Some people buy a Norton commando and make it much slower by fitting a single carb to itWhat is the point of buying a slow bike and making it slightly less slow?
The b25 motor really is fun and fast (between rebuilds)That's interesting to know. When I first asked the question I was looking at buying a BSA 250 green lane bike but have since aquired a 1959 Royal Enfield 350 Bullet/Clipper. These can be taken out to 612 & even bigger, seemingly without problems.
I think you mean a 71mm 650 Bonneville piston?On you tube there is a B 25 conversion 300 cc by Michael Waller. I did something similar from 250 cc to 320 cc by using an old Bonneville 650 big bore piston which is 67 mm. (60 grams heavier). And I used an 850 Norton sleeve. I drilled crank 4 x 5/16 holes to make the same balance factor as a 250 piston. It runs very smooth, nice low end and lots of power. I'm using also a 32 mm mikuni carb. It still really wants to rev and it hasn't blown yet