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so I have a street legal Crf250x and it's 5000rpm at 70km/h with 14tooth front and 50rear, thinking about putting 46 in the back and big bore so it's not as gutless and stressed for street riding?


Will a big bore help out the top end a bit because it's all bottom end because of the "250x" torquey cams?


How often will the top end blow up? I won't race it just getting around town and short highway trips...

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Less stress, (lower RPM, lower weight, lower drag) good maintenance typically will result in longevity. 

this is a good calculator for calculating gearing changes, http://www.gearingcommander.com/

A big bore will give more displacement, more power. A cam will have a bigger effect on the tuning where the power comes from, torque/HP and at what RPM range.  

Dirt bikes are not equipped with cush rear hubs, so additional HP ( Big Bore) and hard acceleration on pavement will add stress to drive components that were designed for dirt with less traction. on pavement riding will impact everything from the wrist pin to the tread on the tire.  

Probably a better question to ask in the 250X forum.



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