Finding an aftermarket piston with correct dimensions is always a bit of a crap shoot. Looking at the amount of metal on that barrel pretty much confirms that the piston grew too big for the bore and of course it stuck.
For sure, ring end gap should be bigger on a larger bore. On 4 strokes they say 4 thou per inch on the top and 5 thou on lower. On high performance motors they say 6-7 thou. That would be in the 20- 23thousandths range. But That's unlikely to have been the issue. How much piston to bore clearance did you have to start with and what did the manufacturer recommend ad was that for a water cooled or air cooled motor.
Forged aluminum pistons typically expand more than cast pistons because of the material used and air cooled motors are less temperature stable than water cooled, but the worst possible combination is a forged piston in a cast iron barrel which will not expand as much as the piston. The coefficient of expansion of cast iron is less than half the coefficient of Aluminum. Add to that the fact that the temperature rise in the piston will be much faster than the barrel and it's easy to see why a piston can outgrow its home.
By comparison, an H2 has steel liners but an aluminum cylinder. Remember the old drag race Harleys years ago with all the fins basically removed from the barrels. That is for two reasons. Firstly to remove the "girdle" and allow the barrel to expand and also to reduce heat transfer to the air. They need those barrels to get hot and expand as fast as possible.
Aluminum expands at roughly .000023" per inch of bore per degree F of temperature rise, and cast iron grows by around .000008" per inch per degree. Choose your own temperature differential and it's easy to see why pistons stick.
Interestingly we expect to see a "four corners" seizure when a piston gets hot too fast, but if the guy operating the throttle doesn't know when it's seizing, the temperature will continue to rise as the piston smears itself all over the bore.
So that leaves the question of whether there was enough clearance for a perfectly running motor and did it run too hot, or was there insufficient cold clearance, or should the motor have been warmer before it was subjected to a load - or All of the above?
That is today's existential question.
