Stick a hex wrench through the two slots in the opposite outdrive and hold it and the gear while turning the screw. You should also do this to both outdrives and try to turn the gear when tightening it back up. It should be very difficult to turn. If you can turn it easily, it needs to be tighter.

I got the gear at the lhs, but tower has it [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&W=000626652&I=LXFWF3&P=K]here[/link]
Probably should also get the [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXAMF0&P=M]rebuild kit[/link]