[RC-Monster Mike]

The 52t spurs are a little tighter (not really a bad thing, but it is more difficult to install) - here is a quick suggestion, though:
Replace the bushing on your 46t spur and take the slightly used Teflon bushing from the 46t spur and use it on the 50t spur. This will work like a charm. :)

Also, if you are measuring .1mm - .3mm out of concentricity on the inside of the gear, I suggest you get a new caliper - the blanks were turned on a cnc Haas lathe and should be concentric within +/- .02mm - no way they are out of round by .1mm(maybe on the 46t gear, which is cast, but not likely on the cnc'd 52t spur).