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KX500 motor in '91 KDX250?

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Forgive me if my searches on this missed a topic already covered, but I didn't find anything....

I've got a California-plated 1991 KDX 250 that needs a total rebuild. I'm having a hard time finding a bottom-end rebuild kit for this bike, and got to thinking that it may be easier to just swap the engine out entirely. I know the kdx's engine is essentially an '89 kx 250 with different porting, but I don't know if or how either the kx 250 or 500 engines will fit in. Can anyone enlighten me? Any help is very much appreciated.

The bike's on it's way to being a motard. The license plate is what's keeping me tied to this particular frame - I know simply getting another bike would be much easier.

Thanks for any help.

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Personally, I wouldn't even mess with trying to swap a 500 engine into that frame. There is a lot of fabrication involved in doing so. A 250 engine would require less significant mods (probably would still need to do some welding for the mounts) but still very doable.

However, I don't see the problem with rebuilding the KDX250 engine. Am I missing something here? If the crank, cases, and trans are good then everything else can be bought through any parts supplier. I know I have bought every bearing, bushing, clip, washer, etc. for a '90 KDX200 on Friday so I'm sure you can get them for the KDX250 as well.

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the motor was never the issue with the bike. It was the jetting, the lower compression gaskets and suspension of the bike that held it back.

check with motosport.com for a rebuild kit for a KDX or KX of the years.

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If you like the bike and it is CA plated I think the 500 swap would be good.

The 500 would be a much better motard. 500 smokers are fun, but not the best street rides. They get terribles gas mileage, run premix and vibrate.JMO

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the 500 swap would be easy as its a near bolt in, the biggest hurdle for some would be the pipe. You would need a modified cr500 pipe just like the kx500 AF. A front head stay would also need to be added to the 500 head. If it were mine i would just rebuild the KDX with a crankshaft and cylinder from a newer kx250.. I have a '98 setup on mine, it makes a great engine.

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Personally, I wouldn't even mess with trying to swap a 500 engine into that frame. There is a lot of fabrication involved in doing so. A 250 engine would require less significant mods (probably would still need to do some welding for the mounts) but still very doable.

However, I don't see the problem with rebuilding the KDX250 engine. Am I missing something here? If the crank, cases, and trans are good then everything else can be bought through any parts supplier. I know I have bought every bearing, bushing, clip, washer, etc. for a '90 KDX200 on Friday so I'm sure you can get them for the KDX250 as well.

The right side cases will have to be replaced. The PO was a tweaker that didn't have much case-repair talent.

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A rod kit for an '86-'93 KX250 works.

Aside from some tuning changes, different ignition and the wide-ratio tranny, the '91-'94 KDX 250 engine is actually a '90 KX250 engine. Nearly every part from the '90-'93 KX will bolt right on.

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