I purchased a prop flex shaft from Aeromarine for the Thunder Cat. It does not have a 'flat' machined in the prop shaft like the flex shaft that came with the Thunder Cat for the prop drive dog collar set screw. Has anyone modified such a drive shaft and if so how did you do it?

Since the prop shaft is likely dead soft, you can file the flat on the shaft with the narrow edge on a flat file. Alternate method; if you have a high speed grinder (Dremel, Black & Decker, etc), use a cut off wheel and grind the flat.

I have all of the above and an air die grinder. That is what I will do. Thanks.