The cylinders, once produced, are verified and according to the final verified measurement, 
they are selected in "A", "B", "C" usually "A" is the smallest (nominal measurement)
 "B" is 0.01mm larger and "C" is 0.02mm larger than "A".

For this reason pistons of different sizes are also manufactured by "A", "B", "C" 
to be able to assemble the piston with the most suitable clearance in each cylinder.

So if the piston or cylinder is "B" we always recommend using piston "B". The original pistons are marked on the piston dome, and most manufacturers usually stamp the cylinder just at the bottom of the cylinder back ledge (just above where the reed cage is).

Modern cylinders with Nicasil do not wear out if the use is in normal conditions,
but if the cylinder is very worn (this happens when dust enters the engine) 
in this case we recommend getting a new Nicasil coating for the cylinder, but if not: 
you can mount a piston one size larger to "compensate" for cylinder wear.