I have seen guys use a ball anvil micrometer and measure rod bearing shells to .ooo1 and mix and match to get nervana.
Randy, here is a toolmaker tip. You do not need a ball mic. Take a 1/4 dowel pin, mic that and you should get a.250 minus .0001 or 2.
mic the dowel and bearing and do a little subtraction. Working in tenths (toolie speak) requires extreme cleanliness, thermal control,and a whole bunch of patients. When I have to hit a number I set a gage block stack and double check my mic's to the stack.
The only place most people see two tent(h)s is camping.
David 89 turbo cab