[(F5)]

I've had to gently file a little bit from ends of ring. Have to be super careful in vice so ends supported.
Piston is shaped like that so when at operating temperature the top part which gets a fire lit next to it and has bulk of the material expands to same size as the bottom.
To be precise you really need to measure with a bore gauge and know how to use it.
But largely the difference is real small and another A would be fine. Might get away with a B.
Best would be to Diamond hone from a specialist and get them to measure but plenty of people don't bother.