Originally Posted by Bill Kapaun
8mm = .31496
5/16 = .31250
Difference is less than .0025"!! It doesn't make sense that one is too big and one is too small.
Sounds like something other than an Allen head.
Thinking about it a bit more, my guess is:
It's been "buggered" by using too large of a wrench. Try LIGHTLY tapping an 5/16" into the hex.
Else the OP has got his wrench sizes mixed up.
Could it be a Brit bike with the old Whitworth? size? It shouldn't matter though.