⚡ Key Takeaways
- Machine screws M: used with a nut or a pre-tapped hole, reassembled freely, GB/T 3098.1.
- Self-tapping screws ST: cut straight into plastic or thin sheet, not intended for repeated assembly, GB/T 3098.5.
- Thread designation: ST is a 55° coarse-pitch tapping thread and does not interchange with M.
- Point types: sharp A (plastic), blunt B (thin metal), wide thread C.
- The pilot hole decides everything — follow the recommended table.
📑 Contents
1. The real difference
2. ST pilot hole table
3. Choosing the point type
4. Selection FAQ
1. The real difference
2. ST pilot hole table
3. Choosing the point type
4. Selection FAQ
1. The real difference
| Item | Machine screw M | Self-tapping ST |
|---|---|---|
| Thread form | 60° metric | 55° tapping |
| Mates with | A nut or a pre-tapped hole | The parent material directly |
| Repeated assembly | Yes | No — strips after 3–5 cycles |
| Material | Steel / stainless | Case-hardened carbon steel, stainless |
| Standard | GB/T 3098.1 | GB/T 3098.5, GB/T 5280 |
| Typical use | Metal structures | Plastic housings, sheet metal, appliances |
2. ST pilot hole table
| Size | Pitch P (mm) | Pilot hole, steel (mm) | Pilot hole, aluminium (mm) | Sheet thickness (mm) |
|---|---|---|---|---|
| ST2.2 | 0.8 | 1.9 | 1.8 | 0.4–0.8 |
| ST2.9 | 1.1 | 2.4 | 2.3 | 0.7–1.2 |
| ST3.5 | 1.3 | 2.9 | 2.8 | 0.8–1.5 |
| ST4.2 | 1.4 | 3.4 | 3.3 | 1.0–1.8 |
| ST4.8 | 1.6 | 3.9 | 3.8 | 1.2–2.0 |
| ST5.5 | 1.8 | 4.6 | 4.4 | 1.2–2.5 |
| ST6.3 | 1.8 | 5.3 | 5.1 | 1.5–2.5 |
Important: too small a hole and the screw snaps; too large and it strips. For plastics adjust the pilot hole by 0.1–0.2 mm to suit the material hardness.
3. Choosing the point type
- Sharp point A (type A): plastic and wood, easy to locate;
- Blunt point B (type AB): thin metal sheet, fewer chips;
- Wide thread C: fast tapping into plastic;
- Self-drilling (type B): integrated drill point, no separate punching, GB/T 3098.11.
4. Selection FAQ
Q: Can a self-tapping screw be used with a nut? No. ST threads do not interchange with M.
Q: What do I use in a plastic housing? ST2.2–ST3.5, with the pilot hole from the table above.
Q: Are stainless self-tapping screws available? Yes, to GB/T 3098.21, but their lower hardness makes them unsuitable for metal.
🛠️ Engineering support: Toprecision supplies the full ST range, including stainless and self-drilling versions.
👉 Engineering services ｜ 👉 Request a technical quote
👉 Engineering services ｜ 👉 Request a technical quote