Down thru the years China bike engine factories have always advertised their biggest engine as 80cc 

for that reason, we have elected to listed them the same way. Also by western standards the volume of the engine head combustion chamber is not figured in the engine’s displacement rating, nevertheless some Chinese factories do include this volume in their engine displacement ratings therefore making the engine appear bigger than it actually is.

However, the bore and stroke of an 80cc being 47x38mm only equals 66cc. 

To figure displacement for this example:  47mm bore x 38mm stroke; we use a standard formula to figure an engine’s cc displacement; ( multiply 3.142 x radius of bore mm squared x stroke length mm divided by 1000. )

Bore diameter divided by 2 = 23.5 mm radius 

To square radius multiply 23.5mm x 23.5mm = 552.25 radius mm squared

552.25mm x 3.142 = 1735.17mm

1735.17mm x 38mm = 65936 cubic milliliters;

To get cubic centimeter divide 65936 by 1000 = 65.936cc or round off to 66cc.

As a result we see there is no such thing as a 80cc:

Gen-II

Gen-II-A

GT4