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Curse you, W. N. Woodruff!
Late in the nineteenth century, a Connecticut inventor named W.N. Woodruff invented the Woodruff Key, a way to mate parts to rotating shafts. Mr. Woodruff came up with the ingenious idea of inserting a semi-circular key into a keyway cut longitudinally into a shaft so that a portion of the key protrudes as a tab from the keyway. The tab fits into a notch on a part placed over the shaft and prevents the part slipped over the shaft from freely rotating about the shaft.
Another benefit of this setup is it eliminates the need for expensive milling of an integrated tab into the shaft which concentrates stress. This configuration also helps to concentrically align the shaft and mated part which is critical to reducing wear and vibration in high speed applications. According to Wikipedia, the woodruff key is widely used in machine tools, automotive applications, snowblowers (but how widely used could that be?), and marine propellers.
How ingenious was this idea? Well…Mr. Woodruff’s invention was SO ingenious that in 1888 he was presented with the John Scott Medal by the prestigious Franklin Institute, where over the years many famous scientists have demonstrated groundbreaking new technology. For example Nikola Tesla demonstrated wireless telegraphy at the Institute in 1893, and in 1934, Philo Taylor Farnsworth gave the world’s first public demonstration of an all-electronic television system.
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So stop slamming the Woodruff key. IT has been around a lot longer than you! LOL.
If you drain the oil you could also use a magnet on a long flexible wand to try to fish it out. I don't think it would come out with the oil. You could also put in a magnet drain plug if you can't fish it out.
I find that by keeping the forward part of the the key a little low (angled slightly towards the front of the car) it might stay in place better.