Most are engine specific. GY6 uses the 18X14 size and the other sizes you've seen would fit Yamaha or other makes of scooter engines. Yes, the gram weight is the adjustable part. (heavier is slower at take off with more top speed, and lighter = more take off speed with loss of top speed)
However, there is a larger size that is also used in certain application variators for the GY6. Like this one
http://www.ebay.com/itm/SCOOTER-150C...63609f&vxp=mtr
It uses a 20 X 17 roller and is used on the Gy6 engine.
So long story short, if your variator is stock or uses most of the common aftermarket variators it's 18X14. But if it's and aftermarket variator, it MAY have different sized rollers and you would HAVE to use that size in order to work correctly.
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My Yerban assault vehicle: 2.2mm stroker crank, 62mm Nikasil cyl, Taida large vavle head, American made valve springs, ported intake manifold, TM 28mm carb, stock CDI, Bando coil, TK exhaust, stock CVT except for the 14g sliders, 13/40 internal gears, 16T drive, 31T axle sprocket, 22" rear tires, and ONE BIG POTATO CANNON mounted on top..........
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