I am getting ready to have my son tear down a 340 and rebuild it for his small engines class. We were thinking of boring out. How big can we go, 390, 420, 440? Does anyone know if the 340 bore is nickel treated or is it a cast iron sleeve? Can I just bore out my 340 and put in the new pistion? I'm wanting to keep it pretty much stock, maybe just some porting since it is coming apart.