Primarily used in narrow spaces that are difficult to reach (e.g. in machines or vehicles when installing engine envelopes), because the head need not be reached from the outside during installation. To install, use L-shaped screwdriver-like tools or small bending tools, called bits.
Full and partial thread versions are available. In function of the internal drive, there are several types: hexagonal drive, TORX drive (the tool and the screw do not meet at the corner points of the hexagon but on the full plate surface). The internal drive screws are made of steels of great solidity. The screws listed so far are normally manufactured with special screw machines, nowadays usually without machining.
Categorization based on material or material quality
- toughened steel
- anti-corrosion steel (A2)
- acid-resistant steel (A4)
Most common types based on quality
- 8.8 screw
- 10.9 screw
- DIN 912 – cylinder head screw
- DIN 6912 – low head, internal drive screw
- DIN 7984 – low head, small drive screw
- DIN 7380 – hemisphere head, internal drive screw
- DIN 7991 – recessed head, internal drive screw