synthiam.com/Community/Questions/2257

EZ-Robot servos use the Futuba standard spline , Hitec servos have a slightly different spline but the threads in the shaft are the same so many servo horns made of plastic you will be able to press on because they are close enough. I prefer to just use horns that match the spline that are usually included with the servo. The difference is going to be important if you try to use a metal servo horn on a metal spline that doesn't perfectly match. At that point you want be able to get the horn on the spline with a sledge hammer lol - Josh S

Awesome That is what I ordered 3" wheels Thanks