Hepolite pistons are heavy. I have seen a lot of old BHB brand Norton pistons that were much lighter than the Hepolites.
I weighed a set of .060 over Hepolite 650 pistons with pins and rings and they came in at 329 grams. A standard 650 Hepolite came in at 299 grams, but a BHB 650 stc. with rings and pin came in at 280g.
A real shocker is that I weighed a std. size BHB dished Atlas piston which was original to an early Atlas engine I have in the basement, and it came it at 317g, lighter than the .060 650 Hepolite, this would from a balance point of view convert the 650 in which it was installed into an early Atlas!
It is noteworthy to mention that all the early Atlas BHB pistons I have on hand, four, have the nice taper-wall wrist pins, while the Hepolite Commando pistons have heavy straight-wall pins.
The Commando flat top Hepolites that so many have plugged into old Atlas bikes are heavy at 353g in .040 os. A set of domed Commando Powermax racing pistons with taper wall pins came in at 356g.
IF you look at the old road test of the 1962 650ss in the english Motorcycle publication the tester complained about how the bike vibrated between 3500 and 4500 rpm, right where he liked to cruise at. My old 650 Manxman which was all original with 5000 miles on it when I got it started to vibrate towards 4000 rpm and never seemed to get any better as the speed went higher, which is why I geared it up so it would do 70mph at 3500 rpm.
With this accumulated data it looks like there may be nothing you are going to be able to do with modern replacement pistons to get the bike much smoother than it was when it was new, which may have been more smooth than a period Atlas, but not as smooth as the old smaller displacement 88 and 99.
I tried to save the old BHB pistons to re-use on my 650ss but they were well worn and had old rings stuck solidly in some parts of them, I threw in a set of used stc. Hepolites that were in pretty good shape with some new Hastings compression rings and it is running very well.
BHB pistons are just about non-existent. Aside from the std. ones I have taken out of old original engines, I have only seen a small handful of new ones over the years come up for sale. It seems Hepolite was much more popular in later years as a replacement piston and eventually as an oem piston.
If I had a bored oversize 650 Norton I would make sure that I have the taper-wall piston pins to use in it and I would drill a few holes and dimples in the piston off the thrust faces to get it as light as I could. A really sharp racing machinist could probably put a taper in the wall of straight wall wrist pins, but unless you found someone who had previous experience doing exactly that I would not trust them.
Jim Schmidt engineering has offered very light Carrillo/JC rod and piston packages for the Dominator in the past, but I think they are special order which would probably eliminate most of the vibration a 650 Norton has, but you will be looking at quite a bit over $1000 for the package if it is available at all. He would probably also balance your crank for you no matter what parts you decide on. There is no one that will not agree he is one of the top Norton engineers in the country.
In the early 1960s John Gregory was one of the earliest and foremost tuners of the Norton 650 in the country. He raced Model 88, 650 and 750 Nortons at drag strips, on the street and on road race tracks. He told me he balanced all his engines to 78% dry, which would be very close to 70% wet. This is what I shot for when balancing my 650 with it's std. Hepolites and it seems good enough. I have no tach on it right now but when I get some data on how it behaves at different rpm and speeds I will certainly share it.
1962 650ss road test here:
https://www.facebook.com/photo.php?fbid ... =3&theater
John Gregory's secret Norton 650 tuning notes here:
https://www.facebook.com/photo.php?fbid ... =3&theater
P.S. Heinz Kegler, the head technician for the U.S. Norton distributor Berliner and a rabid Dominator fan, recommended 65.5% balance factor for the 650 Norton. Unfortunately he is dead and I can not ask him if that was wet or dry, my guess is dry, but it is only a guess today.