Very good info here, even if we have gotten a little mixed up on which end of the shaft. The pictures I posted are the coupler side. GN7 and I mentioned shaft's breaking at the prop end between the strut and hub. I originally posted the picture to get opinions and knowledge about the type of slots cut. We did'nt have a 5/16 saw cutter so we used a regular end mill bit in a vertical mill. I was told it would be fine but the saw cut slot done in the horizontal mill was less likely to lead to a crack forming at the end of the slot, the ramped up area. Westernaero has a different opinion. I won't try the full rounded bottom slot because I don't have a way to shape a key to match. Other than a bench grinder, which would not create good results. I like his advice about the fillet slot. I can sacrifice a Craftsman screwdriver for key material that will match that slot. Sangerdan mentions milling a flat surface for the set screws to seat against. Is that better than just dimpling the spot where the screws hit? Flat milled area would probably be better if you experiment with spacers and the screws don't always hit the same spot.