I need to enlarge the mounting holes in a set of cast aluminium wheels
from
5/8 to 17.5mm. I have a new reduced shank drill that I am using in an air
drill (hand held) as I don't want to demount the tyres.

It sounds like might be trying to adapt a set a wheels to a different hub
type. Be careful here, the center hole of the wheel is what locates it
accurately on the hub, not the wheel studs, they just clamp the wheel to the
hub. You need to make sure that the hub center is the same diameter on both
the new hub and the old hub or these wheels will shake and vibrate and
eventually come off.

Paul T.