you have an old frame that you are trying to fit a modern fitting brake to.
My suggestion would be to get an old brake style that has a bolt that will go all the way through the fork crown and fasten with a conventional bolt.
Drilling out forks seldom goes well unless you have a pillar drill and some tough bits, even then you won't get the exact fit a modern countersunk bolt needs.
Good morning,
you have an old frame that you are trying to fit a modern fitting brake to.
My suggestion would be to get an old brake style that has a bolt that will go all the way through the fork crown and fasten with a conventional bolt.
Drilling out forks seldom goes well unless you have a pillar drill and some tough bits, even then you won't get the exact fit a modern countersunk bolt needs.
Have a look at these :
http://www.ebay.co.uk/itm/Vintage-Weinmann-Type-730-Brakes-/180792282390?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item2a180f4d16
or these :
http://www.ebay.co.uk/itm/WEINMANN-TYPE-500-BRAKE-CALIPERS-SINGLE-PIVOT-BRAKES-VINTAGE-/190625390630?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item2c6228a026
and you will see the different fitting for the bolts.
Pop round to your LBS, they are bound to have one lying about in the back shop or call Ivor and ask him.
regards Allister