long story on the engine . First one they shipped ups ,was a 150cc and damaged in shipping . So i either had to pay for another or get this one back to them ,before they would sent the right one . because it was damaged had to wait on ups to do a damage claim before i could send it back . that all held me up for two weeks . the day it got back to them ,they shipped the right one ,. part # on the first engine was also fml163 ,so i don't know if there really a difference . install was no problem at all . wish i had known if a 250cc would fit ,since it had to be replaced anyway . i did some searching and couldn't find any 250cc swap post . i played it safe