To some degree, maybe both. Most .40 sized engines take a 2 to 2 1/2" spinner and are generally running a 10 to 12 inch prop.. On bigger engines, a bigger spinner may be required due to the size prop you are using or because an electric starter may slip on a smaller spinner, the bigger the spinner, the more grip the starter can get on it. My Magnum .46 has a 12x4 apc prop that wouldn't fit in my 2" nylon spinner, so I'm using a 2 1/2" aluminum spinner on it. Just my experience, some one else may have a more exact answer. Hope this helps.