Is it a good idea.

[postman]

I have been using the Giant Rincon mountain bike these past couple of weeks.I am not too keen on the straight bars,i have some bar ends on.They are uncomfortable.
So i am thinking of investing in some butterfly bars,now the bars that are on seem to be 25.+ where the brakes attach,then the bar gets thicker and might be 31.+ in the middle.
So if i buy 25.4 bars then put a shim in,will it be safe.
I need thinner ends to get the brakes/gears on.If that makes sense.

 

[Jon2]

All handle bars will be the same diameter where you clamp the brake and gear levers. Where the stem clamps the handle bars will either be 25.4mm that you've seen or 31.8 which is what you say is on your bike. The stem clamp diameter has no effect on the diameter of the bars where you clamp the brakes.