ST3.9 self-tapping screws
sheet-metal (ISO 1478)
3 documented uses, taken from manufacturer catalogs, service manuals and teardowns.
Pilot hole
3.3-3.5 mm in 1.0-2.0 mm steel
Too tight and the head snaps or the boss splits; too loose and the thread strips on the first removal.
Buy ST3.9 self-tapping screws
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Watch out for
- Non-preferred size in some regions; ST3.5 or ST4.2 are more common alternatives.
- Most TV, appliance, HVAC and power-tool service documents list only a driver type or a part number, never a thread size - so a screw kit sold as 'fits brand X' is usually a seller's guess, not an OEM spec. Measure the old screw before buying.
Where ST3.9 is used
Volkswagen Golf Interior
A-pillar trim and upper door lining screws
20-25 mm · confirmed · car interior trim
View sourceVW Panel Assembly
Mounting interior panels and noise insulation
25 mm · confirmed · car interior trim
View sourceDaikin Inverter Pair (FTXS-L)
Securing the front grill and service panels
10-16 mm · confirmed · HVAC ducting
View source