6v barrel

Waynus, do you know if a std 6v barrel, 47mm, will bore out to take a 51mm piston? VMax

Don't think so...
I guess (not 100% sure) that the sleeves oustide diameter is smaller as the 51mm barrels.
I know for sure this is the case with a 52mm barrel, so I imagine it to be the same for the 51mm.
Anyways, it's often smarter to get the top end from the same manufacturer.
Because they all have different tolerances.
Example:
Takegawa piston: 52mm, barrel: 52,03mm
Kitaco piston: 51,97mm, barrel: 52mm

So make sure you don't end with a bored barrel which is a fraction too small or wide.

Thanks Waynus, was hoping someone had already tried boring out a standard barrel, a 51mm alloy barrel is £75, plus £88.50 for a piston. Will put a quick road cam in and an 18mm carb and see how much difference that makes first. Vmax

It will make a big difference if you run the correct piston, with a OT70 head.
(Keihin PC20 is the way to go)
If you have the stock 100% Honda OT oil pump, you might want to upgrade this to >220% (don't forget to drill out the oil channel)
Oh, and the alu barrel only has a small advantage (and disadvantage as well!) compared to the cheaper iron version.