Since I had a little spare time...
A small correction for the machinists out there. The actual specification in inches is not some of the numbers posted above.
The specification for ball diameter as stated in the rules (52.5 mm) is 2 17/254 inches exactly. That is not perfectly represented by the long decimal above, which is about 2 1/15. Closer is 2.066929134, which is correctly rounded in the last place shown.
The traditional pre-metric specification, which is still the number used by some retailers, is 2 1/16 inches. That is 2.0625 inches exactly.
It is clear that the metric specification is roughly 0.005 inches larger than the traditional spec in inches. If they had chosen instead 52.4 mm that would be 2.063 inches very nearly, and slightly closer to the traditional size.
Any complete specification says how far off from nominal the item can be and still remain acceptable. In the rules of snooker, that tolerance is +-0.05 mm. That is very close to 0.002 inches. This means that a ball that is exactly the traditional size of 2 1/16 inches is small and out of spec by about 0.0025 inches.
The fractional size mentioned above, 2 1/15 inches, turns out to be within spec since it is small by only about 0.00026 inches. So, I suppose you could say that the spec in traditional units has changed from 2 1/16 inches to 2 1/15 inches. Or, you could amaze people by quoting the 2 17/254 number and win bar bets. You could also win a bar bet that 2 1/16 is out of spec if you can find a traditional person who thinks in inches.
