Mine needed a 3mm allen key. They seem to have loctite on them, one came out fine the other the flats were a bit damaged and the first 3mm allen key I tried just turned in the screw I tried another 3mm key and it came out OK, put back in with copper slip.
Oops, replied in a hurry and mixed up the cambox cover bolt and the headlight screw
It is 3mm
