The Sperzel guitar tuners "post housings" are approx .382" this is only .007" above a precision made brad point 3/8" drill bit.
So, I usually find that the best results come from drilling with a high quality 3/8" Brad Point drill bit first.
And then slightly boring the holes out by hand to just slightly over .382" using a tapered reamer.
-OR-
Some luthiers will just use a 25/64" high quality brad point drill bit and avoid using the tapered reamer all together.
25/64" = .390" so it's only .008" over the size of a .382" Sperzel guitar post housing.
This .008" oversize, really wont cause any problems, and will allow for expansion and contraction of the wood.
So perhaps a 25/64" extremely high quality Brad Point drill bit (-IF- you can find one!) may actually be the best choice here.
Also:
I have heard of some luthiers using 10MM drill bit.
However, a 10MM drill bit is at .394"
In my opinion that may be a bit to much oversized (.012" oversized) for a hole that is supposed to be at around .382"
Then again, I tend to be a bit "over precise" about these kinds of things sometimes!
