b18c5/b17/b16 have just under 43 cc combustion chamber
b18c1 has just under 42cc combustion chamber
b18a1/b1/b20b/z have just under 46cc combustion chamber

all of this is an average of everything i have gathered and measured myself.

the difference between b20z and b20b are:
b20b has p3f pistons and p75 head
b20z has phk pistons and p8r head
the only difference is higher compression piston and bigger valves in the b20z.