IDK...even if it is, the choices will be very limited and we all know what that means. 49 & 72cc heads (6v flavor) are easy enough to find. For that reason, alone, I'd opt for stock 49cc, OEM 72cc or aftermarket 88cc configurations over what it has as-received.
Also pictured is a 70
piston next to a 65, the 65
piston squishes into it's head differently than the 70's into it's,don't understand what the advantage/disadvantage of one over the other is or why they would add another machining step to the newer/more popular model. Any thoughts on that Bob?
The bigger the piston dome, the bigger the impediment to smooth, rapid, combustion. Efficiency suffers, big time....just not as much as it would if CR fell too low. Can you imagine the lawnmower-like power curve the would result from, say, 6:1? Ideally, a flat top, or dished, piston allows for unimpeded flame front propagation, depending upon port flow characteristics and combustion chamber shape + volume. Dished pistons have come on strong over the years; having that squish band forces the air:fuel charge toward the center without interference. It's one parameter that engineers have used to increase efficiency and reduce octane requirement.
Going by the running changes we've seen over the years (decades), it appears that Honda engineers did a lot of ongoing R&D. The running changes have been subtle, but significant. The 12v bikes not only got CDI (a big improvement all by its lonesome), they also got a longer connecting rod(which improved rod/stroke ratio...in terms of higher rpm) and a revised head (smaller combustion chamber) paired with a smaller piston dome (more efficient combustion) with the impact being increased efficiency, including more top end power. The new-gen motors have retained the long rod configuration...and gone to an undersquare configuration - small bore, very tight combustion chamber (less valve shrouding) plus dished piston to maintain reasonable (~9:1) static CR. It's safe to assume that they also included port design, to induce swirl and tumble inside the combustion chamber; the result being engines that "punch above their weight class". By extension (working backwards through the timeline) the S65 was just the first iteration of what became the 72 & 89cc (C90, not sold in North America) motors. They started with a large combustion chamber, to allow large valves & ports, that required a big piston dome to get reasonable static CR...a system/balanced assembly approach. It was a decent first iteration of what would become the most successful engine family in history. That they continued refining/improving the basic design is, imho, classic Honda. The short answer: "they had to start somewhere"...
