I dont have that model but have a few ST.90s with spinners. All the plastic ones have to be trimmed. I don't know what the spinner companies are thinking. Making a 3-1/2" spinner that's only big enough for a 9x6 prop. It sound like yours is metal. I have several Tru-Turn spinners. They are more than big enough. What brand is yours? If it is metal(ALUM) you could open it up your self. All the ALUM ones I have seen have a hole in the center to bolt on. This hole can be used to run a prop balancer through.

David