the sg is a standard shaft that will fit most clutch bells, the other is like the duratrax 21 and needs a adaptor to hold clutchbells,garbage .the sg if you were to chose is the way to go.you would need something like this to make it work if you didnt buy the engine with the sg shaft http://www2.towerhobbies.com/cgi-bin...?&I=LXBCK5&P=X