Possible KDX200-RM125 hybrid. Need cases for mockup.

Discussion for swapping a KDX motor in a MX frame...

Possible KDX200-RM125 hybrid. Need cases for mockup.

Postby TheRadBaron » 10:45 pm Nov 07 2012

Hey guys, this is my first post here. I have a pristine '96 RM125 chassis that I'm thinking about swapping a KDX200 motor into. I've never ridden a KDX but I like everything that I hear about them. My RM even runs, but I don't like the idea of a 10-hr re-ring interval and I'd like to make it into more of a trail bike than a motocrosser. It might be easier to just get a KDX, but everything is so nice about this RM chassis that I'd really like to use it. That and I live to modify things. I was thinking that I'd really be breaking new ground with this build, but when I found this forum I was pleasantly surprised to see that it's commonly done. Swapping the KDX engine into MX chassis, that is. I searched and found mention of a few people wanting to do a swap into an RM125, but no one seems to have gone forward with it and reported back.
Anyway, before I start getting too fired up with the project I'd like to see if it will be doable without any MAJOR modifications. I know that I need a liquid-cooled KDX200 or maybe 220 engine, but is there a particular year or series that will lend itself to this swap better? Also, would there be anyone here who would be willing to lend me a set of bare cases that I could use for a few days for a mockup? I live in central Illinois, but I'd be happy to pay for shipping both ways, of course. I'd also be happy to put down a deposit for the cases until you get them back in good shape. I know it's an odd request, but I don't really want to buy a set of cases that I won't use. If I do go ahead with the swap I'd get a complete motor, and if I decide not to I'd have a set of KDX cases that i couldn't use. Thanks very much.
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Possible KDX200-RM125 hybrid. Need cases for mockup.

Postby rbates9 » 11:09 pm Nov 07 2012

As far as the year of engine to use I would have to say that the 95+ would be the best bet for no reason other then parts availability. The E series (89-94) as I hear it are great but have some power valve issues but people tend to say that they make more low end power. The H (95+) seems to be a little more reliable and the parts should be available longer. I'm not starting a E VS H debate, just do a search and see what the common problems are for your self.
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Possible KDX200-RM125 hybrid. Need cases for mockup.

Postby Tedh98 » 07:55 am Nov 08 2012

TheRadBaron wrote:Anyway, before I start getting too fired up with the project I'd like to see if it will be doable without any MAJOR modifications.


What is your definition of major?

I chose the H series for the exact reason rbates9 stated.
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Possible KDX200-RM125 hybrid. Need cases for mockup.

Postby TheRadBaron » 08:10 am Nov 08 2012

You're right. MAJOR is a relative term, even when it's spelled with all capitals. I guess that I mean substantial frame mods going beyond just engine mounts or some reworking of the frame cradle, professional-level machine shop or welding work, fabrication above what a pretty skilled guy can do in this garage, that sort of thing. Another thing is the exhaust location. I've had potential engine swaps foiled in the past on vintage dirt bikes by the exhaust port exiting right into a frame tube.
I don't expect it to be an easy, bolt-in swap. But I also don't want to get involved if it's going to take $1000 worth of precision machine and one-off fabricating.
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Possible KDX200-RM125 hybrid. Need cases for mockup.

Postby Tedh98 » 08:36 am Nov 08 2012

As you mentioned, there is more to an engine swap than just physically bolting the engine in a different frame.

To really understand if the swap is reasonable I think you need to get the complete engine.

Cases alone won't tell you what is involved in the key areas of fitment:
    cylinder head has to clear the frame when the c/s is lined up with the swingarm bolt
    what kind of clearance you have between the head and gas tank
    what/if any modifications need to be made to the pipe once the engine is lined up properly
    what/if any modifications need to be made to the airbox and boot to mate up to the KDX carb

Those are the areas that come to mind right now. There are always the little things that vary between one build and the next.

There is a lot of overlap between Kawasaki and Suzuki. An RM65 and KX65 are identical except for the color of plastic. I've found I can use RM125 parts on my KX. Plus Suzuki parts are cheaper for some reason. I bring this up because while there aren't any RM builds on the site, there could be a high degree of similarity to KX builds of that year.
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Possible KDX200-RM125 hybrid. Need cases for mockup.

Postby TheRadBaron » 08:48 am Nov 08 2012

That's encouraging to hear. I agree with you about needing the whole engine to truly get an idea, but it's not really practical at this point. A complete KDX200 engine is a sizable investment and I'm not prepared to make it at this point. I think that a set of cases will give me a good enough idea about whether or not I want to go forward with the hybrid. Once (if) I determine that the cases can be made to fit, I'll study pictures of the rest of the engine to see if there will be any serious obstacles. If that comes up clear I'll probably buy a KDX motor at that point.
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Re: Possible KDX200-RM125 hybrid. Need cases for mockup.

Postby KarlP » 08:45 am Nov 12 2012

It can definately be done and would make a really nice hybrid.

If you are serious a low risk approach is to buy a KDX in decent shape. You'll want a platform to get the motor running properly in, anyway. You'll also need all kinds of bits off the KDX for the greatest chance of success.
'08 KTM200xc
'99 CR/KDX Hybrid with that RB stuff done to it
KX100 for the boy
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