An aluminium bracket had to be attached to a curved casting ny countersunk head Allen screws in a position out of line-of-sight, which required bending over to ankle height and fitting and removing the bracket many times.
It was most difficult to locate the key into the head of the screw and also line up the hexagon hole and it often speared out of the fingers and always seemed to fall into an inaccessible place.
The screws had to be pulled up a little at a time, which required many insertions and much frustration until it was discovered by accident that an imperial key of slightly smaller size than the metric hexagon cavity could do most of the tightening, then reverting to the metric size for the final pinch which speeded up the job and removed most of the frustration as it was so much easier to locate into the hexagon cavity.