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Old skool engine in modern chassis

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Hi

Just joined

Thinking of fitting and old skool air cooled single e.g. XR600 or XT/TT500 into a modern MX chassis e.g. Honda CRF or WR/YZ frame - most likely pre aluminium as I don't think there's space in the aluminium frame. 

Anybody done this or seen it done?

 

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I have thought of the same thing using my XR4 engine. I think a KTM steel frame might be a better choice. The room in most al frames is very limited in modern bikes and our older engines are quite large.

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While it is a neat idea, I think engine height in a frame designed for a water cooled engine is going to cause you a lot of grief. You should do some measurements of the engines you want to try and some frames you think might be what you want to use. A few inches, you can probably squeeze, more than that and I think you'll find serious issues

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yes its a "good" idea.

biggest trouble areas, based on my own mini version.    1982 XR 80motor in  2002 KX 60 frame

1. engine height, especially if its a 2 stroke frame.  Frames often have a gusset at the top, if you cut it out of the way you will want to reinforce the neck/ head tube area.

2. counter shaft location relative to swing arm pivot.  It needs to be close to where the original motor would have been.

3. frame width vs motor width at the back.  You need to have the space to line the front and rear sprockets up.  If the engine width at the counter shaft sprocket is to large it may be  deal breaker.  Easiest way to see if it might work is check the rear wheel hub width of the donor bike and the project bike.  If they are close you can probably make it work.  A few mm off will not hurt anything. 

4. Front frame down tube or tubes.  It will most likely have to be removed or modified to fit the new engines and exhaust, also check for full suspension travel with exhaust mounted.  If the engine is to long there may not be enough room between the swing arm pivot/ counter shaft location and the front wheel.  

5. don't be afraid to cut or modify the lower frame cradle.  

6. don't forget foot peg, gear shift, and rear brake pedal clearance with the new engine, if its kick start, that just complicates it even more.  This can be the difference between getting the engine to fit, and getting the bike to work. 

7. The carb has to fit, this can be a issue if you are sticking a big tall four stroke in a spot made for a 2 stroke with a much lower carb location.  plan on making a custom intake boot. 

 

 

 

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IMG_3016.JPG.939524384547ad1fd3bf5c2625321015.JPG


Thanks for your responses. Clearly a lot to think about and some measuring to be done!
I'm going to need to use a steel frame most likely 4 stroke for space and ease of modification.
I like the idea of the xr400 motor as it should be shorter in height and I can hop it up to c. 460cc
Cheers
G

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the xr 400 motor carries its oil in the frame, so if you use one, you have to figure external oil tank and the hose routing/locations.  A xr 400 is my primary ride and has a great motor. 

I started a dr 250 motor swap into a KTM 400 exc frame,  the project got stalled out because of the oil tank issue and the fact that my donor motor was not in as good of shape as I first thought.   I would think the best engine would be a wet sump ( no oil in frame or external oil tank ) and e- start.   This eliminates two potential problems right off the bat. 

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KTM frames are the most forgiving.  I have an 03 engine in my 13 chassis, not as old school as you mentioned, but it was as close to a direct bolt in as you're going to get.  

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I like the KTM frame idea as I run a 200EXC so I have some experience of their chassis having run them since '99
What's my choice of air cooled wet sump motors?
Hondas are out so I guess XT/TT Yams or DR350 with a big bore kit?

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I always wanted to put my modified KDX250 engine in a modern MX chassis. That was a sweet engine hobbled by a heavy chassis with poor geometry.

I think you'll be hard pressed to find a modern frame that is going to accept an old air-cooled four-stroke engine without a LOT of work, those engines were tall and long in comparison to water cooled engines. You're probably better off just trying to fit modern brakes and suspension to the original chassis.

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Thx
May end up having to go down that route but would prefer modern frame with modern geometry and ergos if possible.
I need to find some bikes to measure but I think the KTM single cam 525 motor is quite tall so that might be a good donor chassis.
A mate of mine is just finishing a KTM 640 Adventure build which is a pretty big engine so that chassis might work also.

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I saw a VERY sanitary bike at an AHRMA event a couple of weeks ago.  An XR200 motor in a 78 RM125 chassis.  The guy did a great job on it, looked factory and worked very well on the track.

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