well...say for example you want to build a 350 chevrolet motor. stock they have a 5.7 inch rod. lets say you want to use a 6 inch rod and still have a reasonable c/r. if you look in jegs or summit you will see pistons that are designed specifically for that application. hence the revised pin location. the reason for the longer rods is to slow down the piston speed and to keep the piston at top dead center longer!! make any sense???