Self-Tapping Screw Pilot Hole Size Chart for Metal and Plastic
For a self-tapping screw, the pilot hole must be smaller than the screw's outside diameter but large enough to prevent excessive driving torque. In 1 to 3 mm mild steel, a 4.2 mm screw normally starts with a 3.2 to 3.3 mm pilot hole. A 4.8 mm screw normally starts with 3.7 to 3.8 mm. Aluminium usually needs a hole about 0.1 mm larger. Plastic needs a different approach because grade, wall thickness and boss design change the answer.
The charts below are practical starting points for non-critical work. Test on an offcut first. For load-bearing, safety-critical or repeat production joints, use the screw maker's data and prove the assembly with torque and pull-out testing.
Self-tapping screw pilot hole chart for mild steel
This simplified chart covers the sizes most often used for pan and countersunk tapping screws. It is based on published sheet-metal guidance for thread-forming self-tappers. The correct figure changes with sheet thickness and tensile strength, so do not treat a single drill size as universal.
| Screw size | Approx. gauge | Mild steel, 0.9-1.4 mm | Mild steel, 1.5-3.0 mm | Useful first drill |
|---|---|---|---|---|
| 3.5 mm | No. 6 | 2.7-2.8 mm | 2.8-2.9 mm | 2.8 mm |
| 4.2 mm | No. 8 | 3.2 mm | 3.3 mm | 3.2 or 3.3 mm |
| 4.8 mm | No. 10 | 3.7 mm | 3.8 mm | 3.7 or 3.8 mm |
For thicker sheet, the hole generally increases by another 0.1 to 0.3 mm. If a hole is punched rather than drilled, published guidance commonly calls for an extra 0.1 to 0.3 mm, with the screw driven in the direction the hole was punched. Burr shape changes how the first thread forms.
Pilot hole sizes for aluminium
Aluminium is softer than mild steel and more likely to have its new thread stripped by excessive torque. Start slightly larger than the comparable mild-steel hole:
- 3.5 mm screw: about 2.8 to 2.9 mm.
- 4.2 mm screw: about 3.3 to 3.4 mm.
- 4.8 mm screw: about 3.9 to 4.0 mm.
Use the smaller end only in thin, ductile material where you need stronger thread engagement. Move up a drill size if the screw becomes hard to drive, the recess starts to cam out or the aluminium puckers around the hole. If the fixing will remain outdoors, material compatibility matters as much as hole size. Our guide to stainless steel screws in aluminium explains moisture, area ratio and isolation washers.
What about plastic?
There is no honest one-size chart for plastic. ABS, nylon, polypropylene, polycarbonate and glass-filled compounds have very different stiffness and crack resistance. Standard 60-degree self-tapping threads can also create more outward stress than a screw designed for thermoplastics.
For a purpose-made plastics screw, a useful engineering starting point is a pilot diameter near 80% of the screw's outside diameter. That gives these trial sizes:
| Nominal screw | 80% starting hole | Practical trial drill |
|---|---|---|
| 3.5 mm | 2.8 mm | 2.8 mm |
| 4.2 mm | 3.36 mm | 3.3 or 3.4 mm |
| 4.8 mm | 3.84 mm | 3.8 or 3.9 mm |
These are development values, not guarantees. Hard or brittle plastic generally needs a larger hole. Softer plastic may accept a smaller one. A moulded boss should have enough wall around the hole, a lead-in or counterbore to relieve the top edge, and at least two screw diameters of usable thread engagement. Do not let a screw bottom out in a blind hole.
Why a pilot hole fails
The hole is too small
The screw has to displace too much material. Driving torque rises sharply, the drive recess can strip and a stainless screw can twist off. Plastic may split around the boss. In metal, a blunt screw can simply polish the hole without forming a clean thread.
The hole is too large
There is too little flank engagement. The screw may feel easy to install but pull out under a modest load, or spin before the head clamps the parts. This is common after an installer "fixes" a tight hole by jumping several drill sizes at once.
The sheet is outside the screw's useful range
A self-tapper needs enough material to form useful thread, but not so much that forming torque becomes excessive. Very thin sheet may need a clip nut, rivet nut or through-bolt. Thick plate may be better drilled and tapped for a machine screw. If you are unsure about head and drive selection, use the fastener head and drive glossary before choosing the joint.
Stainless self-tappers need a little more care
Austenitic A2 and A4 stainless steel resists corrosion well, but it is not case-hardened like a conventional carbon-steel self-tapper. Do not assume an A2 tapping screw will cut a reliable thread into stainless sheet. That application normally needs a suitable hardened or bi-metal drilling point, or a pre-tapped hole and machine screw. Match the screw to the substrate, not just the exposure.
For mild steel, aluminium, plastics and many enclosures, stainless pan-head tapping screws give a broad bearing surface. A countersunk version is useful only where the top sheet has enough thickness or a formed countersink to support the head. The distinction between tapping and drilling points is covered in our DIN, ISO and BS fastener standards guide.
A reliable five-step method
- Identify the material and thickness. Measure rather than guess.
- Choose the screw size and point. Confirm it is suitable for the substrate.
- Drill the chart starting size. Deburr the hole without turning it into a countersink.
- Test one joint at low speed. The screw should form steadily and clamp before the recess is damaged.
- Inspect and adjust by 0.1 mm. Increase the hole if torque is excessive. Decrease only if pull-out or stripping is poor.
Drive stainless fasteners slowly and keep the bit square. High speed adds heat and makes seizure more likely in threaded assemblies. For bolts and nuts, see how to stop stainless steel bolts seizing.
Continue through the stainless fixing series
Useful sizes for a test setup
- 4.2 x 13 mm A2 pan tapping screws - a compact size for panels, covers and light enclosures.
- 4.8 x 25 mm A2 pan tapping screws - a larger diameter where the material and edge distance allow it.
- Stainless flat washers - spread the head load where a clearance hole is used in the top component.
Frequently asked questions
What size pilot hole for a 4.2 mm self-tapping screw?
Start with 3.2 mm in thin mild steel, 3.3 mm in 1.5 to 3 mm mild steel, or about 3.3 to 3.4 mm in aluminium. Test an offcut and move in 0.1 mm steps.
What size pilot hole for a 4.8 mm self-tapping screw?
Use roughly 3.7 to 3.8 mm in common mild-steel sheet and 3.9 to 4.0 mm in aluminium. Sheet thickness, alloy and screw point can change the result.
Can a self-tapping screw go into plastic?
Yes, but a screw designed for thermoplastic gives lower radial stress than a standard sheet-metal thread. Start near 80% of nominal diameter and prove the joint on the actual plastic.
Do self-drilling screws need a pilot hole?
Usually not within their rated drilling capacity. A self-drilling screw has a drill-shaped point. A self-tapping screw forms or cuts thread but often needs a prepared hole.
Can stainless self-tapping screws tap stainless sheet?
Do not assume they can. Austenitic stainless screws are not case-hardened like carbon-steel tapping screws. Use a specified bi-metal self-driller or a drilled and tapped machine-screw joint.
Next step: measure the sheet, drill one test hole from the chart, then inspect the formed thread before committing the rest of the job. Browse Simfix pan tapping screws by diameter and length.
