My son, who is in graduate school, uses a high quality but no longer made microscope for his home studies. Recently, one of the small brasspinions broke in the focusing unit and he asked me if I could repair it. I was able to disassemble to access the pinion which is on a rod. Unfortunately, since the scope is no longer made, the exact part is no longer available, but I have determined that a 64p 16T 0.4 modulus would fit it almost exactly except that the bore is too small (it is 3mm and I need it to be 4mm... actually a bit wider at #21 drill bit sized). 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? Thanks!