First, slide the prop on without the key, mark the shaft at the rear of the prop hub. Now install the key in the groove, and slide the prop back on making sure the key stays put. The prop should reach back to the fully seated mark you made, if not sand/file the key on the side facing up until the prop slides back to your mark. If you can, fit the new prop with a new key and save the original key and prop for easy swap out later if needed because once you sand down that key, it won't work with the original prop. It is not as important for the key to be super tight in the groove as it is for the prop to go on all the way.
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