What is Spur Gear?
 

What is a spur gear and where should it be located? I am very new to this hobby and I kinda need some help. Also, would a .12 engine fit in a .15 engine space?

heh
the spur gear is the large gear that connects to the engines clutch

ill try and answer your second question, im guessing your asking if they are the same size and they are just about the same size but if you get an engine make sure the shaft on the engine will fit onto the car.

What exactly this thing for?
 

I did saw on option parts that did selling this spur gear but what really this spur gear do? What are the gear improve? Acceleration or...:tire:

It can add Acceleration or top speed depends on which one you buy. Are you talking HPI?