Does a JE 12:1 std 85mm high compression piston add power to a 06 400ex? And will it effect the reliability of it at all? Also am i able to make the std bore from 85mm to 91mm without doing anything else beside putting the piston in?

 

Yes it will add power. Yes it will probably effect reliability a little. No 91mm piston will not fit in a 85mm cylinder without getting it bored.