Hey, I was thinking about putting a 250+4t engine to an yzf-r125 body. Has anyone done this? What engine would fit? What do I need to change? How much modding is needed?
Hey, probably going to do the same thing with ktm rc125 and rc390/250. Doing it because of my age(age 16 can have a max 125cc moto, 125 frame so it wont be reqocnizeable). RC125 and 390 have the same frame(apparenrly) same brakes, etc.I think the main issue you're likely to face with this is not so much whether the engine will physically fit but that then the other items around the engine (frame, forks, brakes etc) will be starting to get to their structural limits once you start adding that much more power + weight.
My bike is only a tuned 180cc and already she's feeling kinda under-braked and that's only adding ~10mph. With an even bigger motor you would face similar issues. You could arguably fit a later fork or a fork with a bigger disk but then you're getting into pretty extensive (and expensive) mods.
There's a (french i think) R125 on youtube which is running a 250 cylinder (with extensive casing work to make it fit) and around 35bhp (apparently) so it's possible, but be prepared for exponentially increasing sketchiness, expense & complexity as you head that way.