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1983 honda cr125 roller with xr100 motor

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Has any body swapped  motors from 2 stroke to 4 stroke? I have a old 1983 honda cr125 roller no motor and wish to install a xr100 (120cc) big bore motor.  I set the motor in and looks like it would work. I have welders ,mills, lathes  etc. so it should be easy swap. Any input would be appreciated.   

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Could prob work but why swap a 4 stroke into it. I'd at least find a decent motor to put into it like a newer 125 2 stroke engine. If you ever would decide to sell it with that 4 stroke motor in it it would prob end up being worthless. Up to you. But I am sure there have been plenty of swaps like this before. 

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Saved the bike from the scrap guy 40$  .  I'll try to get some pics of the bike up.  

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1. It will fit, will most likely have to modify the lower fame rails.   

2. The carb location relative to the  frame and shock may be a problem,  4 stroke carbs sit higher on the engine.

3. The foot pegs may interfere with the kick starter, so custom foot peg brackets might be needed. 

4. Make sure you can line up the sprockets.

I fit a XR 80 motor into a kx 60 frame, it took some creativity, and a custom fuel tank,  the CR125 is a full size bike, so it should be easier.  I bet a XR 100 is physically larger then a 125 cc 2 stroke.   Getting the engine to "fit " is the easy part, getting the little things like pegs, kick start, gear shift.... to work well is the hard part. 

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These XR 100 engines are usually swapped into 80 MX frames to make nice little racers or hobby bikes that don't have to be revved to the heavens to make power. I'm sure it would be less work (still not a drop in and go) to put one into a 125mx chassis, but don't quite see the appeal, unless it's just to practice fabrication skills, but there is certainly nothing wrong  with that. 

Maybe you could take it to the next level with one of the super high performance big bore kits from webike.com in Japan. They stuff far above and beyond the TB and BBR 120 kits readily available in the US market.

Edited by HappyAndy
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I have an 84 cr125 that i was thinking of putting an xr250 into.  I have enough stuff to build an extra engine.   got better forks for it too.   Kind of a tastey idea.  It is a pretty roomy frame compared to now.  

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Here's some pics xr80 motor installed for reference . This is just a project bike (spare).  I rebuilt my sons honda xr100 with an Tb 120cc kit which goeshonda 1 042.jpghonda 1 005.jpg pretty good.   

honda 1 041.jpg

honda 1 001.jpg

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I like it.  It seems like my stock xr100 will skin a ttr125,  and yours built up will certainly do that and handle better.   Oh, but lol, your RESALE!!!  It looks like a fun project, little expense,  and fun to play on or go trail riding.  And you are going to get some conversations going.   I have some 43mm cartridge forks to put on the one I have,  now I am starting to think of one more project that I DONT NEED!  (thanks)

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I don't think an xr250 motor will fit without cutting and welding the frame. I took some more pics and the oem gas tank should fit. The bottom motor mounts will need to be removed and the motor needs to go over 1 1/2 but looks good.

honda 2 013.jpg

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The xr250 will definitely take some cutting.   I watched a friend of mine make a pattern jig to put a cr500 into a crf250,  and wondered why I had been working the way I had forever.  Takes about 30 minutes (to make the jig)  if you have the cutoff saws and scraps that I have,  and of course welders.   he had his aluminum frame cut two hours after he started.  Smartest thing I could do is leave my "stuff" with him.  

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Ossagp, the xr100 motor is close in dimensional size to the cr125 motor. Everything revolves around sprocket alignment and the gear box on the xr250 motor is a lot bigger. The xr250 has dual exhaust ports that might cause a problem. You may want to try an 84 xr250 frame and weld the lower half if the parts are not interchangeable . You would need to make a jig for alignment and heat warp age. I'll try to get some dimensions this weekend.            

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I don't know why I have the frame and one of the lower ends fully assembled on the lot now.  Frame is out in the snow darn it.   Joel simply took the cr500 engine out of its frame,  Made some jig ends by cutting some flat,  welded tabs to the flat,  drilled to fit the mounting bolts and put them in the frame.  took tube stock and cut to connect the frame spacers.  Cut and fit his cr500 into the aluminum frame the first time.  Might find it if you google eskimo joe's cr500 snow cycle.    

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On 3/27/2017 at 1:15 PM, ossagp said:

I have an 84 cr125 that i was thinking of putting an xr250 into.  I have enough stuff to build an extra engine.   got better forks for it too.   Kind of a tastey idea.  It is a pretty roomy frame compared to now.  

It will fit. A local Honda shop use to do those conversions.

On 3/30/2017 at 10:56 AM, ossagp said:

The xr250 will definitely take some cutting.   I watched a friend of mine make a pattern jig to put a cr500 into a crf250,  and wondered why I had been working the way I had forever.  Takes about 30 minutes (to make the jig)  if you have the cutoff saws and scraps that I have,  and of course welders.   he had his aluminum frame cut two hours after he started.  Smartest thing I could do is leave my "stuff" with him.  

Yes and no. The 2T chassis have a tighter radius on the frame cradle because of the 2T smaller crankcase so will need to be reshaped for the 4T crankcase.  I, and 4 others, installed XR200 engines into 81 RM Full floater chassis'. I  rebent the frame cradle to suit the XR crankcase and added a 1-1/2" splice to the down tube, plus new motor mounts.  They were so successful that today I know location and owners of 4 of the bikes.   

58e5a371877ab_Honzuki(1).JPG.7e3bb21ccfb29089a44888bb5ed0172c.JPG

 

 

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Here's some pics of my 1987 xr250 motor (just rebuilt going back in bike) compare to the chassis of the 1983 cr125. The xr100 is a better fit and less work . The pegs need to be moved up and the sprocket on the 100 is 420 and cr is 520. I will have to modify a sprocket to fit or change to 428 sprockets.  

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yes the 250 would be some work,

but it would be a much better handling home..............

 

curious,

does anyone know if there is a physical size difference between an xr200 and 250 engine? i have never seen them side by side, just always assumed they shared some parts

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How is your XR/CR conversion coming along? I am considering a similar project using a '77 CR125M with an XR100 mill punched out to a 120 with a few other trick mods.

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I put the project on hold for other projects (house) . The last think I remember is changing the chain to 428 . The OEM is 520 and the rear sprocket would need to change to . I was looking at yamaha ttr125 rear sprocket mod. My son has an xr100 and xr80 the 100 has the bbr 120 kit on it . It will power wheelie in first and and bump the wheel in second. If i every get my house projects done i hope to start working on the bike again .     

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You might want to check into using a rear wheel from the '74-'78 CR125M. The early 125s came stock with a 428 sprocket. I have built a couple XR75's, one with an XR80 motor punched out to 92cc's and a twin-shock XR100, with the 120 kit, for my boys to race. I also have had some fun on them. Last year my oldest son started racing the '77 Elsinore 125, but I have discovered (as others cautioned me) those early 125 motors are fragile. I have a couple XR100 parts motors lying around and a friend of mine built an XR100 motor and stuffed it into a '76 Elsinore 125. Duane Brown, of BBR, did much of the fab work for him though. Taking all this into consideration I've decided to drop an MR175 motor into the Elsinore. Maybe it won't blow up after a season.

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Ill  keep my out for and cr125m rear wheel . That's great your boys race mine is still learning how to ride. On the xr motors I drilled out the oil passage on crank cover .185 so more oil goes to the head. I've seen a lot of oil starvation on cam journals.   I might try using xr80 cylinder head on 100. I think it has a smaller combustion camber than the 100.       

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