Originally Posted by
bulgie
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
Well, this is a one time deal. IOW's, it's the only frame/fork I have that even uses road calipers. Everything else I have is canti'd. So, if I can find a counter bore bit cheaply enough, even if only stays sharp enough to do this one job, I'll be satisfied. I'm seeing some piloted bits on ebay for not much money. But, I'll see if Harbor Freight or Northern Tools also has something. There again, hmmm...I may have to buy a set...which would be pointless.
Failing that, chopping the end off a normal bit may be worth exploring. Cut it off with a dremel tool and cutting disc. And, go slow on the fork...like you said.
I've done the regular drill out of a fork crown before leaving the nut head exposed on another frame I had years ago. Meh...looked fine and it's not like trying to get the full recessed effect is direly important this time around. Just thought it'd be nice since I'm going to blast the old paint and nominal rust off and repainting the entire frame/fork. You know....get the nice detail of matching front and rear brake holes to top it off.
BTW, do you think some 3 in 1 oil would work for cutting oil? Is that even important?