Had time to read your B4/5 log, i d'nt get to use my PC often so miss things, its most interesting and gets the brain ticking. My bro races lambrettas lol, they bore n stroke all the time and use thimble type spacers to take the small end pin size out to fit the bigger pistons, they do it as standard with no problems, so thats one way as you have mentioned to get round the 14/16mm piston problem, your primary/clutch gear is a problem that can be overcome with new gears etc hope you solve the prob, could the clutch off a B4 and a GSX be split and rerivited to make one?. But the big problem i see is that the cranks are flipped having the ends opposite, on a race bike you should be ok, but is yours for the road as how will you fit the signal generator and the gene flywheel, its possible to machine the long side down but how can you add it back to the otherside? I hope it is possible to do, keep the post going, i run 59mm already and have two other VVM motors as spare so will have a go myself if it is a possability. Ive always fancied an RF900 motor as the top end is quite narrow and ive already got the spare frame to chop!!!