which one of these pistons would i be better off get for my buddys 400EX. we want the stoke reliability. will it give a notiable amount of power gane.

 

10:1 isn't much over stock and I don't think you will feel any difference. Go with 11:1 and make sure you use the steel head gasket. The fiber gaskets are about .023" thicker, causing less compression. I read where Dunlap doesn't run a base gasket with this setup and with all that is only a little over 11:1. 11:1 you should run about 100 octane (mix 50/50) pump/VP red or VP C12. I am running the base gasket, but I have also had my cylinder deck 010 and my head decked 010, so I need it.