08 yz 250f blown motor swap to 450f.

hey guys. i have a yamaha 2008 yz250f that i currently blew my motor. the motors shot and is not repairable. i am looking to drop a 450f motor into my 250 frame. After a little bit of researh i have discovered that the 450 motor will fit into my frame but my issue is the 250 electrical such as the c.d.i unit or magneto and other parts will not support or give the 450 motor enough power to run. I’m not a total expert on doing this and need some guidance on where to go and what i need to get to make this swap doable. if anyone has done this before and can help me it would be greatly appreciated!

I'm just guessing here but I'd bet you would need all the electronics from the 450

You can go to any of the many online parts vendors that have OEM parts breakdowns and see if the part numbers for the electrical parts match between the two models. You might be surprised. I doubt the cdi will match, but the coil and other stuff very well might. (This is assuming both are kick-start & carb).

On a different note, what makes you so sure that the 250 engine can't be saved? Did the connecting rod fly out the cases? I've brought more than a few unrepairable engines back to life. Usually I find unrepairable really means I can't afford it, or I want an excuse to get rid of it, or I don't understand what's wrong.

Buy all new oem 450 electrical except like kill switch and stuff like that

I'd be very surprised if it was a bolt-in affair, but I'm sure you can make the 450 fit with a little work. I'm not sure if you'll be able to service it reasonably though - pretty sure it's taller, so the frame might be in the way of the valve cover coming off. 


I imagine some of the electronics are compatible, others almost certainly aren't. The wiring harness from your 250 cross references to 06-09 250s only. The CDI is different than a 450 too (rev limiter is lower for one, also a different spark advance curve I imagine). Stator/rotor appear to be 250 only. Ignition coil appears to be 250 only. I agree with above that you need pretty much the entire 450 electrical system. You'll also want the 450 carb, the 250 carb is smaller (although you might be able to use it, you'd have less top end and need to do some tuning).

This would be an interesting swap, I am curious if there would be a weight savings. 


PS I think I have all of the electrical components off an 08 450 in my garage, shoot me an offer. 

I doubt the '06-'09 engines will work out.  Their cylinder orientation is more vertical than the YZ250F.  OTOH, the Gen 1 '03-'05 engines are almost indistinguishable from a YZ250F; same oil lines and cylinder tilt. 


As far as electrical, the generator assembly is unique to the YZ450, and the CDI needs to match the engine, but outside of the stator connectors, the rest of the system should be compatible. 

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