I have the Thunder Tiger 91 Four stroke and I'm having trouble fitting a spinner on. The thickness of the backplate along with the prop thickness doesn't give room for both of the lock nuts to secure the propeller. The shaft becomes too short. The prop is a Master Airscrew 14x6. The backplate of the Great Planes Spinner doesn't look unusually thick.
Has anyone had the same problem and solved it?

Yeah, switch to an APC or wooden prop. Those MAS props are way too thick. I simply ran without a spinner until I was comfortable with the plane then switched to APC. On the next plane I went straight to the APC.

Thnks I'll get the APC prop.