I need to loosen M2 hex head screw. None of my Allen bits fits. 1.27mm is too small, 1.5mm is too big, I really don't feel like sanding a 1.5mm bit down to fit. I am not aware of any intermediate size.
7/128 = 1.43mm Not sure that SAE is made.
This is where a high quality item made to tight tolerances is needed. I suspect yours is not, or the head, is buggered up. Probably 1.5. The other posts are good also.
Bondhus ball types are all I use.