any 40mm piston and ring set should work. So don't get too picky. Just check the ring gap. I'm actually using a qt50 piston and rings on this Express. The qt50 piston doesn't have that stupid divot in the piston crown like the stock or stock-like Honda piston does. And the qt50 piston comes with better circlips for easier installation instead of the "C" circlips. Amazon sells a bunch of scooter 40mm pistons which will work as well. They are a little bit taller from the digwell to the crown. Just install and check where the piston is at TDC and maybe add a thicker base gasket or a second base gasket if needed.