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I did mine 5 years ago and saved the directions. I just reread them and it simply assumes the impeller has been removed and at the end says reinstallation of impeller will position seal to correct working length. I do not recall having any difficulty removing the impeller, so it must have been a tapered shaft & nut, or tapered shaft & a pin, or a snug fit on a straight shaft & a pin. I'm pretty sure it was a stainless steel pin. I do recall using a piece of PVC pipe that had the inside diameter that fit over the shaft to press the new seal on. Just inspect the impeller looking for ways it it can be secured.
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It would appear that you have a problem with spacial reasoning as it applies to mechanics. Tapered shafts have a wider diameter in the back and narrower diameter in th front. A straight shaft has the same diameter for its entire lenght. Pins can be used for securing to any shaft by drilling a hole thru the impeller/gear/fan/whatever device and the shaft & then sticking a pin in the drilled holes. Another method is by threading. Look at the impeller and see which way the motor turns it. To unscrew turn the impeller in the opposite direction of rotation while holding the shaft from turning.
Summary: To Remove the impeller-if you see a nut or pin, remove it and pull the impeller off. If there is no nut or pin- unscrew it.
General C&ontractor
35 years of troubleshooting construction, remodel, component & material failures. What to do next