first off, any comments similar to 'why?' or 'its cheaper to buy new/used instead' don't bother. im going to start building a 14-15 yz450f from scratch. my first question, would a 2010-2013 motor (namely cylinder) work in a 2014+ frame with the different exhausts? i already compared part numbers, they are different. but the numbers i found were for head assemblies where the valves and such were included. so i am not sure if the numbers are different for something small that wont matter, or because they are drastically different. i have owned a 2010 previously. loved the motor. but rode a 15 and loved the handling more so. i just dont have either bike to sit side by side and look to see how they compare. and yes, obviously if i come to the conclusion they are different, ill simply go with the 15 motor.