Counterbore.
Get one with a pilot the size of the existing hole, to keep the counterbore on-center. That can be the existing 6 mm hole, or the 8 mm hole you will drill it out to.
On some, the pilot is ground from the same chunk o' steel, where others have a hollow that lets you insert different pilots, which is more versatile. Mine is the former type, made by Silva for bike frame builders, but I got it ~30 years ago and I don't know if they're even still in business.
The through-hole (where the pilot goes) is nominal 8 mm, the head of the nut is nominal 10 mm.
If you have trouble sourcing one with metric dimensions, you can get by with fractional-inch as follows:
pilot 5/16" (7.9 mm)
bore 13/32" (10.3 mm)
For using a 5/16" pilot, I would drill the hole 5/16" also, so the pilot fits snug in the hole, reducing chatter. If the 8 mm nut then is too snug a fit, you can open up the hole a little after counterboring. You probably won't need to though; a nominal 8 mm nut will usually go in a 5/16" hole no problem.
The 13/32" bore, being a little large, may not look quite like how they do it in Milan, but it'll be 100% functional, and the extra clearance can even help on holes that get painted (or PC) after drilling, i.e. most any bike that comes with recessed-nut brakes.
I think once you see the prices on good counterbores (roughly the same as what an '85 Super LeTour usually sells for), you may start looking for cheaper alternatives. A normal 13/32" twist drill (preferably the short-length variety for rigidity) will do a half-assed job if it, but I've seen worse. It will leave a conical bottom instead of the prefferred flat-bottom hole, but it'll hold your brake to the fork anyway, and no one will ever know the hole was made by a drill. The drill can also be hand-ground to a flat-bottom pilotless counterbore, which will work OK if you can clamp the fork accurately on-center. Best used only to flatten the bottom of the hole after drilling it to 13/32" with a regular drill. But that's a lot of work just to turn that little bit of conical shelf into a flat-bottom shelf. The shelf, (bottom of a 10 mm bore with an 8 mm hole through it) is only 1 mm wide, so the difference between flat and conical is trivial.
Mark B
Last edited by bulgie; 12-22-21 at 07:24 PM.