Step 10: Mount the motors
Take one motor and put it into the frame (like you did when marking the mounting holes for the U-bolts). Add a 4 1/2" 12-13 bolt and lock nut, and tighten everything so that the motor is pulled up against the frame, but you are still able to move pivot the motor about the bolt.
Now, if your holes weren't drilled perfectly (mine weren't), then the the head of the drive bolt will be hitting the Center cross member. Before I discussing the solution to this problem, I'd like to point back to Step 4 where I mentioned that I couldn't change the step size on my robot. This is why. As you can clearly see, if the bolt were placed in any other hole, the head of the bolt would hit either the Center cross member or the frame rail. This problem is a design flaw that came about from my neglecting the size of the bolt head when I made my CAD model. Keep this in mind if you decide to make the robot; you may want to alter the size or position of components so that this doesn't happen.
The immediate bolt head clearance problem can be alleviated by adding a small riser under the barrel of the motor over the cross member. Since the motor can pivot about the main mounting bolt, raising the barrel of the motor raises the drive shaft, so we can get the necessary clearance. Cut a small piece of scrap wood or metal that lifts the motor enough to provide clearance. Then, add the U-bolt and secure it with lock nuts. Also secure the nut on the main mounting bolt.
Repeat this step for the other motor.