If two set screws are placed Diametrally opposite they will drive by the contact of their ends only, and not by reason of their inducing frictional contact between the bore and the shaft.
"Modern Machine-Shop Practice, Volumes I and II"
Joshua Rose
To have made the left-hand crank lead, when the engine runs in the direction of the arrow D, we should, supposing the keyway F to be already cut, have to cut the keyway E on the directly opposite side of the shaft; or, what is the same thing, supposing the keyway E to be already cut, the keyway F would require to be cut on the Diametrally opposite side of the shaft.
"Modern Machine-Shop Practice, Volumes I and II"
Joshua Rose
Diametrally opposite is another split passing partly through as at b.
"Modern Machine-Shop Practice, Volumes I and II"
Joshua Rose