what size drill bit?

[petemoore]

For drilling holes for jacks [Switchcraft or RS]? 1/4'' Phone...
 My good bit snapped, I threw the shank away without looking to see what size it was...dohh...

[Samuel]

I use a 3/8".  <--Decided upon by the careful careful process of holding the shaft up next to the jack.

[JohnM]

I think a Unibit type of bit is almost mandatory.  A bit expensive, but worth it.  If you don't know what it is, go to amazon.com and do a search on Unibit 10234 to see what it is.  I think drilling a small pilot hole (1/16) helps, too.