Arvid Nilsson 5085204000 external serrated lock washer (fan washer) to DIN 6798 Form A, for M4 bolts, zinc-plated steel. Inner diameter 4.3 mm, outer diameter 8 mm, thickness 0.5 mm. The external teeth bite into the bearing surface to resist loosening under vibration. Manufacturer's own designation: Arvid nilsson 5085204000 D6798A 350HV-EL 4 (4,3x8x0,5) external-tooth serrated washer.