250cc 2 stroke engine swap in a 125 Chassis

Apr 13, 2004
I was wondering if anyone here has ever put a two stroke 250cc motor in a 125cc dirtbike? I am thinking about doing this but I don't know if the motor mounts are the same for both an 88 KX125 and an 88 KX250. I am new to dirtbikes so I don't know much about the differences between the 250 chassis and the 125 chassis. Is the motor the same for both 250cc and 125cc but with a different cylinder, piston, and connecting rod? In other words the bottom half of the motor & trans. So for example, does Honda, Yamaha, Kawasaki, and/or Suzuki use the same bottom half for both 250s and 125s and simply bolt on a 125 or 250 cylinder? Here is another idea: Could you use a 250 cylinder, piston, and connecting rod to replace the 125 motors cylinder, piston, and connecting rod?
Any info would be really appreciated.



Sponsoring Member
Jul 15, 2000
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Why? A modern 250 frame is really not much different than a 125 frame. The frame weights are so close, it wouldnt really even be an issue. You would need to retune the 125 suspension for the added weight of the 250 engine, though.

There really is no way to get a 250 cylinder to bolt to a 125 bottom end.
Feb 20, 2004
you would also have to make sure the pipe would fit. If you are able to weld there should not be much of a problem, because you could just weld new mounts on. It sounds like an awesome idea keep me posted. R.J.


Jan 17, 2001
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The 88 KX125 also only has one radiator and the 250 has 2 radiators. You WILL have a cooling issue if you only use one on the 250. I had both bikes 88 KX125 and an 88 KX250. Wish I still had them both!
Mar 29, 2005
So, how did it work out?

I'm curious about what happened with this idea. I may be interested in doing the same thing with my 2002 kx 125, to put a 250 in it. :aj:
Jul 10, 2016
What's up 11yrs later did that 250cc swap work. I want to redo a 250 or most likely a 125cc bike with no motor tearing it apart and power coating evert thing after I get any monts made if need for the new motor and mostly likely going to change the rims out and street tires of yah super motor street bike custom build I got it check but I think I can run a 2 stroke on the streets in ks but in going to clean and paint every thing and get new forks rims every thing frame off build in my garage also check me out got a YouTube channel going and want to build a super moto and I got a moped I am going to build a 150cc gy6 motor out fuel pump like 5 different sensers Zack #turboed scooter

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