Inner cone is 41mm /-0.5mm outside diameter as I do not have a dial micrometer.
i use this spinner with a Turnigy Aerodrive SK3 - 3530-1340kv Brushless Outrunner Motor 4mm shaft. the spinner parts fit like a glove. an added bonus is the spinner can be used with different motor shafts. in my opinion, this is a fine product worth every penny.
Fits very well with the NTM 3536 mounted on a Multiplex Cularis. The weight did not adversely affect the CG and a short flight confirmed it was okay. Well balanced and looks great with the matte finish.