Just about everywhere in the world measures displacement by cc's, except China way over exaggerates these engines for marketing purposes I'd guess but here's the real deal, most engines marketed as 80cc bicycle engines are actually 65.93 cc's with a 38mm stroke and 47mm bore or 69.4cc's with a 40mm stroke and 47mm bore, the 48cc engines are 38mm stroke and 40mm bore with a 50cc being 40mm stroke and bore, then the 100cc engines are similar in having a 38mm stroke and 50mm bore which is 74.61cc's these are the bt and yt versions and a 40mm stroke with 50mm bore is 78.54cc's, and what difference does this make? In the variants with 2mm of stroke difference not much really and the difference between 48 and 78 cc's well it's pretty obvious more displacement makes more torque and therefor useable power in theory but then herein lies the problems of tuners world wide, what do you want to get from the engine, torque monster, screamer or somewhere in between? while displacement with big bores can improve torque output it's usually at the cost of rpm gains because it's harder to turn more reciprocating mass and not do damage to other internal parts such as the wrist pin it's bearing and the big end bearing along with the crank bearings and even connecting rod itself, an engine that has more bore than stroke is referred to as being over square and over square engines are not considered as good high rpm engines mainly due to the afore mentioned but are great torque monsters over a narrower rpm range. The normal 2 stroke is slightly over square and can be made to still have decently higher rpm but again this comes with a cost of losing torque due to having to highly alter port timing and changing the flow characteristics to better suite the higher rpm range. A square engine like a true 50cc with equal bore and stroke of 40mm and lower reciprocating mass on the same weight crank as the larger 66-69's will rev to the moon and get there quick if not over loaded while lasting much longer doing so. It's all about what you want to get from the engine that should direct your research in getting there.