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mix match motor '12X top and '04R bottom

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I have a 2012 CRF450X engine with a shagged 3rd gear and upon stripping it down found cracks in the right hand crank case, but the top end is all good

I have a 2004 CRF450R engine with a destroyed top end but cases and and gearbox are all good.

Question: Can i use the X top half and the r bottom half?

As far as I've seen the crankshafts have the same part number and I have one spare plus other parts. I would much prefer the x gear ratios (once 3rd is fixed to be in the final frakenstein motor but it's not necessary) 

Is this possible and importantly would the '04 R bottom end fit in the '12 frame?? (my extremely rough measurement indicates it would be close)

Just asking to save time and money, time being the biggest factor.



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