RE: Zenoah G26 rebuild questions?
The gap? I'm not sure at all. When not under tension, about a mm? Ane installed, maybe less than half a mm as teh ring expands to meet the cylinder walls? That is a good question, I never bothered to measure that gap, I just installed teh piston into teh cylinder.
As for a piston, I think you buy the head and piston together as a unit. If ou can buy the piston separately, make sure you buy the one with the correct diameter for bore of your cylinder head.
The bore of teh Zen 23cc is 32mm. The bore for a Zen 26cc is 34mm. So for you engine displacement, go for the respective pats that would fit teh respective diameters.
As for teh Chang Yangs, I think they are dimensionally cloned directly off of the Zen, but before you take my word on that, be sure to research it yourself.