Cannot tell you the size of the hole, but removal is not a problem. There is a bolt with an allen key head which locks into the end of the prop drive shaft. Once this has been removed i simply put a large phillips screwdriver into the hole in the cone and it comes free with a good sharp tap with a hammer. Hold a block of wood against the prop to stop it turning as you use the hammer. I always grease everything with a waterproof grease on re-assembly and have never had a problem. If i had to guess the hole size, i would think it will take a 5 or 6 mm bar.