Originally Posted by
joejack951
That would be a 5mm (as in 5 millimeter) hex. Keep in mind that you'll need a long 5mm hex key to reach the hidden clamp that holds the shifters to the bars. There's no need to remove the screw either. You need to only loosen it just as much as necessary to slide the brifter off the handlebar.
I've exchanged Shimano brifters without unwrapping the bars. First, remove the cables. Next, unscrew the hex bolt all the way and lift the brifter right off of the clamp, which will remain on the bar. Unscrew the clamp from the new brifter and set it aside, then thread the brake housing into the new brifter, line up the bolt with the clamp and attach. You'll want to push the brifter up snug against the brake housing (e.g. "up" the handlebar) when tightening the attachment bolt. Reinstall your cables, adjust, and you're done.
One last step, thread the "new" clamp onto the old brifter and list on ebay.