Due to its stringent size tolerance, hexagon socket head screw is very likely to cause awkward situations where the hexagon wrench can’t go into to socket head or strips during the insertion. M2.6 requires very high precision. Particularly when it comes to electroplating, the width of coat on the hexagon socket is very likely to cause the problems mentioned above, so makers usually don’t want to produce such screws. (…)
Japanese Nagai Seimitsu Company Solves Problems of Hexagon Socket Head Screws | Fastener World