What is the exterior diameter of a recessed brake nut?
 

I'm considering drilling my rear brake mount on my old raleigh to accept a recessed nut. yeah, i know its not strictly necessary but i think i might get a better fit this way. i already drilled the front and i'm very happy with the results.

actually i'd be more interested in the diameter since that'll match my drill bit.

It's 8 mm. A 5/16" in. drill is close enough.

Remember you only want to open up the front face of the rear brake bridge, not drill all the way through. It's tricky since you can't get a regular drill in there as the seattube blocks it. A short drill bit held sideways in a Visegrip and rotated a fraction of a turn at a time will eventually get it done. And, yes, an 8 mm or 5/16" is the correct size.

yes, thats exactly the plan

anyone have a suggestion on the make of drill bit to use? since i'm drilling into steel i assume i'll want something of higher quality.

Any decent quality drill bit should work since all but wood bits are intended to drill steel as long as it isn't extremely hard and bike tubing isn't. A new, sharp bit will make the job go faster.