A self-tapping screw has sharper, wider-spaced threads and a pointed or specially shaped tip designed to cut or push its way into material without a pre-threaded hole. If you’re holding a screw and wondering whether it’s self-tapping, the threads and the tip are the two features that give it away. A standard machine screw, by comparison, has uniform, closely spaced threads and a flat or blunt end, because it’s designed to fit into a hole that’s already threaded or to be secured with a nut.
Check the Threads First
The fastest way to identify a self-tapping screw is to look at its threads. Self-tapping screws have a noticeably larger thread pitch, meaning there’s more space between each thread. This wider spacing is what lets the screw bite into raw material and pull itself in. Machine screws have finer, more tightly packed threads because they’re engineered to mate with existing threads in a nut or a tapped hole.
Self-tapping threads also tend to be deeper and sharper at the crest (the outermost edge of each thread). Run your finger lightly along the threads: a self-tapping screw feels more aggressive, almost like a wood screw, while a machine screw feels smoother and more uniform. The threads on a self-tapping screw are fully formed all the way down the shank, whereas some standard wood screws leave a smooth, unthreaded section near the head.
Look at the Tip
The tip is the other dead giveaway. Self-tapping screws come in several tip styles, and each one tells you something about how the screw works and what material it’s made for.
- Sharp, tapered point (Type A): Looks like a needle or a gimlet. This is the most recognizable self-tapping tip. It pierces soft materials like thin sheet metal, softwood, or plastic and pushes material aside as it enters. These have coarse, widely spaced threads.
- Blunt point with close threads (Type B): The tip is slightly rounded rather than needle-sharp, but the threads are still aggressive. These are common for thicker sheet metal and plastic.
- Blunt point with slots or flutes (Type F, Type 23, Type 25): Look closely at the tip area and you’ll see small vertical slits or a spiral groove cut into the threads. These are thread-cutting screws. Instead of pushing material aside, they actually cut and remove material like a tiny tap. If you see metal shavings when driving one of these, that’s by design.
A machine screw, by contrast, has a flat or slightly chamfered end with no tapering, no point, and no cutting features. It just stops.
Thread Forming vs. Thread Cutting
Self-tapping screws work in two fundamentally different ways, and you can tell which type you have by inspecting the tip and the first few threads.
Thread-forming screws (Types A, AB, and B) displace material as they drive in. They don’t remove anything from the hole. The material gets pushed outward, and the displaced material grips the screw threads tightly. These screws typically have smooth, continuous threads with no interruptions. Some thread-forming screws have a slightly triangular cross-section (called trilobular) rather than a perfectly round one, which reduces the force needed to drive them in.
Thread-cutting screws (Types F, BT, 23, and 25) actually remove material from the hole, like a drill bit or a tap. You can spot these by looking for a slot, notch, or flute carved into the lower threads near the tip. That flute serves a specific purpose: it gives the cut material somewhere to go. Without it, debris would pack into the hole and jam the screw. Thread-cutting types are used in harder materials like thick metal, dense hardwood, or rigid plastics where displacing material would require too much force or risk cracking.
Self-Tapping vs. Self-Drilling
This is where people get confused most often. All self-drilling screws are self-tapping, but not all self-tapping screws are self-drilling.
A self-drilling screw (sometimes called a Tek screw) has a tip that looks exactly like a small drill bit, complete with a split point and flutes. It can bore through sheet metal without any pilot hole at all. You can’t miss this tip once you know what to look for: it’s wider, chunkier, and shaped like a miniature twist drill. The flute length on a self-drilling screw determines how thick a piece of metal it can handle. A longer flute drills through thicker material.
A standard self-tapping screw, even one with a sharp point, usually still needs a pilot hole when used in metal or hardwood. It can create its own threads in that hole, but it can’t drill through solid material the way a self-drilling screw can. If the tip looks like a point or a blunt cone rather than a drill bit, you have a self-tapping screw that is not self-drilling.
When a Pilot Hole Is Still Needed
Even though self-tapping screws create their own threads, many applications still call for a pilot hole. The screw taps threads into the sides of the hole, but the hole itself needs to exist first in harder materials.
In softwood, the smallest self-tapping screws (No. 2 and No. 4) can go in without a pilot hole. Larger sizes, starting around No. 6 (3.5mm), benefit from a pilot hole of about 1.6mm in softwood and 2mm in hardwood to prevent splitting. In sheet metal, a pilot hole is almost always required unless you’re using a self-drilling type. The recommended pilot hole size falls between the screw’s minor diameter (the inner core) and major diameter (the outer thread edge), typically around 60 to 80 percent of the screw’s outer diameter depending on the material’s hardness.
For plastics, the pilot hole size depends on rigidity. Softer plastics like ABS or PETG use a smaller hole, while rigid plastics like nylon or polycarbonate need a hole closer to the screw’s outer diameter to avoid cracking. An M4 self-tapping screw, for example, needs about a 3.2mm pilot hole in soft plastic but a 3.45mm hole in rigid plastic.
Quick Identification Checklist
If you’re sorting through a bin of mixed screws, here’s what to look for:
- Wide thread spacing and a sharp point: Self-tapping, thread-forming type. Designed for sheet metal, wood, or plastic.
- Wide thread spacing and a blunt point with slots or grooves: Self-tapping, thread-cutting type. Designed for harder materials.
- Drill bit-shaped tip: Self-drilling screw. No pilot hole needed in sheet metal.
- Fine, closely spaced threads and a flat end: Machine screw. Needs a nut or a pre-tapped hole.
The head style won’t help you here. Self-tapping screws come in the same head shapes as machine screws: pan head, flat head, hex head, and others. The head dimensions are often identical between the two types. The distinction is entirely in the threads and the tip.