Sizing Pistons

[Paul Jones <nrg@domain.elided>]

Hi,
    I am presently reading alot and working out how to rebuild an
engine, i am presently having trouble trying to figure out what size
pistons one would order.
    For example if i was rebuilding a 2 litre Beta engine which has a
84mm bore, would I then ask for a 84mm piston, or a piston to suit an
84mm bore, or is there a special sizing when the actual width of the
piston has to be calculated?
    Also if i was increasing the bore size by theoreticaly +0.4,
would I
then just ask for a 0.04 oversize piston?
    Any help on these questions would be much appreciated.

Thank-You
--
Paul Jones, aka Nrg.
nrg@domain.elided