On Fri, 19 Feb 2016 22:08:11 -0500, Bill Baxter wrote:

Unfortunately, web searching cannot locate a pinion of the proper bore
size (all are either 2 or 3mm bore), so I am thinking of getting one of
the 3mm ones and drilling it out to #21 sized. The problem is that this
needs to be very precise and if off by even a little will cause binding
and other problems in the focus mechanism. So I pose the question: How
would I go about drilling this out from 3 mm to #21 accurately using
ONLY hand drill, vice, etc as I have no access to lathes, presses, etc?

Well, hand drill would be hard. You could go in steps; #21 is .1590" or
4.04mm; 3mm is around #31, so one way would be to use a bunch of
intermediate drills that shave very little in each step.

If you have a lathe, I'd use a 4-jaw chuck, indicate on the existing
hole, and bore it out.

BTW, are you sure it's not metric 4mm?