Self-Tapping Screw Pilot Hole Size Chart for Aluminium and Plastic
A self-tapping screw pilot hole must be smaller than the screw's outside diameter, but large enough to avoid excessive driving torque. Aluminium and plastics need different starting sizes, and the final choice depends on material grade, thickness and joint design.
The figures below are practical starting points for non-critical work. Always test on an offcut first. For load-bearing, safety-critical or repeat-production joints, use the screw manufacturer's data and prove the assembly with torque and pull-out testing.
Self-tapping screw pilot hole chart for aluminium
Aluminium is relatively soft, so an undersized pilot hole can raise driving torque or strip the newly formed thread. These starting sizes suit the common 3.5, 4.2 and 4.8 mm tapping screws in the Simfix range.
| Nominal screw | Starting pilot hole | Adjustment range |
|---|---|---|
| 3.5 mm | 2.8 mm | 2.8-2.9 mm |
| 4.2 mm | 3.3 mm | 3.3-3.4 mm |
| 4.8 mm | 3.9 mm | 3.9-4.0 mm |
Use the smaller end only where testing shows that the material accepts the driving torque and gives reliable thread engagement. Move up by 0.1 mm if the screw becomes hard to drive, the recess starts to cam out or the aluminium puckers around the hole. For outdoor joints, our guide to stainless steel screws in aluminium explains moisture, area ratio and isolation washers.
Starting sizes for compatible plastics
There is no honest one-size chart for plastic. ABS, nylon, polypropylene, polycarbonate and glass-filled compounds have different stiffness and crack resistance. A screw designed specifically for thermoplastics can also create less outward stress than a standard self-tapping profile.
For a purpose-made plastics screw, a useful development starting point is a pilot diameter near 80% of the screw's outside diameter:
| Nominal screw | 80% calculation | 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, while 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 the 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 the screw can twist off. Plastic may split around the boss, while aluminium can pucker or deform around the hole.
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.
The material is outside the screw's useful range
A self-tapper needs enough material to form a useful thread without demanding excessive forming torque. If testing does not produce a sound thread, choose a fixing system designed for that substrate rather than forcing the screw.
A reliable five-step method
- Identify the material and thickness. Measure rather than guess.
- Confirm compatibility. Check that the screw is suitable for the actual aluminium alloy or plastic grade.
- Drill the starting size. Deburr the hole without enlarging it unnecessarily.
- 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; only decrease it if testing shows insufficient engagement.
Drive stainless fasteners slowly and keep the bit square. 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 aluminium panels, compatible plastics 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 in aluminium?
Start at about 3.3 mm, test an offcut and move towards 3.4 mm if driving torque is excessive. Alloy, thickness and screw point can change the result.
What size pilot hole for a 4.8 mm self-tapping screw in aluminium?
Start at about 3.9 mm and test the actual joint. Moving towards 4.0 mm can reduce torque if the first trial is too tight.
Can a self-tapping screw go into plastic?
Only where the screw profile and plastic grade are compatible. For a purpose-made thermoplastics screw, start near 80% of nominal diameter and prove the joint on the actual material.
Next step: identify the substrate, drill one test hole from the relevant starting point, then inspect the formed thread before committing the rest of the job. Browse Simfix pan tapping screws by diameter and length.
