Can we talk about "center bore" for a minute?

There has been a lot of talk here about hub centering rings without any dimensional data to support their use. So how accurate is a plastic ring? So far I have seen numbers kicked around like 5 to 8 mils being good enough. Two diameters on a ring would need to +/- 2.5 to 4 mills each to make that. Seems like that is pretty tight for a piece of plastic, particularly if it is molded.