Assuming that this is the top bolt on a quill (threaded) stem - I've never seen a 6mm Allen on any other cockpit application except a treadless expander plug, and they're too low torque to be a concern. If you have a ratchet handle, square-drive (3/8" or 1/2") hex heads are readily available. Alternatively, any sort of narrow pipe of tube to extend the handle of your current hex key will give you more torque