They are the same motors. Being here in Japan I have seen them all run side by side. The only difference is basically the head. The worlds II also has built-up piston and sleeve that is supposed to make it last longer. It is not that much of a difference when you hold them up side by side, but supposedly it is supposed to last longer. I already have a couple of gallons on my worlds II and it still has a ton of pinch. I also sucked some dirt into the thing and it still runs like a champ. Second to none if you ask me, but that is my opinion.