351 JE piston weights 244 grams a few more than stock. Bill designed it that way, as close to stock weight as possible.

I even had the piston coated by Swain top and sides, still weighted 244 grams.

Bore is 85.5mm with 61.2mm stroke

I didn't notice any difference between stock and the 351 as far as vibration.

Only noticed the grunt it has now!

Heres some pix

 

The BB 331 is an 83mm bore, same stroke as stock. No idea on piston weight though.

 

Good info to know. I'm inputting it into an Excel spreadsheet. I would prefer to keep the piston weight within 3% of orginal 250, 5% max.

Volume...250........Kawi 300....B&B 331....KK 331.....B&B 351
Bore......72 mm.....78 mm.......83 mm....................85.5 mm
Stroke...61.2 mm...61.2 mm....61.2 mm...61.2 mm...61.2 mm
Weight............................................ .................244 g

So far the numbers. Sorry for all the periods. Quick way to make sure the numbers are more readable and aligned.

 

I can attest too that there's no more noticeable vibrations with the 351 over the 250 slug.

 

I am leaving town this weekend I will give you the actual weight on all but the 340 kit next week.
The 300 piston is actually matches the crank balence the the bike started out as a 300 cc the 250 piston is lighter & has a little vibration so I had the 351 piston made as close to the 300 piston as possable. It came out a little heavier than I wanted but it is close. I could have the piston made the same as the 300 but it would cost quite a bit more because of the extra machining. We havent noticed any excess vibration on the 351 kit.
Bill
bandbcyclerestorations

 

ya, i was going to answer about the piston weights, but it looks like thats been done (close to the same, keep it that way for vibrations). one thing i sawe though was bore vs stoke, and keeping it square. this isnt going to be a problem, the problem you might be thinking of is rod/stroke ratio, but that isnt effected till you change one of those 2. with my back ground of hondas, you might be able to use those changes with cams, but then your moving power around and i like the power band of this bike.

 

Why?

According to the parts list for both the 250 and 300, the balance shaft is the same.
Kawasaki uses the same balancer because these two engine's crankshaft assembly parts are the same. Except for the piston. The weight of the crankshaft assembly is many, many times that of 10 or 20 grams difference between the pistons. Let's say the difference is 50 grams. Less than 2 ounces. The crankshaft assembly, for the sake of discussion, weighs 15 lbs. 16 ounces to the pound equals 240 ounces or 6720 grams. More than 100 times the piston weight difference.

 

There is a point to piston weight as apposed to crank weight.
The piston creates totally different forces as it must accelerate and decelerate first one way and then the other.
This takes energy for one thing and creates vibration.
The crank rotates and therefore weight is important, hence the flywheel.
Adding weight to the piston will increase vibration, its just whether or not it is significant or noticeable.
Adding weight to the crank/flywheel, as long as the weight is not itself unbalanced, will reduce vibration by using inertia to smooth out the firing pulses etc.

But, on this bike its a non issue.
There is plenty of margin to allow for a few more grams in the piston.

 

The rod and wrist pin and rings, all steel parts, all do the same reversing acceleration as the piston, and weigh far more than a few grams difference.

I'm still curious as to the OP's interest of keeping the piston weights within 2~5% of each other.

If a piston weighs 244 grams, then his tightest tolerance of a 2% difference is 4.9 grams, or .17 ounce. 17/100th of an ounce. That's a very small amount of weight considering all that steel flying back and forth.

 

A little weight in a fast moving part can mean a lot of stress. The precentage which I posted seemed reasonable, not sciencetific fact, just my gut feeling.

On a side note, future problems may not let me upgrade. I may have to settle with the 250 for now until next year.