It's not so much that the rod is better, it's that the big end bearing being much, much better. The rod is 2mm wider and 1mm longer (on a 250, I haven't done a 490). You need to machine 1mm from the inner flange of each flywheel and use a thicker base gasket to allow for the different rod dimensions. The Yamaha YZ J rod kit is also shitloads cheaper, or at least it was back in the 90s when I last did the conversion. Because of the machining of the flywheels, you can't go back to a Maico rod later on. You're stuck with a Yammy rod for life but so what? The Yamaha rod/big end in my 350 lasted from 1992 to 2009...not bad.