Self-drilling (self-tapping “TEK”) screws drill their own pilot and cut their own thread in one drive. The two things that go wrong are the point burning out in thick steel and the head corroding.
Match the drill point to the base metal
Self-drilling screws are graded by point number for the total thickness of metal they can drill:
- #1/#2 point — thin sheet, up to ~2–3 mm total.
- #3 point — up to ~4–5 mm.
- #4/#5 point — structural steel, 5–12 mm.
If the point is too small for the steel, it work-hardens the hole and stops cutting. Into aluminium any point drills easily, but keep drive speed moderate so the flutes clear swarf.
Thread type
- Fine thread for steel-to-steel and thicker sections.
- Coarse / hi-lo thread for aluminium, timber sub-frames and soft materials — more grip per turn.
Head and washer
For anything weather-exposed use a bonded EPDM sealing washer under a hex or pancake head so the hole is sealed. For sheet cladding, a wide flange or a top-hat washer spreads the load.
Coating and corrosion
- Zinc-plated — dry internal only.
- Ruspert / Climaseal / equivalent multi-coat — external fixings, good salt-spray life.
- Stainless (A2/A4) with a carbon-steel drill tip (bi-metal) — coastal and long-life external work; solid stainless will not self-drill steel.
Do not mix stainless screws hard against bare aluminium in a wet joint without isolation — galvanic corrosion.
Tell us the two materials, their thickness and the exposure, and we will match the Point screw and washer.
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