go here
http://www.robinsonracing.com/catalog/
or better yet here
http://www.robinsonracing.com/catalog/64pitchspur.html
You can see the little holes in the gear. You see 8 in a cir and two oppset each other. You want to make sure that you have the ones in the circle. some might have two row of circles. If it dose you want to use the out side ones. If you racing 1/12th then go with the in side ones.
Sorry if thire are miss typing but it 2 am here and i need some sleep.