I went from a 50cc 40mm cylinder and piston to a 47mm cylinder and piston but now I'm having trouble getting it started. It was pretty simple the only trouble I had was the cylinder bolt pattern was a little different but all I had to do was take a little bit bigger drill bit and drill both sides of the cylinder about a inch down and I also had to make my crank case a little bit wider for the cylinder to go down all the way which adding another gasket took care of that and a couple of brass shims on each side of my crank