Allen Key Imperial

An imperial Allen key is sometimes your last chance of rescuing a grubscrew whose hexagon socket has given up. This Allen key is only one tenth of a millimetre larger than the standard metric key, but in most cases it will successfully loosen a screw where the socket has been damaged.
Heat up the Allen key before you apply it, and the heat will help to ease the grip of the thread-lock fluid. Your imperial friend will usually solve the problem; if not, drilling out the screw is your only remedy!
Imperial Allen key for removing damaged M 3 grubscrews