Wera Tools 4mm Ball End Hex-Plus L-Key

A ball end hex L-key will allow you to get into otherwise impossible areas to tighten up a screw.  The ball-shape on the long end of the key allows for a misalignment of up to 20 degrees. The short end of the key is square cut to provide maximum surface area when you have a clear shot at the screw and want to use the leverage of the key.