WELL YOU NEED TO GRIND AWAY A LITTLE MATRIAL AROUND THE OPENING.NOT MUCH!AND THEN 45 DAGREE THE EDGES.THIS IS TO ALLOW THE WHEEL ON YOUR STARTER BOX NOT TO HIT THE BOTTOM OF YOUR CHASIS.SOUNDS WEIRD I KNOW BUT BELIVE ME YOU WANT TO DO IT.OR YOU WILL RUIN THE WHEEL ON YOUR STARTER BOX.[&o]THE REASON IS THAT THE OPENING ON THE PRO IS BIGGER