Can I put a YZ400F motor in my 250F frame?

Ok, I may be buying a YZ400F motor with all the other related engine items. The idea is to use this motor in my wifes street bike -- basically a GP style racer.

But, it may more more sense for me to put my WR250F motor in her bike and use the 400 in mine.

Will the 400 motor fit the WR250F frame?


Sure, got a saws-all? Ego has a flame thrower he might lend ya :)

Unfortunatley, you wont end up with the bike your shooting for.

MXA did the test on the SX 520 and SX400 back to back with the 400 being first. They all loved the handling of the 400. So quick and easy to steer. The said "if only it had more power". They all rode the 520 and said "man this thing handles wierd, vague in front, rear shock bad, fork harsh. The kicker: 520 and 400 are same bike, with a different cylinder and crank. Thats all.

Bottom line, if you wedge that 400 into your 250, its not going to handle like a 250, or a 400 for that matter. It will be fun for sure, but it wont be what you might expect.

Well, I already have 426 springs front and rear and I've got the valves to install as well. I'm not trying to make my 250 a 400. It is just chance that the 400 will work better in my dirtbike than it would in my wifes street bike.

As long as it is a bolt it up operation it would be worth a try.


sorry, but no it wont fit. the 250 motor si shorter than the 426 motor and ahs the same fram clearance, you might be able to jam that 400 int here, but i guarentee youd need ot make your own engine mounts, probobly modify the head pipe, and i still dont think it would fit under the gastank.

just put the 400 motor in your wifes streetbike and retard the valve timing and the ignition timing, yo can also throw on a smaller carb. You might also try doubling up on head gaskets to reduce the compression. this would all help tame the engine.

Not selling my 250 motor.


You can do anything if you put your mind to it.

There is a B class rider from Florida that has a CRF 450 with a YZ 250F motor. It was a neat, clean swap, and is a whole lot stronger in the 125 class than the anemic CR 125 motor.

Was just on the MX Sports website... he will be in the 125 B Mod class at Loretta Lynn's in a couple of weeks.

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