my b2 piston is diff. then the b2....
as far as I know u can swap everything out between the b5 and b6 but I could be totaly off as I have never needed to swap anything over between the 2

but Ron will chime in on the do's and dont's...
I know carbs,cranks,back plates buttons, heads,rods....all interchange but dont know about the p/s and crank case..
hope this helps