I have seen this done and was wondering what the benefits are?

 

they say the cylinder has more cooling and better port layout for making power..plus the sleeve is nikasiled so you don't bore it, just put pistons in. On the trx cases they have to block the powervalves off, as the trx has no gearing in the bottom end to run the valves.

 

I have saw a 250r where they just removed the power valves fromt he cr250r cylinder then it just bolts right on.