The above mentioned Rotostart is for the EZstart style OWB.
For a standard 12mm Hex OWB, I'd suggest using this one instead:
http://www2.towerhobbies.com/cgi-bin...&I=LXERF2&P=ML
If you do get this one, you'll also need a 12mm Hex OWB:
http://www2.towerhobbies.com/cgi-bin...&I=LXDLN7&P=ML
Be sure to place a spacer (about 6mm thick) between the OWB and the Rotostart cover to prevent the bearing from backing off the shaft, this is vital to prevent the OWB from binding and failing prematurely.